diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index e403ed994e566..e30178e987bb0 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -173,6 +173,14 @@ namespace ts {
             return node.name ? declarationNameToString(node.name) : getDeclarationName(node);
         }
 
+        /**
+         * Declares a Symbol for the node and adds it to symbols. Reports errors for conflicting identifier names.
+         * @param symbolTable - The symbol table which node will be added to.
+         * @param parent - node's parent declaration.
+         * @param node - The declaration to be added to the symbol table
+         * @param includes - The SymbolFlags that node has in addition to its declaration type (eg: export, ambient, etc.)
+         * @param excludes - The flags which node cannot be declared alongside in a symbol table. Used to report forbidden declarations. 
+         */
         function declareSymbol(symbolTable: SymbolTable, parent: Symbol, node: Declaration, includes: SymbolFlags, excludes: SymbolFlags): Symbol {
             Debug.assert(!hasDynamicName(node));
 
@@ -181,10 +189,11 @@ namespace ts {
 
             let symbol: Symbol;
             if (name !== undefined) {
+
                 // Check and see if the symbol table already has a symbol with this name.  If not,
                 // create a new symbol with this name and add it to the table.  Note that we don't
                 // give the new symbol any flags *yet*.  This ensures that it will not conflict 
-                // witht he 'excludes' flags we pass in.
+                // with the 'excludes' flags we pass in.
                 //
                 // If we do get an existing symbol, see if it conflicts with the new symbol we're
                 // creating.  For example, a 'var' symbol and a 'class' symbol will conflict within
@@ -202,10 +211,10 @@ namespace ts {
                 symbol = hasProperty(symbolTable, name)
                     ? symbolTable[name]
                     : (symbolTable[name] = createSymbol(SymbolFlags.None, name));
-                
+
                 if (name && (includes & SymbolFlags.Classifiable)) {
-                    classifiableNames[name] = name;   
-                } 
+                    classifiableNames[name] = name;
+                }
 
                 if (symbol.flags & excludes) {
                     if (node.name) {
@@ -314,6 +323,7 @@ namespace ts {
 
                 addToContainerChain(container);
             }
+
             else if (containerFlags & ContainerFlags.IsBlockScopedContainer) {
                 blockScopeContainer = node;
                 blockScopeContainer.locals = undefined;
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d72e74f748eb6..d07764045a57b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12034,6 +12034,12 @@ namespace ts {
                 grammarErrorOnFirstToken(node, Diagnostics.A_class_declaration_without_the_default_modifier_must_have_a_name);
             }
             checkClassLikeDeclaration(node);
+
+            // Interfaces cannot be merged with non-ambient classes.
+            if (getSymbolOfNode(node).flags & SymbolFlags.Interface && !isInAmbientContext(node)) {
+                error(node, Diagnostics.Only_an_ambient_class_can_be_merged_with_an_interface)
+            }
+
             forEach(node.members, checkSourceElement);
         }
 
@@ -12306,6 +12312,16 @@ namespace ts {
                         checkIndexConstraints(type);
                     }
                 }
+
+                // Interfaces cannot merge with non-ambient classes.
+                if (symbol && symbol.declarations) {
+                    for (let declaration of symbol.declarations) {
+                        if (declaration.kind === SyntaxKind.ClassDeclaration && !isInAmbientContext(declaration)) {
+                            error(node, Diagnostics.Only_an_ambient_class_can_be_merged_with_an_interface);
+                            break;
+                        }
+                    }
+                }
             }
             forEach(getInterfaceBaseTypeNodes(node), heritageElement => {
                 if (!isSupportedExpressionWithTypeArguments(heritageElement)) {
@@ -12313,6 +12329,7 @@ namespace ts {
                 }
                 checkTypeReferenceNode(heritageElement);
             });
+
             forEach(node.members, checkSourceElement);
 
             if (produceDiagnostics) {
diff --git a/src/compiler/diagnosticInformationMap.generated.ts b/src/compiler/diagnosticInformationMap.generated.ts
index 24b182c7134d8..a17f885759081 100644
--- a/src/compiler/diagnosticInformationMap.generated.ts
+++ b/src/compiler/diagnosticInformationMap.generated.ts
@@ -408,6 +408,7 @@ namespace ts {
         Non_abstract_class_0_does_not_implement_inherited_abstract_member_1_from_class_2: { code: 2515, category: DiagnosticCategory.Error, key: "Non-abstract class '{0}' does not implement inherited abstract member '{1}' from class '{2}'." },
         All_declarations_of_an_abstract_method_must_be_consecutive: { code: 2516, category: DiagnosticCategory.Error, key: "All declarations of an abstract method must be consecutive." },
         Constructor_objects_of_abstract_type_cannot_be_assigned_to_constructor_objects_of_non_abstract_type: { code: 2517, category: DiagnosticCategory.Error, key: "Constructor objects of abstract type cannot be assigned to constructor objects of non-abstract type" },
+        Only_an_ambient_class_can_be_merged_with_an_interface: { code: 2518, category: DiagnosticCategory.Error, key: "Only an ambient class can be merged with an interface." },
         Duplicate_identifier_0_Compiler_uses_declaration_1_to_support_async_functions: { code: 2520, category: DiagnosticCategory.Error, key: "Duplicate identifier '{0}'. Compiler uses declaration '{1}' to support async functions." },
         Expression_resolves_to_variable_declaration_0_that_compiler_uses_to_support_async_functions: { code: 2521, category: DiagnosticCategory.Error, key: "Expression resolves to variable declaration '{0}' that compiler uses to support async functions." },
         The_arguments_object_cannot_be_referenced_in_an_async_arrow_function_Consider_using_a_standard_async_function_expression: { code: 2522, category: DiagnosticCategory.Error, key: "The 'arguments' object cannot be referenced in an async arrow function. Consider using a standard async function expression." },
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 9b58e3143b2ee..94be4f53c9717 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1621,6 +1621,10 @@
         "category": "Error",
         "code":2517
     },
+    "Only an ambient class can be merged with an interface.": {
+        "category": "Error",
+        "code": 2518
+    },
     "Duplicate identifier '{0}'. Compiler uses declaration '{1}' to support async functions.": {
         "category": "Error",
         "code": 2520
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 7b683f2eeb5d0..4617ad98b0240 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -1597,8 +1597,8 @@ namespace ts {
         PropertyExcludes = Value,
         EnumMemberExcludes = Value,
         FunctionExcludes = Value & ~(Function | ValueModule),
-        ClassExcludes = (Value | Type) & ~ValueModule,
-        InterfaceExcludes = Type & ~Interface,
+        ClassExcludes = (Value | Type) & ~(ValueModule | Interface), // class-interface mergability done in checker.ts
+        InterfaceExcludes = Type & ~(Interface | Class),
         RegularEnumExcludes = (Value | Type) & ~(RegularEnum | ValueModule), // regular enums merge only with regular enums and modules
         ConstEnumExcludes = (Value | Type) & ~ConstEnum, // const enums merge only with const enums
         ValueModuleExcludes = Value & ~(Function | Class | RegularEnum | ValueModule),
diff --git a/tests/baselines/reference/augmentedTypesClass2.errors.txt b/tests/baselines/reference/augmentedTypesClass2.errors.txt
index 5881e49dae92d..278dca816fa30 100644
--- a/tests/baselines/reference/augmentedTypesClass2.errors.txt
+++ b/tests/baselines/reference/augmentedTypesClass2.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/augmentedTypesClass2.ts(4,7): error TS2300: Duplicate identifier 'c11'.
-tests/cases/compiler/augmentedTypesClass2.ts(10,11): error TS2300: Duplicate identifier 'c11'.
+tests/cases/compiler/augmentedTypesClass2.ts(4,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/augmentedTypesClass2.ts(10,11): error TS2518: Only an ambient class can be merged with an interface.
 tests/cases/compiler/augmentedTypesClass2.ts(16,7): error TS2300: Duplicate identifier 'c33'.
 tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate identifier 'c33'.
 
