Improved error messaging for index signature parameters

src/compiler/checker.ts

@@ -25775,6 +25775,21 @@ namespace ts {
                 return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_must_have_a_type_annotation);
             }
             if (parameter.type.kind !== SyntaxKind.StringKeyword && parameter.type.kind !== SyntaxKind.NumberKeyword) {
+                const type = getTypeFromTypeNode(parameter.type);
+
+                if (type.flags & TypeFlags.String || type.flags & TypeFlags.Number) {
+                    return grammarErrorOnNode(parameter.name,
+                                              Diagnostics.An_index_signature_parameter_type_cannot_be_a_type_alias_Consider_writing_0_Colon_1_Colon_2_instead,
+                                              getTextOfNode(parameter.name),
+                                              typeToString(type),
+                                              typeToString(getTypeFromTypeNode(node.type)));
+                }
+
+                if (allTypesAssignableToKind(type, TypeFlags.StringLiteral, /*strict*/ true)) {
+                    return grammarErrorOnNode(parameter.name,
+                                              Diagnostics.An_index_signature_parameter_type_cannot_be_a_union_type_Consider_using_a_mapped_object_type_instead);
+                }
+
                 return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_must_be_string_or_number);
             }
             if (!node.type) {

src/compiler/diagnosticMessages.json

@@ -939,6 +939,14 @@
         "category": "Error",
         "code": 1335
     },
+    "An index signature parameter type cannot be a type alias. Consider writing '[{0}: {1}]: {2}' instead.": {
+        "category": "Error",
+        "code": 1336
+    },
+    "An index signature parameter type cannot be a union type. Consider using a mapped object type instead.": {
+        "category": "Error",
+        "code": 1337
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",

tests/baselines/reference/indexerConstraints2.errors.txt

@@ -1,9 +1,17 @@
 tests/cases/compiler/indexerConstraints2.ts(9,5): error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
 tests/cases/compiler/indexerConstraints2.ts(17,5): error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
 tests/cases/compiler/indexerConstraints2.ts(26,5): error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
+tests/cases/compiler/indexerConstraints2.ts(34,6): error TS1336: An index signature parameter type cannot be a type alias. Consider writing '[n: number]: A' instead.
+tests/cases/compiler/indexerConstraints2.ts(40,6): error TS1336: An index signature parameter type cannot be a type alias. Consider writing '[s: string]: A' instead.
+tests/cases/compiler/indexerConstraints2.ts(46,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
+tests/cases/compiler/indexerConstraints2.ts(52,6): error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead.
+tests/cases/compiler/indexerConstraints2.ts(58,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
+tests/cases/compiler/indexerConstraints2.ts(64,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
+tests/cases/compiler/indexerConstraints2.ts(70,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
+tests/cases/compiler/indexerConstraints2.ts(74,6): error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead.
 
 
-==== tests/cases/compiler/indexerConstraints2.ts (3 errors) ====
+==== tests/cases/compiler/indexerConstraints2.ts (11 errors) ====
     class A { a: number; }
     class B extends A { b: number; }
 
@@ -37,4 +45,67 @@ tests/cases/compiler/indexerConstraints2.ts(26,5): error TS2413: Numeric index t
         ~~~~~~~~~~~~~~~
 !!! error TS2413: Numeric index type 'A' is not assignable to string index type 'B'.
         [s: string]: B;
+    }
+
+
+    type AliasedNumber = number;
+
+    interface L {
+        [n: AliasedNumber]: A;
+         ~
+!!! error TS1336: An index signature parameter type cannot be a type alias. Consider writing '[n: number]: A' instead.
+    }
+
+    type AliasedString = string;
+
+    interface M {
+        [s: AliasedString]: A;
+         ~
+!!! error TS1336: An index signature parameter type cannot be a type alias. Consider writing '[s: string]: A' instead.
+    }
+
+    type AliasedBoolean = boolean;
+
+    interface N {
+        [b: AliasedBoolean]: A;
+         ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
+    }
+
+    type IndexableUnion = "foo" | "bar";
+
+    interface O {
+        [u: IndexableUnion]: A;
+         ~
+!!! error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead.
+    }
+
+    type NonIndexableUnion = boolean | {};
+
+    interface P {
+        [u: NonIndexableUnion]: A;
+         ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
+    }
+
+    type NonIndexableUnion2 = string | number;
+
+    interface Q {
+        [u: NonIndexableUnion2]: A;
+         ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
+    }
+
+    type NonIndexableUnion3 = "foo" | 42;
+
+    interface R {
+        [u: NonIndexableUnion3]: A;
+         ~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
+    }
+
+    interface S {
+        [u: "foo" | "bar"]: A;
+         ~
+!!! error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead.
     }

