Leading and middle rest elements in tuple types

src/compiler/diagnosticMessages.json

@@ -843,10 +843,6 @@
         "category": "Error",
         "code": 1255
     },
-    "A rest element must be last in a tuple type.": {
-        "category": "Error",
-        "code": 1256
-    },
     "A required element cannot follow an optional element.": {
         "category": "Error",
         "code": 1257
@@ -875,6 +871,14 @@
         "category": "Error",
         "code": 1264
     },
+    "A rest element cannot follow another rest element.": {
+        "category": "Error",
+        "code": 1265
+    },
+    "An optional element cannot follow a rest element.": {
+        "category": "Error",
+        "code": 1266
+    },
 
     "'with' statements are not allowed in an async function block.": {
         "category": "Error",
@@ -2469,9 +2473,25 @@
         "category": "Error",
         "code": 2621
     },
-    "Element at index {0} is variadic in one type but not in the other.": {
+    "Source provides no match for required element at position {0} in target.": {
+        "category": "Error",
+        "code": 2623
+    },
+    "Source provides no match for variadic element at position {0} in target.": {
+        "category": "Error",
+        "code": 2624
+    },
+    "Variadic element at position {0} in source does not match element at position {1} in target.": {
+        "category": "Error",
+        "code": 2625
+    },
+    "Type at position {0} in source is not compatible with type at position {1} in target.": {
+        "category": "Error",
+        "code": 2626
+    },
+    "Type at positions {0} through {1} in source is not compatible with type at position {2} in target.": {
         "category": "Error",
-        "code": 2622
+        "code": 2627
     },
 
     "Cannot augment module '{0}' with value exports because it resolves to a non-module entity.": {

src/compiler/types.ts

@@ -5166,18 +5166,21 @@ namespace ts {
     }
 
     export const enum ElementFlags {
-        Required  = 1 << 0,  // T
-        Optional  = 1 << 1,  // T?
-        Rest      = 1 << 2,  // ...T[]
-        Variadic  = 1 << 3,  // ...T
-        Variable  = Rest | Variadic,
+        Required    = 1 << 0,  // T
+        Optional    = 1 << 1,  // T?
+        Rest        = 1 << 2,  // ...T[]
+        Variadic    = 1 << 3,  // ...T
+        Fixed       = Required | Optional,
+        Variable    = Rest | Variadic,
+        NonRequired = Optional | Rest | Variadic,
+        NonRest     = Required | Optional | Variadic,
     }
 
     export interface TupleType extends GenericType {
         elementFlags: readonly ElementFlags[];
-        minLength: number;
-        fixedLength: number;
-        hasRestElement: boolean;
+        minLength: number;  // Number of required or variadic elements
+        fixedLength: number;  // Number of initial required or optional elements
+        hasRestElement: boolean;  // True if tuple has any rest or variadic elements
         combinedFlags: ElementFlags;
         readonly: boolean;
         labeledElementDeclarations?: readonly (NamedTupleMember | ParameterDeclaration)[];

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2590,7 +2590,10 @@ declare namespace ts {
         Optional = 2,
         Rest = 4,
         Variadic = 8,
-        Variable = 12
+        Fixed = 3,
+        Variable = 12,
+        NonRequired = 14,
+        NonRest = 11
     }
     export interface TupleType extends GenericType {
         elementFlags: readonly ElementFlags[];

tests/baselines/reference/api/typescript.d.ts

@@ -2590,7 +2590,10 @@ declare namespace ts {
         Optional = 2,
         Rest = 4,
         Variadic = 8,
-        Variable = 12
+        Fixed = 3,
+        Variable = 12,
+        NonRequired = 14,
+        NonRest = 11
     }
     export interface TupleType extends GenericType {
         elementFlags: readonly ElementFlags[];

tests/baselines/reference/for-of39.errors.txt

@@ -7,7 +7,7 @@ tests/cases/conformance/es6/for-ofStatements/for-of39.ts(1,11): error TS2769: No
             Type 'IteratorYieldResult<[string, number] | [string, true]>' is not assignable to type 'IteratorYieldResult<readonly [string, boolean]>'.
               Type '[string, number] | [string, true]' is not assignable to type 'readonly [string, boolean]'.
                 Type '[string, number]' is not assignable to type 'readonly [string, boolean]'.
-                  Types of property '1' are incompatible.
+                  Type at position 1 in source is not compatible with type at position 1 in target.
                     Type 'number' is not assignable to type 'boolean'.
   Overload 2 of 3, '(entries?: readonly (readonly [string, boolean])[]): Map<string, boolean>', gave the following error.
     Type 'number' is not assignable to type 'boolean'.
@@ -25,7 +25,7 @@ tests/cases/conformance/es6/for-ofStatements/for-of39.ts(1,11): error TS2769: No
 !!! error TS2769:             Type 'IteratorYieldResult<[string, number] | [string, true]>' is not assignable to type 'IteratorYieldResult<readonly [string, boolean]>'.
 !!! error TS2769:               Type '[string, number] | [string, true]' is not assignable to type 'readonly [string, boolean]'.
 !!! error TS2769:                 Type '[string, number]' is not assignable to type 'readonly [string, boolean]'.
-!!! error TS2769:                   Types of property '1' are incompatible.
+!!! error TS2769:                   Type at position 1 in source is not compatible with type at position 1 in target.
 !!! error TS2769:                     Type 'number' is not assignable to type 'boolean'.
 !!! error TS2769:   Overload 2 of 3, '(entries?: readonly (readonly [string, boolean])[]): Map<string, boolean>', gave the following error.
 !!! error TS2769:     Type 'number' is not assignable to type 'boolean'.