@@ -10,7 +10,7 @@ tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate iden
     // class then interface
     class c11 { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'c11'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         foo() {
             return 1;
         }
@@ -18,7 +18,7 @@ tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate iden
     
     interface c11 { // error
               ~~~
-!!! error TS2300: Duplicate identifier 'c11'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         bar(): void;
     }
     
diff --git a/tests/baselines/reference/augmentedTypesInterface.errors.txt b/tests/baselines/reference/augmentedTypesInterface.errors.txt
index a9fc28dd5ae57..690bf8b7a991c 100644
--- a/tests/baselines/reference/augmentedTypesInterface.errors.txt
+++ b/tests/baselines/reference/augmentedTypesInterface.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/augmentedTypesInterface.ts(12,11): error TS2300: Duplicate identifier 'i2'.
-tests/cases/compiler/augmentedTypesInterface.ts(16,7): error TS2300: Duplicate identifier 'i2'.
+tests/cases/compiler/augmentedTypesInterface.ts(12,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/augmentedTypesInterface.ts(16,7): error TS2518: Only an ambient class can be merged with an interface.
 tests/cases/compiler/augmentedTypesInterface.ts(23,11): error TS2300: Duplicate identifier 'i3'.
 tests/cases/compiler/augmentedTypesInterface.ts(26,6): error TS2300: Duplicate identifier 'i3'.
 
@@ -18,13 +18,13 @@ tests/cases/compiler/augmentedTypesInterface.ts(26,6): error TS2300: Duplicate i
     // interface then class
     interface i2 { // error
               ~~
-!!! error TS2300: Duplicate identifier 'i2'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         foo(): void;
     }
     
     class i2 {  // error
           ~~
-!!! error TS2300: Duplicate identifier 'i2'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         bar() {
             return 1;
         }
diff --git a/tests/baselines/reference/class1.errors.txt b/tests/baselines/reference/class1.errors.txt
deleted file mode 100644
index b90fae9cb402e..0000000000000
--- a/tests/baselines/reference/class1.errors.txt
+++ /dev/null
@@ -1,11 +0,0 @@
-tests/cases/compiler/class1.ts(1,11): error TS2300: Duplicate identifier 'foo'.
-tests/cases/compiler/class1.ts(2,7): error TS2300: Duplicate identifier 'foo'.
-
-
-==== tests/cases/compiler/class1.ts (2 errors) ====
-    interface foo{ } // error
-              ~~~
-!!! error TS2300: Duplicate identifier 'foo'.
-    class foo{ } // error
-          ~~~
-!!! error TS2300: Duplicate identifier 'foo'.
\ No newline at end of file
diff --git a/tests/baselines/reference/class1.js b/tests/baselines/reference/class1.js
deleted file mode 100644
index 7a104e237b6c0..0000000000000
--- a/tests/baselines/reference/class1.js
+++ /dev/null
@@ -1,10 +0,0 @@
-//// [class1.ts]
-interface foo{ } // error
-class foo{ } // error
-
-//// [class1.js]
-var foo = (function () {
-    function foo() {
-    }
-    return foo;
-})(); // error
diff --git a/tests/baselines/reference/classAbstractMergedDeclaration.errors.txt b/tests/baselines/reference/classAbstractMergedDeclaration.errors.txt
index ca2cc3b8c2d08..b3a2dec5e9c52 100644
--- a/tests/baselines/reference/classAbstractMergedDeclaration.errors.txt
+++ b/tests/baselines/reference/classAbstractMergedDeclaration.errors.txt
@@ -1,19 +1,26 @@
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(7,16): error TS2300: Duplicate identifier 'CI'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(8,11): error TS2300: Duplicate identifier 'CI'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(10,11): error TS2300: Duplicate identifier 'IC'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(11,16): error TS2300: Duplicate identifier 'IC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(7,16): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(8,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(10,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(11,16): error TS2518: Only an ambient class can be merged with an interface.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(13,16): error TS2300: Duplicate identifier 'CC1'.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(14,7): error TS2300: Duplicate identifier 'CC1'.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(16,7): error TS2300: Duplicate identifier 'CC2'.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(17,16): error TS2300: Duplicate identifier 'CC2'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(19,1): error TS2511: Cannot create an instance of the abstract class 'CM'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(20,1): error TS2511: Cannot create an instance of the abstract class 'MC'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(21,1): error TS2511: Cannot create an instance of the abstract class 'CI'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(22,5): error TS2304: Cannot find name 'IC'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(23,1): error TS2511: Cannot create an instance of the abstract class 'CC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(25,24): error TS2300: Duplicate identifier 'DCC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(26,15): error TS2300: Duplicate identifier 'DCC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(28,15): error TS2300: Duplicate identifier 'DCC2'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(29,24): error TS2300: Duplicate identifier 'DCC2'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(31,1): error TS2511: Cannot create an instance of the abstract class 'CM'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(32,1): error TS2511: Cannot create an instance of the abstract class 'MC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(33,1): error TS2511: Cannot create an instance of the abstract class 'CI'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(34,1): error TS2511: Cannot create an instance of the abstract class 'IC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(35,1): error TS2511: Cannot create an instance of the abstract class 'CC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(37,1): error TS2511: Cannot create an instance of the abstract class 'DCI'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(38,1): error TS2511: Cannot create an instance of the abstract class 'DIC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(39,1): error TS2511: Cannot create an instance of the abstract class 'DCC1'.
 
 
-==== tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts (13 errors) ====
+==== tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts (20 errors) ====
     abstract class CM {}
     module CM {}
     
@@ -22,17 +29,17 @@ tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbst
     
     abstract class CI {}
                    ~~
-!!! error TS2300: Duplicate identifier 'CI'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     interface CI {}
               ~~
-!!! error TS2300: Duplicate identifier 'CI'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     
     interface IC {}
               ~~
-!!! error TS2300: Duplicate identifier 'IC'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     abstract class IC {}
                    ~~
-!!! error TS2300: Duplicate identifier 'IC'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     
     abstract class CC1 {}
                    ~~~
@@ -48,6 +55,26 @@ tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbst
                    ~~~
 !!! error TS2300: Duplicate identifier 'CC2'.
     