tests/baselines/reference/indexerConstraints2.js

@@ -26,6 +26,53 @@ class J {
 class K extends J {
     [n: number]: A;
     [s: string]: B;
+}
+
+
+type AliasedNumber = number;
+
+interface L {
+    [n: AliasedNumber]: A;
+}
+
+type AliasedString = string;
+
+interface M {
+    [s: AliasedString]: A;
+}
+
+type AliasedBoolean = boolean;
+
+interface N {
+    [b: AliasedBoolean]: A;
+}
+
+type IndexableUnion = "foo" | "bar";
+
+interface O {
+    [u: IndexableUnion]: A;
+}
+
+type NonIndexableUnion = boolean | {};
+
+interface P {
+    [u: NonIndexableUnion]: A;
+}
+
+type NonIndexableUnion2 = string | number;
+
+interface Q {
+    [u: NonIndexableUnion2]: A;
+}
+
+type NonIndexableUnion3 = "foo" | 42;
+
+interface R {
+    [u: NonIndexableUnion3]: A;
+}
+
+interface S {
+    [u: "foo" | "bar"]: A;
 }
 
 //// [indexerConstraints2.js]

tests/baselines/reference/indexerConstraints2.symbols

@@ -62,3 +62,96 @@ class K extends J {
 >s : Symbol(s, Decl(indexerConstraints2.ts, 26, 5))
 >B : Symbol(B, Decl(indexerConstraints2.ts, 0, 22))
 }
+
+
+type AliasedNumber = number;
+>AliasedNumber : Symbol(AliasedNumber, Decl(indexerConstraints2.ts, 27, 1))
+
+interface L {
+>L : Symbol(L, Decl(indexerConstraints2.ts, 30, 28))
+
+    [n: AliasedNumber]: A;
+>n : Symbol(n, Decl(indexerConstraints2.ts, 33, 5))
+>AliasedNumber : Symbol(AliasedNumber, Decl(indexerConstraints2.ts, 27, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type AliasedString = string;
+>AliasedString : Symbol(AliasedString, Decl(indexerConstraints2.ts, 34, 1))
+
+interface M {
+>M : Symbol(M, Decl(indexerConstraints2.ts, 36, 28))
+
+    [s: AliasedString]: A;
+>s : Symbol(s, Decl(indexerConstraints2.ts, 39, 5))
+>AliasedString : Symbol(AliasedString, Decl(indexerConstraints2.ts, 34, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type AliasedBoolean = boolean;
+>AliasedBoolean : Symbol(AliasedBoolean, Decl(indexerConstraints2.ts, 40, 1))
+
+interface N {
+>N : Symbol(N, Decl(indexerConstraints2.ts, 42, 30))
+
+    [b: AliasedBoolean]: A;
+>b : Symbol(b, Decl(indexerConstraints2.ts, 45, 5))
+>AliasedBoolean : Symbol(AliasedBoolean, Decl(indexerConstraints2.ts, 40, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type IndexableUnion = "foo" | "bar";
+>IndexableUnion : Symbol(IndexableUnion, Decl(indexerConstraints2.ts, 46, 1))
+
+interface O {
+>O : Symbol(O, Decl(indexerConstraints2.ts, 48, 36))
+
+    [u: IndexableUnion]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 51, 5))
+>IndexableUnion : Symbol(IndexableUnion, Decl(indexerConstraints2.ts, 46, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type NonIndexableUnion = boolean | {};
+>NonIndexableUnion : Symbol(NonIndexableUnion, Decl(indexerConstraints2.ts, 52, 1))
+
+interface P {
+>P : Symbol(P, Decl(indexerConstraints2.ts, 54, 38))
+
+    [u: NonIndexableUnion]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 57, 5))
+>NonIndexableUnion : Symbol(NonIndexableUnion, Decl(indexerConstraints2.ts, 52, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type NonIndexableUnion2 = string | number;
+>NonIndexableUnion2 : Symbol(NonIndexableUnion2, Decl(indexerConstraints2.ts, 58, 1))
+
+interface Q {
+>Q : Symbol(Q, Decl(indexerConstraints2.ts, 60, 42))
+
+    [u: NonIndexableUnion2]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 63, 5))
+>NonIndexableUnion2 : Symbol(NonIndexableUnion2, Decl(indexerConstraints2.ts, 58, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+type NonIndexableUnion3 = "foo" | 42;
+>NonIndexableUnion3 : Symbol(NonIndexableUnion3, Decl(indexerConstraints2.ts, 64, 1))
+
+interface R {
+>R : Symbol(R, Decl(indexerConstraints2.ts, 66, 37))
+
+    [u: NonIndexableUnion3]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 69, 5))
+>NonIndexableUnion3 : Symbol(NonIndexableUnion3, Decl(indexerConstraints2.ts, 64, 1))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}
+
+interface S {
+>S : Symbol(S, Decl(indexerConstraints2.ts, 70, 1))
+
+    [u: "foo" | "bar"]: A;
+>u : Symbol(u, Decl(indexerConstraints2.ts, 73, 5))
+>A : Symbol(A, Decl(indexerConstraints2.ts, 0, 0))
+}