+    declare abstract class DCI {}
+    interface DCI {}
+    
+    interface DIC {}
+    declare abstract class DIC {}
+    
+    declare abstract class DCC1 {}
+                           ~~~~
+!!! error TS2300: Duplicate identifier 'DCC1'.
+    declare class DCC1 {}
+                  ~~~~
+!!! error TS2300: Duplicate identifier 'DCC1'.
+    
+    declare class DCC2 {}
+                  ~~~~
+!!! error TS2300: Duplicate identifier 'DCC2'.
+    declare abstract class DCC2 {}
+                           ~~~~
+!!! error TS2300: Duplicate identifier 'DCC2'.
+    
     new CM;
     ~~~~~~
 !!! error TS2511: Cannot create an instance of the abstract class 'CM'.
@@ -58,9 +85,19 @@ tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbst
     ~~~~~~
 !!! error TS2511: Cannot create an instance of the abstract class 'CI'.
     new IC;
-        ~~
-!!! error TS2304: Cannot find name 'IC'.
+    ~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'IC'.
     new CC1;
     ~~~~~~~
 !!! error TS2511: Cannot create an instance of the abstract class 'CC1'.
-    new CC2;
\ No newline at end of file
+    new CC2;
+    new DCI;
+    ~~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'DCI'.
+    new DIC;
+    ~~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'DIC'.
+    new DCC1;
+    ~~~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'DCC1'.
+    new DCC2;
\ No newline at end of file
diff --git a/tests/baselines/reference/classAbstractMergedDeclaration.js b/tests/baselines/reference/classAbstractMergedDeclaration.js
index 1b4d6fea342f0..f6a91f870d299 100644
--- a/tests/baselines/reference/classAbstractMergedDeclaration.js
+++ b/tests/baselines/reference/classAbstractMergedDeclaration.js
@@ -17,12 +17,28 @@ class CC1 {}
 class CC2 {}
 abstract class CC2 {}
 
+declare abstract class DCI {}
+interface DCI {}
+
+interface DIC {}
+declare abstract class DIC {}
+
+declare abstract class DCC1 {}
+declare class DCC1 {}
+
+declare class DCC2 {}
+declare abstract class DCC2 {}
+
 new CM;
 new MC;
 new CI;
 new IC;
 new CC1;
-new CC2;
+new CC2;
+new DCI;
+new DIC;
+new DCC1;
+new DCC2;
 
 //// [classAbstractMergedDeclaration.js]
 var CM = (function () {
@@ -71,3 +87,7 @@ new CI;
 new IC;
 new CC1;
 new CC2;
+new DCI;
+new DIC;
+new DCC1;
+new DCC2;
diff --git a/tests/baselines/reference/classAndInterface1.errors.txt b/tests/baselines/reference/classAndInterface1.errors.txt
deleted file mode 100644
index 8d31e847d71e7..0000000000000
--- a/tests/baselines/reference/classAndInterface1.errors.txt
+++ /dev/null
@@ -1,11 +0,0 @@
-tests/cases/compiler/classAndInterface1.ts(1,8): error TS2300: Duplicate identifier 'cli'.
-tests/cases/compiler/classAndInterface1.ts(2,11): error TS2300: Duplicate identifier 'cli'.
-
-
-==== tests/cases/compiler/classAndInterface1.ts (2 errors) ====
-     class cli { } // error
-           ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
-    interface cli { } // error
-              ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
\ No newline at end of file
diff --git a/tests/baselines/reference/classAndInterface1.js b/tests/baselines/reference/classAndInterface1.js
deleted file mode 100644
index 5efabded83813..0000000000000
--- a/tests/baselines/reference/classAndInterface1.js
+++ /dev/null
@@ -1,10 +0,0 @@
-//// [classAndInterface1.ts]
- class cli { } // error
-interface cli { } // error
-
-//// [classAndInterface1.js]
-var cli = (function () {
-    function cli() {
-    }
-    return cli;
-})(); // error
diff --git a/tests/baselines/reference/classAndInterfaceMerge.d.symbols b/tests/baselines/reference/classAndInterfaceMerge.d.symbols
new file mode 100644
index 0000000000000..84fc2b129c145
--- /dev/null
+++ b/tests/baselines/reference/classAndInterfaceMerge.d.symbols
@@ -0,0 +1,39 @@
+=== tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts ===
+
+interface C { }
+>C : Symbol(C, Decl(classAndInterfaceMerge.d.ts, 0, 0), Decl(classAndInterfaceMerge.d.ts, 1, 15), Decl(classAndInterfaceMerge.d.ts, 3, 19), Decl(classAndInterfaceMerge.d.ts, 5, 15))
+
+declare class C { }
+>C : Symbol(C, Decl(classAndInterfaceMerge.d.ts, 0, 0), Decl(classAndInterfaceMerge.d.ts, 1, 15), Decl(classAndInterfaceMerge.d.ts, 3, 19), Decl(classAndInterfaceMerge.d.ts, 5, 15))
+
+interface C { }
+>C : Symbol(C, Decl(classAndInterfaceMerge.d.ts, 0, 0), Decl(classAndInterfaceMerge.d.ts, 1, 15), Decl(classAndInterfaceMerge.d.ts, 3, 19), Decl(classAndInterfaceMerge.d.ts, 5, 15))
+
+interface C { }
+>C : Symbol(C, Decl(classAndInterfaceMerge.d.ts, 0, 0), Decl(classAndInterfaceMerge.d.ts, 1, 15), Decl(classAndInterfaceMerge.d.ts, 3, 19), Decl(classAndInterfaceMerge.d.ts, 5, 15))
+
+declare module M {
+>M : Symbol(M, Decl(classAndInterfaceMerge.d.ts, 7, 15), Decl(classAndInterfaceMerge.d.ts, 20, 1))
+
+    interface C1 { }
+>C1 : Symbol(C1, Decl(classAndInterfaceMerge.d.ts, 9, 18), Decl(classAndInterfaceMerge.d.ts, 11, 20), Decl(classAndInterfaceMerge.d.ts, 13, 16), Decl(classAndInterfaceMerge.d.ts, 15, 20))
+
+    class C1 { }
+>C1 : Symbol(C1, Decl(classAndInterfaceMerge.d.ts, 9, 18), Decl(classAndInterfaceMerge.d.ts, 11, 20), Decl(classAndInterfaceMerge.d.ts, 13, 16), Decl(classAndInterfaceMerge.d.ts, 15, 20))
+
+    interface C1 { }
+>C1 : Symbol(C1, Decl(classAndInterfaceMerge.d.ts, 9, 18), Decl(classAndInterfaceMerge.d.ts, 11, 20), Decl(classAndInterfaceMerge.d.ts, 13, 16), Decl(classAndInterfaceMerge.d.ts, 15, 20))
+
+    interface C1 { }
+>C1 : Symbol(C1, Decl(classAndInterfaceMerge.d.ts, 9, 18), Decl(classAndInterfaceMerge.d.ts, 11, 20), Decl(classAndInterfaceMerge.d.ts, 13, 16), Decl(classAndInterfaceMerge.d.ts, 15, 20))
+
+    export class C2 { }
+>C2 : Symbol(C2, Decl(classAndInterfaceMerge.d.ts, 17, 20), Decl(classAndInterfaceMerge.d.ts, 22, 18))
+}
+
+declare module M {
+>M : Symbol(M, Decl(classAndInterfaceMerge.d.ts, 7, 15), Decl(classAndInterfaceMerge.d.ts, 20, 1))
+
+    export interface C2 { }
+>C2 : Symbol(C2, Decl(classAndInterfaceMerge.d.ts, 17, 20), Decl(classAndInterfaceMerge.d.ts, 22, 18))
+}
diff --git a/tests/baselines/reference/classAndInterfaceMerge.d.types b/tests/baselines/reference/classAndInterfaceMerge.d.types
new file mode 100644
index 0000000000000..baab121ca6ea6
--- /dev/null
+++ b/tests/baselines/reference/classAndInterfaceMerge.d.types
@@ -0,0 +1,39 @@
+=== tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts ===
+
+interface C { }
+>C : C
+
+declare class C { }
+>C : C
+
+interface C { }
+>C : C
+
+interface C { }
+>C : C
+
+declare module M {
+>M : typeof M
+
+    interface C1 { }
+>C1 : C1
+
+    class C1 { }
+>C1 : C1
+
+    interface C1 { }
+>C1 : C1
+
+    interface C1 { }
+>C1 : C1
+
+    export class C2 { }
+>C2 : C2
+}
+
+declare module M {
+>M : typeof M
+
+    export interface C2 { }
+>C2 : C2
+}
diff --git a/tests/baselines/reference/classAndInterfaceMergeConflictingMembers.errors.txt b/tests/baselines/reference/classAndInterfaceMergeConflictingMembers.errors.txt
new file mode 100644
index 0000000000000..48e52faae1e78
--- /dev/null
+++ b/tests/baselines/reference/classAndInterfaceMergeConflictingMembers.errors.txt
@@ -0,0 +1,44 @@
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts(2,12): error TS2300: Duplicate identifier 'x'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts(6,5): error TS2300: Duplicate identifier 'x'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts(10,15): error TS2300: Duplicate identifier 'x'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts(14,5): error TS2300: Duplicate identifier 'x'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts(18,13): error TS2300: Duplicate identifier 'x'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts(22,5): error TS2300: Duplicate identifier 'x'.
+
+
+==== tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts (6 errors) ====
+    declare class C1 {
+        public x : number;
+               ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    interface C1 {
+        x : number;
+        ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    declare class C2 {
+        protected x : number;
+                  ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    interface C2 {
+        x : number;
+        ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    declare class C3 {
+        private x : number;
+                ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    interface C3 {
+        x : number;
+        ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/classAndInterfaceMergeConflictingMembers.js b/tests/baselines/reference/classAndInterfaceMergeConflictingMembers.js
new file mode 100644
index 0000000000000..07b425383f439
--- /dev/null
+++ b/tests/baselines/reference/classAndInterfaceMergeConflictingMembers.js
@@ -0,0 +1,26 @@
+//// [classAndInterfaceMergeConflictingMembers.ts]
+declare class C1 {
+    public x : number;
+}
+
+interface C1 {
+    x : number;
+}
+
+declare class C2 {
+    protected x : number;
+}
+
+interface C2 {
+    x : number;
+}
+
+declare class C3 {
+    private x : number;
+}
+
+interface C3 {
+    x : number;
+}
+
+//// [classAndInterfaceMergeConflictingMembers.js]
diff --git a/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt b/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt
index aa3e687cf4d3f..332024b74fdf0 100644
--- a/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt
+++ b/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt
@@ -1,27 +1,39 @@
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(1,7): error TS2300: Duplicate identifier 'C'.
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(2,11): error TS2300: Duplicate identifier 'C'.
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(5,11): error TS2300: Duplicate identifier 'D'.
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(9,15): error TS2300: Duplicate identifier 'D'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(1,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(1,11): error TS2300: Duplicate identifier 'foo'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(2,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(2,15): error TS2300: Duplicate identifier 'foo'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(5,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(6,9): error TS2300: Duplicate identifier 'bar'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(9,15): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(10,9): error TS2300: Duplicate identifier 'bar'.
 
 
-==== tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts (4 errors) ====
+==== tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts (8 errors) ====
     class C { foo: string; }
           ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
+              ~~~
+!!! error TS2300: Duplicate identifier 'foo'.
     interface C { foo: string; } // error
               ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
+                  ~~~
+!!! error TS2300: Duplicate identifier 'foo'.
     
     module M {
         class D {
               ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
             bar: string;
+            ~~~
+!!! error TS2300: Duplicate identifier 'bar'.
         }
     
         interface D { // error
                   ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
             bar: string;
+            ~~~
+!!! error TS2300: Duplicate identifier 'bar'.
         }
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/classImplementsMergedClassInterface.errors.txt b/tests/baselines/reference/classImplementsMergedClassInterface.errors.txt
new file mode 100644
index 0000000000000..3e62e72be8422
--- /dev/null
+++ b/tests/baselines/reference/classImplementsMergedClassInterface.errors.txt
@@ -0,0 +1,41 @@
+tests/cases/conformance/classes/classDeclarations/classImplementsMergedClassInterface.ts(9,7): error TS2420: Class 'C2' incorrectly implements interface 'C1'.
+  Property 'x' is missing in type 'C2'.
+tests/cases/conformance/classes/classDeclarations/classImplementsMergedClassInterface.ts(12,7): error TS2420: Class 'C3' incorrectly implements interface 'C1'.
+  Property 'y' is missing in type 'C3'.
+tests/cases/conformance/classes/classDeclarations/classImplementsMergedClassInterface.ts(16,7): error TS2420: Class 'C4' incorrectly implements interface 'C1'.
+  Property 'x' is missing in type 'C4'.
+
+
+==== tests/cases/conformance/classes/classDeclarations/classImplementsMergedClassInterface.ts (3 errors) ====
+    declare class C1 {
+        x : number;
+    }
+    
+    interface C1 {
+        y : number;
+    }
+    
+    class C2 implements C1 { // error -- missing x
+          ~~
+!!! error TS2420: Class 'C2' incorrectly implements interface 'C1'.
+!!! error TS2420:   Property 'x' is missing in type 'C2'.
+    }
+    
+    class C3 implements C1 { // error -- missing y
+          ~~
+!!! error TS2420: Class 'C3' incorrectly implements interface 'C1'.
+!!! error TS2420:   Property 'y' is missing in type 'C3'.
+        x : number;
+    }
+    
+    class C4 implements C1 { // error -- missing x
+          ~~
+!!! error TS2420: Class 'C4' incorrectly implements interface 'C1'.
+!!! error TS2420:   Property 'x' is missing in type 'C4'.
+        y : number;
+    }
+    
+    class C5 implements C1 { // okay
+        x : number;
+        y : number;
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/classImplementsMergedClassInterface.js b/tests/baselines/reference/classImplementsMergedClassInterface.js
new file mode 100644
index 0000000000000..64a0198cc1627
--- /dev/null
+++ b/tests/baselines/reference/classImplementsMergedClassInterface.js
@@ -0,0 +1,46 @@
+//// [classImplementsMergedClassInterface.ts]
+declare class C1 {
+    x : number;
+}
+
+interface C1 {
+    y : number;
+}
+
+class C2 implements C1 { // error -- missing x
+}
+
+class C3 implements C1 { // error -- missing y
+    x : number;
+}
+
+class C4 implements C1 { // error -- missing x
+    y : number;
+}
+
+class C5 implements C1 { // okay
+    x : number;
+    y : number;
+}
+
+//// [classImplementsMergedClassInterface.js]
+var C2 = (function () {
+    function C2() {
+    }
+    return C2;
+})();
+var C3 = (function () {
+    function C3() {
+    }
+    return C3;
+})();
+var C4 = (function () {
+    function C4() {
+    }
+    return C4;
+})();
+var C5 = (function () {
+    function C5() {
+    }
+    return C5;
+})();
diff --git a/tests/baselines/reference/clinterfaces.errors.txt b/tests/baselines/reference/clinterfaces.errors.txt
index d5eab2c7994df..5960e6ae32b2e 100644
--- a/tests/baselines/reference/clinterfaces.errors.txt
+++ b/tests/baselines/reference/clinterfaces.errors.txt
@@ -1,50 +1,50 @@
-tests/cases/compiler/clinterfaces.ts(2,11): error TS2300: Duplicate identifier 'C'.
-tests/cases/compiler/clinterfaces.ts(3,15): error TS2300: Duplicate identifier 'C'.
-tests/cases/compiler/clinterfaces.ts(4,15): error TS2300: Duplicate identifier 'D'.
-tests/cases/compiler/clinterfaces.ts(5,11): error TS2300: Duplicate identifier 'D'.
-tests/cases/compiler/clinterfaces.ts(8,11): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/clinterfaces.ts(12,7): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/clinterfaces.ts(16,7): error TS2300: Duplicate identifier 'Bar'.
-tests/cases/compiler/clinterfaces.ts(20,11): error TS2300: Duplicate identifier 'Bar'.
+tests/cases/compiler/clinterfaces.ts(2,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(3,15): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(4,15): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(5,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(8,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(12,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(16,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(20,11): error TS2518: Only an ambient class can be merged with an interface.
 
 
 ==== tests/cases/compiler/clinterfaces.ts (8 errors) ====
     module M {
         class C { }
               ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         interface C { }
                   ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         interface D { }
                   ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         class D { }
               ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     }
     
     interface Foo<T> {
               ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         a: string;
     }
     
     class Foo<T>{
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         b: number;
     }
     
     class Bar<T>{
           ~~~
-!!! error TS2300: Duplicate identifier 'Bar'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         b: number;
     }
     
     interface Bar<T> {
               ~~~
-!!! error TS2300: Duplicate identifier 'Bar'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         a: string;
     }
     
diff --git a/tests/baselines/reference/clinterfaces.js b/tests/baselines/reference/clinterfaces.js
index 02ae73498697a..a55924227c9ef 100644
--- a/tests/baselines/reference/clinterfaces.js
+++ b/tests/baselines/reference/clinterfaces.js
@@ -49,3 +49,4 @@ var Bar = (function () {
     }
     return Bar;
 })();
+module.exports = Foo;
diff --git a/tests/baselines/reference/declInput.errors.txt b/tests/baselines/reference/declInput.errors.txt
index 8ec341ce15bb9..179db3598abb3 100644
--- a/tests/baselines/reference/declInput.errors.txt
+++ b/tests/baselines/reference/declInput.errors.txt
@@ -1,17 +1,17 @@
-tests/cases/compiler/declInput.ts(1,11): error TS2300: Duplicate identifier 'bar'.
-tests/cases/compiler/declInput.ts(5,7): error TS2300: Duplicate identifier 'bar'.
+tests/cases/compiler/declInput.ts(1,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/declInput.ts(5,7): error TS2518: Only an ambient class can be merged with an interface.
 
 
 ==== tests/cases/compiler/declInput.ts (2 errors) ====
     interface bar {
               ~~~
-!!! error TS2300: Duplicate identifier 'bar'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     
     }
     
     class bar {
           ~~~
-!!! error TS2300: Duplicate identifier 'bar'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
       public f() { return ''; }
       public g() { return {a: <bar>null, b: undefined, c: void 4 }; }
       public h(x = 4, y = null, z = '') { x++; }
diff --git a/tests/baselines/reference/declaredClassMergedwithSelf.errors.txt b/tests/baselines/reference/declaredClassMergedwithSelf.errors.txt
new file mode 100644
index 0000000000000..a46dfb0d3394a
--- /dev/null
+++ b/tests/baselines/reference/declaredClassMergedwithSelf.errors.txt
@@ -0,0 +1,43 @@
+tests/cases/conformance/classes/classDeclarations/file1.ts(3,15): error TS2300: Duplicate identifier 'C1'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(5,15): error TS2300: Duplicate identifier 'C1'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(7,15): error TS2300: Duplicate identifier 'C2'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(9,11): error TS2300: Duplicate identifier 'C2'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(11,15): error TS2300: Duplicate identifier 'C2'.
+tests/cases/conformance/classes/classDeclarations/file2.ts(2,15): error TS2300: Duplicate identifier 'C3'.
+tests/cases/conformance/classes/classDeclarations/file3.ts(2,15): error TS2300: Duplicate identifier 'C3'.
+
+
+==== tests/cases/conformance/classes/classDeclarations/file1.ts (5 errors) ====
+    
+    
+    declare class C1 {}
+                  ~~
+!!! error TS2300: Duplicate identifier 'C1'.
+    
+    declare class C1 {}
+                  ~~
+!!! error TS2300: Duplicate identifier 'C1'.