tests/baselines/reference/indexerConstraints2.types

@@ -62,3 +62,96 @@ class K extends J {
 >s : string
 >B : B
 }
+
+
+type AliasedNumber = number;
+>AliasedNumber : number
+
+interface L {
+>L : L
+
+    [n: AliasedNumber]: A;
+>n : number
+>AliasedNumber : number
+>A : A
+}
+
+type AliasedString = string;
+>AliasedString : string
+
+interface M {
+>M : M
+
+    [s: AliasedString]: A;
+>s : string
+>AliasedString : string
+>A : A
+}
+
+type AliasedBoolean = boolean;
+>AliasedBoolean : boolean
+
+interface N {
+>N : N
+
+    [b: AliasedBoolean]: A;
+>b : boolean
+>AliasedBoolean : boolean
+>A : A
+}
+
+type IndexableUnion = "foo" | "bar";
+>IndexableUnion : IndexableUnion
+
+interface O {
+>O : O
+
+    [u: IndexableUnion]: A;
+>u : IndexableUnion
+>IndexableUnion : IndexableUnion
+>A : A
+}
+
+type NonIndexableUnion = boolean | {};
+>NonIndexableUnion : NonIndexableUnion
+
+interface P {
+>P : P
+
+    [u: NonIndexableUnion]: A;
+>u : NonIndexableUnion
+>NonIndexableUnion : NonIndexableUnion
+>A : A
+}
+
+type NonIndexableUnion2 = string | number;
+>NonIndexableUnion2 : NonIndexableUnion2
+
+interface Q {
+>Q : Q
+
+    [u: NonIndexableUnion2]: A;
+>u : NonIndexableUnion2
+>NonIndexableUnion2 : NonIndexableUnion2
+>A : A
+}
+
+type NonIndexableUnion3 = "foo" | 42;
+>NonIndexableUnion3 : NonIndexableUnion3
+
+interface R {
+>R : R
+
+    [u: NonIndexableUnion3]: A;
+>u : NonIndexableUnion3
+>NonIndexableUnion3 : NonIndexableUnion3
+>A : A
+}
+
+interface S {
+>S : S
+
+    [u: "foo" | "bar"]: A;
+>u : IndexableUnion
+>A : A
+}