+    
+    declare class C2 {}
+                  ~~
+!!! error TS2300: Duplicate identifier 'C2'.
+    
+    interface C2 {}
+              ~~
+!!! error TS2300: Duplicate identifier 'C2'.
+    
+    declare class C2 {}
+                  ~~
+!!! error TS2300: Duplicate identifier 'C2'.
+    
+==== tests/cases/conformance/classes/classDeclarations/file2.ts (1 errors) ====
+    
+    declare class C3 { }
+                  ~~
+!!! error TS2300: Duplicate identifier 'C3'.
+    
+==== tests/cases/conformance/classes/classDeclarations/file3.ts (1 errors) ====
+    
+    declare class C3 { }
+                  ~~
+!!! error TS2300: Duplicate identifier 'C3'.
\ No newline at end of file
diff --git a/tests/baselines/reference/declaredClassMergedwithSelf.js b/tests/baselines/reference/declaredClassMergedwithSelf.js
new file mode 100644
index 0000000000000..a38391d90dbc8
--- /dev/null
+++ b/tests/baselines/reference/declaredClassMergedwithSelf.js
@@ -0,0 +1,26 @@
+//// [tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts] ////
+
+//// [file1.ts]
+
+
+declare class C1 {}
+
+declare class C1 {}
+
+declare class C2 {}
+
+interface C2 {}
+
+declare class C2 {}
+
+//// [file2.ts]
+
+declare class C3 { }
+
+//// [file3.ts]
+
+declare class C3 { }
+
+//// [file1.js]
+//// [file2.js]
+//// [file3.js]
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt b/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
index 6c80d13b2f44a..731e97482035a 100644
--- a/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(2,22): error TS2300: Duplicate identifier 'I'.
-tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(5,18): error TS2300: Duplicate identifier 'I'.
+tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(2,22): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(5,18): error TS2518: Only an ambient class can be merged with an interface.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(9,21): error TS2300: Duplicate identifier 'f'.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(12,18): error TS2300: Duplicate identifier 'f'.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(37,12): error TS2300: Duplicate identifier 'x'.
@@ -10,12 +10,12 @@ tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(41,16): er
     module M {
         export interface I { }
                          ~
-!!! error TS2300: Duplicate identifier 'I'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     }
     module M {
         export class I { } // error
                      ~
-!!! error TS2300: Duplicate identifier 'I'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     }
     
     module M {
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt
new file mode 100644
index 0000000000000..9867441f7700f
--- /dev/null
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt
@@ -0,0 +1,68 @@
+tests/cases/compiler/file1.ts(2,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/file1.ts(3,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/file1.ts(4,7): error TS2300: Duplicate identifier 'C2'.
+tests/cases/compiler/file1.ts(5,10): error TS2300: Duplicate identifier 'f'.
+tests/cases/compiler/file1.ts(9,12): error TS2300: Duplicate identifier 'x'.
+tests/cases/compiler/file2.ts(1,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/file2.ts(2,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/file2.ts(3,10): error TS2300: Duplicate identifier 'C2'.
+tests/cases/compiler/file2.ts(4,7): error TS2300: Duplicate identifier 'f'.
+tests/cases/compiler/file2.ts(7,8): error TS2433: A namespace declaration cannot be in a different file from a class or function with which it is merged
+tests/cases/compiler/file2.ts(8,16): error TS2300: Duplicate identifier 'x'.
+
+
+==== tests/cases/compiler/file1.ts (5 errors) ====
+    
+    interface I { }
+              ~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    class C1 { }
+          ~~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    class C2 { }
+          ~~
+!!! error TS2300: Duplicate identifier 'C2'.
+    function f() { }
+             ~
+!!! error TS2300: Duplicate identifier 'f'.
+    var v = 3;
+    
+    class Foo {
+        static x: number;
+               ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    module N {
+        export module F {
+            var t;
+        }
+    }
+    
+==== tests/cases/compiler/file2.ts (6 errors) ====
+    class I { } // error -- cannot merge interface with non-ambient class
+          ~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    interface C1 { } // error -- cannot merge interface with non-ambient class
+              ~~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    function C2() { } // error -- cannot merge function with non-ambient class
+             ~~
+!!! error TS2300: Duplicate identifier 'C2'.
+    class f { } // error -- cannot merge function with non-ambient class
+          ~
+!!! error TS2300: Duplicate identifier 'f'.
+    var v = 3;
+    
+    module Foo {
+           ~~~
+!!! error TS2433: A namespace declaration cannot be in a different file from a class or function with which it is merged
+        export var x: number; // error for redeclaring var in a different parent
+                   ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    declare module N {
+        export function F(); // no error because function is ambient
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.js b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.js
new file mode 100644
index 0000000000000..a2509ea97f462
--- /dev/null
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.js
@@ -0,0 +1,110 @@
+//// [tests/cases/compiler/duplicateIdentifiersAcrossFileBoundaries.ts] ////
+
+//// [file1.ts]
+
+interface I { }
+class C1 { }
+class C2 { }
+function f() { }
+var v = 3;
+
+class Foo {
+    static x: number;
+}
+
+module N {
+    export module F {
+        var t;
+    }
+}
+
+//// [file2.ts]
+class I { } // error -- cannot merge interface with non-ambient class
+interface C1 { } // error -- cannot merge interface with non-ambient class
+function C2() { } // error -- cannot merge function with non-ambient class
+class f { } // error -- cannot merge function with non-ambient class
+var v = 3;
+
+module Foo {
+    export var x: number; // error for redeclaring var in a different parent
+}
+
+declare module N {
+    export function F(); // no error because function is ambient
+}
+
+
+//// [file1.js]
+var C1 = (function () {
+    function C1() {
+    }
+    return C1;
+})();
+var C2 = (function () {
+    function C2() {
+    }
+    return C2;
+})();
+function f() { }
+var v = 3;
+var Foo = (function () {
+    function Foo() {
+    }
+    return Foo;
+})();
+var N;
+(function (N) {
+    var F;
+    (function (F) {
+        var t;
+    })(F = N.F || (N.F = {}));
+})(N || (N = {}));
+//// [file2.js]
+var I = (function () {
+    function I() {
+    }
+    return I;
+})(); // error -- cannot merge interface with non-ambient class
+function C2() { } // error -- cannot merge function with non-ambient class
+var f = (function () {
+    function f() {
+    }
+    return f;
+})(); // error -- cannot merge function with non-ambient class
+var v = 3;
+var Foo;
+(function (Foo) {
+})(Foo || (Foo = {}));
+
+
+//// [file1.d.ts]
+interface I {
+}
+declare class C1 {
+}
+declare class C2 {
+}
+declare function f(): void;
+declare var v: number;
+declare class Foo {
+    static x: number;
+}
+declare module N {
+    module F {
+    }
+}
+//// [file2.d.ts]
+declare class I {
+}
+interface C1 {
+}
+declare function C2(): void;
+declare class f {
+}
+declare var v: number;
+declare module Foo {
+    var x: number;
+}
+declare module N {
+    function F(): any;
+}
diff --git a/tests/baselines/reference/interfaceDeclaration2.errors.txt b/tests/baselines/reference/interfaceDeclaration2.errors.txt
index 9739d946bc52b..db525e6f21f25 100644
--- a/tests/baselines/reference/interfaceDeclaration2.errors.txt
+++ b/tests/baselines/reference/interfaceDeclaration2.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/interfaceDeclaration2.ts(4,11): error TS2300: Duplicate identifier 'I2'.
-tests/cases/compiler/interfaceDeclaration2.ts(5,7): error TS2300: Duplicate identifier 'I2'.
+tests/cases/compiler/interfaceDeclaration2.ts(4,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/interfaceDeclaration2.ts(5,7): error TS2518: Only an ambient class can be merged with an interface.
 
 
 ==== tests/cases/compiler/interfaceDeclaration2.ts (2 errors) ====
@@ -8,10 +8,10 @@ tests/cases/compiler/interfaceDeclaration2.ts(5,7): error TS2300: Duplicate iden
     
     interface I2 { }
               ~~
-!!! error TS2300: Duplicate identifier 'I2'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     class I2 { }
           ~~
-!!! error TS2300: Duplicate identifier 'I2'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     
     interface I3 { }
     function I3() { }
diff --git a/tests/baselines/reference/mergeClassInterfaceAndModule.js b/tests/baselines/reference/mergeClassInterfaceAndModule.js
new file mode 100644
index 0000000000000..5b7315031cd21
--- /dev/null
+++ b/tests/baselines/reference/mergeClassInterfaceAndModule.js
@@ -0,0 +1,19 @@
+//// [mergeClassInterfaceAndModule.ts]
+
+interface C1 {}
+declare class C1 {}
+module C1 {}
+
+declare class C2 {}
+interface C2 {}
+module C2 {}
+
+declare class C3 {}
+module C3 {}
+interface C3 {}
+
+module C4 {}
+declare class C4 {} // error -- class declaration must preceed module declaration
+interface C4 {}
+
+//// [mergeClassInterfaceAndModule.js]
diff --git a/tests/baselines/reference/mergeClassInterfaceAndModule.symbols b/tests/baselines/reference/mergeClassInterfaceAndModule.symbols
new file mode 100644
index 0000000000000..a10227ed4618b
--- /dev/null
+++ b/tests/baselines/reference/mergeClassInterfaceAndModule.symbols
@@ -0,0 +1,38 @@
+=== tests/cases/conformance/classes/classDeclarations/mergeClassInterfaceAndModule.ts ===
+
+interface C1 {}
+>C1 : Symbol(C1, Decl(mergeClassInterfaceAndModule.ts, 0, 0), Decl(mergeClassInterfaceAndModule.ts, 1, 15), Decl(mergeClassInterfaceAndModule.ts, 2, 19))
+
+declare class C1 {}
+>C1 : Symbol(C1, Decl(mergeClassInterfaceAndModule.ts, 0, 0), Decl(mergeClassInterfaceAndModule.ts, 1, 15), Decl(mergeClassInterfaceAndModule.ts, 2, 19))
+
+module C1 {}
+>C1 : Symbol(C1, Decl(mergeClassInterfaceAndModule.ts, 0, 0), Decl(mergeClassInterfaceAndModule.ts, 1, 15), Decl(mergeClassInterfaceAndModule.ts, 2, 19))
+
+declare class C2 {}
+>C2 : Symbol(C2, Decl(mergeClassInterfaceAndModule.ts, 3, 12), Decl(mergeClassInterfaceAndModule.ts, 5, 19), Decl(mergeClassInterfaceAndModule.ts, 6, 15))
+
+interface C2 {}
+>C2 : Symbol(C2, Decl(mergeClassInterfaceAndModule.ts, 3, 12), Decl(mergeClassInterfaceAndModule.ts, 5, 19), Decl(mergeClassInterfaceAndModule.ts, 6, 15))
+
+module C2 {}
+>C2 : Symbol(C2, Decl(mergeClassInterfaceAndModule.ts, 3, 12), Decl(mergeClassInterfaceAndModule.ts, 5, 19), Decl(mergeClassInterfaceAndModule.ts, 6, 15))
+
+declare class C3 {}
+>C3 : Symbol(C3, Decl(mergeClassInterfaceAndModule.ts, 7, 12), Decl(mergeClassInterfaceAndModule.ts, 9, 19), Decl(mergeClassInterfaceAndModule.ts, 10, 12))
+
+module C3 {}
+>C3 : Symbol(C3, Decl(mergeClassInterfaceAndModule.ts, 7, 12), Decl(mergeClassInterfaceAndModule.ts, 9, 19), Decl(mergeClassInterfaceAndModule.ts, 10, 12))
+
+interface C3 {}
+>C3 : Symbol(C3, Decl(mergeClassInterfaceAndModule.ts, 7, 12), Decl(mergeClassInterfaceAndModule.ts, 9, 19), Decl(mergeClassInterfaceAndModule.ts, 10, 12))
+
+module C4 {}
+>C4 : Symbol(C4, Decl(mergeClassInterfaceAndModule.ts, 11, 15), Decl(mergeClassInterfaceAndModule.ts, 13, 12), Decl(mergeClassInterfaceAndModule.ts, 14, 19))
+
+declare class C4 {} // error -- class declaration must preceed module declaration
+>C4 : Symbol(C4, Decl(mergeClassInterfaceAndModule.ts, 11, 15), Decl(mergeClassInterfaceAndModule.ts, 13, 12), Decl(mergeClassInterfaceAndModule.ts, 14, 19))
+
+interface C4 {}
+>C4 : Symbol(C4, Decl(mergeClassInterfaceAndModule.ts, 11, 15), Decl(mergeClassInterfaceAndModule.ts, 13, 12), Decl(mergeClassInterfaceAndModule.ts, 14, 19))
+
diff --git a/tests/baselines/reference/mergeClassInterfaceAndModule.types b/tests/baselines/reference/mergeClassInterfaceAndModule.types
new file mode 100644
index 0000000000000..a91691e5bacda
--- /dev/null
+++ b/tests/baselines/reference/mergeClassInterfaceAndModule.types
@@ -0,0 +1,38 @@
+=== tests/cases/conformance/classes/classDeclarations/mergeClassInterfaceAndModule.ts ===
+
+interface C1 {}
+>C1 : C1
+
+declare class C1 {}
+>C1 : C1
+
+module C1 {}
+>C1 : typeof C1
+
+declare class C2 {}
+>C2 : C2
+
+interface C2 {}
+>C2 : C2
+
+module C2 {}
+>C2 : typeof C2
+
+declare class C3 {}
+>C3 : C3
+
+module C3 {}
+>C3 : typeof C3
+
+interface C3 {}
+>C3 : C3
+
+module C4 {}
+>C4 : typeof C4
+
+declare class C4 {} // error -- class declaration must preceed module declaration
+>C4 : C4
+
+interface C4 {}
+>C4 : C4
+
diff --git a/tests/baselines/reference/mergedClassInterface.errors.txt b/tests/baselines/reference/mergedClassInterface.errors.txt
new file mode 100644
index 0000000000000..5b23b046db75c
--- /dev/null
+++ b/tests/baselines/reference/mergedClassInterface.errors.txt
@@ -0,0 +1,67 @@
+tests/cases/conformance/classes/classDeclarations/file1.ts(11,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/file1.ts(13,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/file1.ts(15,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/file1.ts(17,7): error TS2518: Only an ambient class can be merged with an interface.
+
+
+==== tests/cases/conformance/classes/classDeclarations/file1.ts (4 errors) ====
+    
+    
+    declare class C1 { }
+    
+    interface C1 { }
+    
+    interface C2 { }
+    
+    declare class C2 { }
+    
+    class C3 { } // error -- cannot merge non-ambient class and interface
+          ~~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    
+    interface C3 { } // error -- cannot merge non-ambient class and interface
+              ~~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    
+    interface C4 { } // error -- cannot merge non-ambient class and interface
+              ~~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    
+    class C4 { } // error -- cannot merge non-ambient class and interface
+          ~~
+!!! error TS2518: Only an ambient class can be merged with an interface.
+    
+    interface C5 {
+        x1: number;
+    }
+    
+    declare class C5 {
+        x2: number;
+    }
+    
+    interface C5 {
+        x3: number;
+    }
+    
+    interface C5 {
+        x4: number;
+    }
+    
+    // checks if properties actually were merged
+    var c5 : C5;
+    c5.x1;
+    c5.x2;
+    c5.x3;
+    c5.x4;
+    
+==== tests/cases/conformance/classes/classDeclarations/file2.ts (0 errors) ====
+    
+    declare class C6 { }
+    
+    interface C7 { }
+    
+==== tests/cases/conformance/classes/classDeclarations/file3.ts (0 errors) ====
+    
+    interface C6 { }
+    
+    declare class C7 { }
\ No newline at end of file
diff --git a/tests/baselines/reference/mergedClassInterface.js b/tests/baselines/reference/mergedClassInterface.js
new file mode 100644
index 0000000000000..7176255655dbf
--- /dev/null
+++ b/tests/baselines/reference/mergedClassInterface.js
@@ -0,0 +1,117 @@
+//// [tests/cases/conformance/classes/classDeclarations/mergedClassInterface.ts] ////
+
+//// [file1.ts]
+
+
+declare class C1 { }
+
+interface C1 { }
+
+interface C2 { }
+
+declare class C2 { }
+
+class C3 { } // error -- cannot merge non-ambient class and interface
+
+interface C3 { } // error -- cannot merge non-ambient class and interface
+
+interface C4 { } // error -- cannot merge non-ambient class and interface
+
+class C4 { } // error -- cannot merge non-ambient class and interface
+
+interface C5 {
+    x1: number;
+}
+
+declare class C5 {
+    x2: number;
+}
+
+interface C5 {
+    x3: number;
+}
+
+interface C5 {
+    x4: number;
+}
+
+// checks if properties actually were merged
+var c5 : C5;
+c5.x1;
+c5.x2;
+c5.x3;
+c5.x4;
+
+//// [file2.ts]
+
+declare class C6 { }
+
+interface C7 { }
+
+//// [file3.ts]
+
+interface C6 { }
+
+declare class C7 { }
+
+//// [file1.js]
+var C3 = (function () {
+    function C3() {
+    }
+    return C3;
+})(); // error -- cannot merge non-ambient class and interface
+var C4 = (function () {
+    function C4() {
+    }
+    return C4;
+})(); // error -- cannot merge non-ambient class and interface
+// checks if properties actually were merged
+var c5;
+c5.x1;
+c5.x2;
+c5.x3;
+c5.x4;
+//// [file2.js]
+//// [file3.js]
+
+
+//// [file1.d.ts]
+declare class C1 {
+}
+interface C1 {
+}
+interface C2 {
+}
+declare class C2 {
+}
+declare class C3 {
+}
+interface C3 {
+}
+interface C4 {
+}
+declare class C4 {
+}
+interface C5 {
+    x1: number;
+}
+declare class C5 {
+    x2: number;
+}
+interface C5 {
+    x3: number;
+}
+interface C5 {
+    x4: number;
+}
+declare var c5: C5;
+//// [file2.d.ts]
+declare class C6 {
+}
+interface C7 {
+}
+//// [file3.d.ts]
+interface C6 {
+}
+declare class C7 {
+}
diff --git a/tests/baselines/reference/nameCollisions.errors.txt b/tests/baselines/reference/nameCollisions.errors.txt
index 5eb7ea2cf8023..5c32d4640705f 100644
--- a/tests/baselines/reference/nameCollisions.errors.txt
+++ b/tests/baselines/reference/nameCollisions.errors.txt
@@ -11,10 +11,10 @@ tests/cases/compiler/nameCollisions.ts(33,11): error TS2300: Duplicate identifie
 tests/cases/compiler/nameCollisions.ts(34,14): error TS2300: Duplicate identifier 'C'.
 tests/cases/compiler/nameCollisions.ts(36,14): error TS2300: Duplicate identifier 'C2'.
 tests/cases/compiler/nameCollisions.ts(37,11): error TS2300: Duplicate identifier 'C2'.
-tests/cases/compiler/nameCollisions.ts(42,11): error TS2300: Duplicate identifier 'cli'.
-tests/cases/compiler/nameCollisions.ts(43,15): error TS2300: Duplicate identifier 'cli'.
-tests/cases/compiler/nameCollisions.ts(45,15): error TS2300: Duplicate identifier 'cli2'.
-tests/cases/compiler/nameCollisions.ts(46,11): error TS2300: Duplicate identifier 'cli2'.
+tests/cases/compiler/nameCollisions.ts(42,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/nameCollisions.ts(43,15): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/nameCollisions.ts(45,15): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/nameCollisions.ts(46,11): error TS2518: Only an ambient class can be merged with an interface.
 
 
 ==== tests/cases/compiler/nameCollisions.ts (17 errors) ====
@@ -87,15 +87,15 @@ tests/cases/compiler/nameCollisions.ts(46,11): error TS2300: Duplicate identifie
     
         class cli { }
               ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         interface cli { } // error
                   ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     
         interface cli2 { }
                   ~~~~
-!!! error TS2300: Duplicate identifier 'cli2'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         class cli2 { } // error
               ~~~~
-!!! error TS2300: Duplicate identifier 'cli2'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt b/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt
index 7c796d4739869..69d686c31561a 100644
--- a/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt
+++ b/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt
@@ -1,14 +1,13 @@
-lib.d.ts(521,11): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(2,7): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(8,3): error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-  Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(2,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(8,3): error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+  Property 'raw' is missing in type '{}'.
 
 
 ==== tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts (2 errors) ====
     
     class TemplateStringsArray {
           ~~~~~~~~~~~~~~~~~~~~
-!!! error TS2300: Duplicate identifier 'TemplateStringsArray'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     }
     
     function f(x: TemplateStringsArray, y: number, z: number) {
@@ -16,7 +15,7 @@ tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(8,3): error TS2
     
     f({}, 10, 10);
       ~~
-!!! error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-!!! error TS2345:   Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+!!! error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+!!! error TS2345:   Property 'raw' is missing in type '{}'.
     
     f `abcdef${ 1234 }${ 5678 }ghijkl`;
\ No newline at end of file
diff --git a/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt b/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt
index 4b9cb63d9aaeb..02ccd7f2413f8 100644
--- a/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt
+++ b/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt
@@ -1,14 +1,13 @@
-lib.d.ts(521,11): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(2,7): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(8,3): error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-  Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(2,7): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(8,3): error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+  Property 'raw' is missing in type '{}'.
 
 
 ==== tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts (2 errors) ====
     
     class TemplateStringsArray {
           ~~~~~~~~~~~~~~~~~~~~
-!!! error TS2300: Duplicate identifier 'TemplateStringsArray'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     }
     
     function f(x: TemplateStringsArray, y: number, z: number) {
@@ -16,7 +15,7 @@ tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(8,3): error T
     
     f({}, 10, 10);
       ~~
-!!! error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-!!! error TS2345:   Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+!!! error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+!!! error TS2345:   Property 'raw' is missing in type '{}'.
     
     f `abcdef${ 1234 }${ 5678 }ghijkl`;
\ No newline at end of file
diff --git a/tests/cases/compiler/class1.ts b/tests/cases/compiler/class1.ts
deleted file mode 100644
index c7b2115b88b28..0000000000000
--- a/tests/cases/compiler/class1.ts
+++ /dev/null
@@ -1,2 +0,0 @@
-interface foo{ } // error
-class foo{ } // error
\ No newline at end of file
diff --git a/tests/cases/compiler/classAndInterface1.ts b/tests/cases/compiler/classAndInterface1.ts
deleted file mode 100644
index 79b029158572e..0000000000000
--- a/tests/cases/compiler/classAndInterface1.ts
+++ /dev/null
@@ -1,2 +0,0 @@
- class cli { } // error
-interface cli { } // error
\ No newline at end of file
diff --git a/tests/cases/compiler/duplicateIdentifiersAcrossFileBoundaries.ts b/tests/cases/compiler/duplicateIdentifiersAcrossFileBoundaries.ts
new file mode 100644
index 0000000000000..94e5cd94ab188
--- /dev/null
+++ b/tests/cases/compiler/duplicateIdentifiersAcrossFileBoundaries.ts
@@ -0,0 +1,33 @@
+// @declaration: true
+
+// @Filename: file1.ts
+interface I { }
+class C1 { }
+class C2 { }
+function f() { }
+var v = 3;
+
+class Foo {
+    static x: number;
+}
+
+module N {
+    export module F {
+        var t;
+    }
+}
+
+// @Filename: file2.ts
+class I { } // error -- cannot merge interface with non-ambient class
+interface C1 { } // error -- cannot merge interface with non-ambient class
+function C2() { } // error -- cannot merge function with non-ambient class
+class f { } // error -- cannot merge function with non-ambient class
+var v = 3;
+
+module Foo {
+    export var x: number; // error for redeclaring var in a different parent
+}
+
+declare module N {
+    export function F(); // no error because function is ambient
+}
diff --git a/tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts b/tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts
index 160c0de298bb3..6e5686feb2562 100644
--- a/tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts
+++ b/tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts
@@ -16,9 +16,25 @@ class CC1 {}
 class CC2 {}
 abstract class CC2 {}
 
+declare abstract class DCI {}
+interface DCI {}
+
+interface DIC {}
+declare abstract class DIC {}
+
+declare abstract class DCC1 {}
+declare class DCC1 {}
+
+declare class DCC2 {}
+declare abstract class DCC2 {}
+
 new CM;
 new MC;
 new CI;
 new IC;
 new CC1;
-new CC2;
\ No newline at end of file
+new CC2;
+new DCI;
+new DIC;
+new DCC1;
+new DCC2;
\ No newline at end of file
diff --git a/tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts b/tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts
new file mode 100644
index 0000000000000..1ef7724f81d30
--- /dev/null
+++ b/tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts
@@ -0,0 +1,25 @@
+
+interface C { }
+
+declare class C { }
+
+interface C { }
+
+interface C { }
+
+declare module M {
+
+    interface C1 { }
+
+    class C1 { }
+
+    interface C1 { }
+
+    interface C1 { }
+
+    export class C2 { }
+}
+
+declare module M {
+    export interface C2 { }
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts b/tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts
new file mode 100644
index 0000000000000..d11d67c3feabb
--- /dev/null
+++ b/tests/cases/conformance/classes/classDeclarations/classAndInterfaceMergeConflictingMembers.ts
@@ -0,0 +1,23 @@
+declare class C1 {
+    public x : number;
+}
+
+interface C1 {
+    x : number;
+}
+
+declare class C2 {
+    protected x : number;
+}
+
+interface C2 {
+    x : number;
+}
+
+declare class C3 {
+    private x : number;
+}
+
+interface C3 {
+    x : number;
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/classes/classDeclarations/classImplementsMergedClassInterface.ts b/tests/cases/conformance/classes/classDeclarations/classImplementsMergedClassInterface.ts
new file mode 100644
index 0000000000000..d5ac1b005bfd3
--- /dev/null
+++ b/tests/cases/conformance/classes/classDeclarations/classImplementsMergedClassInterface.ts
@@ -0,0 +1,23 @@
+declare class C1 {
+    x : number;
+}
+
+interface C1 {
+    y : number;
+}
+
+class C2 implements C1 { // error -- missing x
+}
+
+class C3 implements C1 { // error -- missing y
+    x : number;
+}
+
+class C4 implements C1 { // error -- missing x
+    y : number;
+}
+
+class C5 implements C1 { // okay
+    x : number;
+    y : number;
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts b/tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts
new file mode 100644
index 0000000000000..7445104e22833
--- /dev/null
+++ b/tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts
@@ -0,0 +1,20 @@
+
+// @Filename: file1.ts
+
+declare class C1 {}
+
+declare class C1 {}
+
+declare class C2 {}
+
+interface C2 {}
+
+declare class C2 {}
+
+// @Filename: file2.ts
+
+declare class C3 { }
+
+// @Filename: file3.ts
+
+declare class C3 { }
\ No newline at end of file
diff --git a/tests/cases/conformance/classes/classDeclarations/mergeClassInterfaceAndModule.ts b/tests/cases/conformance/classes/classDeclarations/mergeClassInterfaceAndModule.ts
new file mode 100644
index 0000000000000..4dc733d28c05f
--- /dev/null
+++ b/tests/cases/conformance/classes/classDeclarations/mergeClassInterfaceAndModule.ts
@@ -0,0 +1,16 @@
+
+interface C1 {}
+declare class C1 {}
+module C1 {}
+
+declare class C2 {}
+interface C2 {}
+module C2 {}
+
+declare class C3 {}
+module C3 {}
+interface C3 {}
+
+module C4 {}
+declare class C4 {} // error -- class declaration must preceed module declaration
+interface C4 {}
\ No newline at end of file
diff --git a/tests/cases/conformance/classes/classDeclarations/mergedClassInterface.ts b/tests/cases/conformance/classes/classDeclarations/mergedClassInterface.ts
new file mode 100644
index 0000000000000..29400c63808b8
--- /dev/null
+++ b/tests/cases/conformance/classes/classDeclarations/mergedClassInterface.ts
@@ -0,0 +1,54 @@
+// @declaration: true
+
+// @Filename: file1.ts
+
+declare class C1 { }
+
+interface C1 { }
+
+interface C2 { }
+
+declare class C2 { }
+
+class C3 { } // error -- cannot merge non-ambient class and interface
+
+interface C3 { } // error -- cannot merge non-ambient class and interface
+
+interface C4 { } // error -- cannot merge non-ambient class and interface
+
+class C4 { } // error -- cannot merge non-ambient class and interface
+
+interface C5 {
+    x1: number;
+}
+
+declare class C5 {
+    x2: number;
+}
+
+interface C5 {
+    x3: number;
+}
+
+interface C5 {
+    x4: number;
+}
+
+// checks if properties actually were merged
+var c5 : C5;
+c5.x1;
+c5.x2;
+c5.x3;
+c5.x4;
+
+// @Filename: file2.ts
+
+declare class C6 { }
+
+interface C7 { }
+
+// @Filename: file3.ts
+
+interface C6 { }
+
+declare class C7 { }
\ No newline at end of file