tests/baselines/reference/genericRestParameters2.types

@@ -408,7 +408,7 @@ f20(42, "hello", ...t3);
 >t3 : boolean[]
 
 f20(42, "hello", ...t2, true);
->f20(42, "hello", ...t2, true) : [number, string, string, ...boolean[]]
+>f20(42, "hello", ...t2, true) : [number, string, string, ...boolean[], boolean]
 >f20 : <T extends unknown[]>(...args: T) => T
 >42 : 42
 >"hello" : "hello"

tests/baselines/reference/genericRestParameters3.errors.txt

@@ -114,7 +114,7 @@ tests/cases/conformance/types/rest/genericRestParameters3.ts(59,5): error TS2345
 
     let a = bar(10, 20);
     let b = bar<CoolArray<number>>(10, 20);  // Error
-                                   ~~
+                                   ~~~~~~
 !!! error TS2345: Argument of type '[10, 20]' is not assignable to parameter of type 'CoolArray<number>'.
 !!! error TS2345:   Property 'hello' is missing in type '[10, 20]' but required in type 'CoolArray<number>'.
 !!! related TS2728 tests/cases/conformance/types/rest/genericRestParameters3.ts:30:5: 'hello' is declared here.
@@ -133,7 +133,7 @@ tests/cases/conformance/types/rest/genericRestParameters3.ts(59,5): error TS2345
 !!! error TS2345:   Property 'hello' is missing in type '[number]' but required in type 'CoolArray<unknown>'.
 !!! related TS2728 tests/cases/conformance/types/rest/genericRestParameters3.ts:30:5: 'hello' is declared here.
     baz(1, 2);   // Error
-        ~
+        ~~~~
 !!! error TS2345: Argument of type '[number, number]' is not assignable to parameter of type 'CoolArray<unknown>'.
 !!! error TS2345:   Property 'hello' is missing in type '[number, number]' but required in type 'CoolArray<unknown>'.
 !!! related TS2728 tests/cases/conformance/types/rest/genericRestParameters3.ts:30:5: 'hello' is declared here.

tests/baselines/reference/iterableArrayPattern28.errors.txt

@@ -7,7 +7,7 @@ tests/cases/conformance/es6/destructuring/iterableArrayPattern28.ts(2,24): error
             Type 'IteratorYieldResult<[string, number] | [string, boolean]>' is not assignable to type 'IteratorYieldResult<readonly [string, number]>'.
               Type '[string, number] | [string, boolean]' is not assignable to type 'readonly [string, number]'.
                 Type '[string, boolean]' is not assignable to type 'readonly [string, number]'.
-                  Types of property '1' are incompatible.
+                  Type at position 1 in source is not compatible with type at position 1 in target.
                     Type 'boolean' is not assignable to type 'number'.
   Overload 2 of 3, '(entries?: readonly (readonly [string, number])[]): Map<string, number>', gave the following error.
     Type 'boolean' is not assignable to type 'number'.
@@ -26,7 +26,7 @@ tests/cases/conformance/es6/destructuring/iterableArrayPattern28.ts(2,24): error
 !!! error TS2769:             Type 'IteratorYieldResult<[string, number] | [string, boolean]>' is not assignable to type 'IteratorYieldResult<readonly [string, number]>'.
 !!! error TS2769:               Type '[string, number] | [string, boolean]' is not assignable to type 'readonly [string, number]'.
 !!! error TS2769:                 Type '[string, boolean]' is not assignable to type 'readonly [string, number]'.
-!!! error TS2769:                   Types of property '1' are incompatible.
+!!! error TS2769:                   Type at position 1 in source is not compatible with type at position 1 in target.
 !!! error TS2769:                     Type 'boolean' is not assignable to type 'number'.
 !!! error TS2769:   Overload 2 of 3, '(entries?: readonly (readonly [string, number])[]): Map<string, number>', gave the following error.
 !!! error TS2769:     Type 'boolean' is not assignable to type 'number'.

tests/baselines/reference/optionalTupleElements1.errors.txt

@@ -1,16 +1,16 @@
 tests/cases/conformance/types/tuple/optionalTupleElements1.ts(11,29): error TS1257: A required element cannot follow an optional element.
 tests/cases/conformance/types/tuple/optionalTupleElements1.ts(15,5): error TS2322: Type 'T2' is not assignable to type 'T1'.
-  Property '2' is optional in type '[number, string, (boolean | undefined)?]' but required in type '[number, string, boolean]'.
+  Source provides no match for required element at position 2 in target.
 tests/cases/conformance/types/tuple/optionalTupleElements1.ts(16,5): error TS2322: Type 'T3' is not assignable to type 'T1'.
-  Property '1' is optional in type '[number, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, boolean]'.
+  Source provides no match for required element at position 1 in target.
 tests/cases/conformance/types/tuple/optionalTupleElements1.ts(17,5): error TS2322: Type 'T4' is not assignable to type 'T1'.
-  Property '0' is optional in type '[(number | undefined)?, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, boolean]'.
+  Source provides no match for required element at position 0 in target.
 tests/cases/conformance/types/tuple/optionalTupleElements1.ts(20,5): error TS2322: Type 'T3' is not assignable to type 'T2'.
-  Property '1' is optional in type '[number, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, (boolean | undefined)?]'.
+  Source provides no match for required element at position 1 in target.
 tests/cases/conformance/types/tuple/optionalTupleElements1.ts(21,5): error TS2322: Type 'T4' is not assignable to type 'T2'.
-  Property '0' is optional in type '[(number | undefined)?, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, (boolean | undefined)?]'.
+  Source provides no match for required element at position 0 in target.
 tests/cases/conformance/types/tuple/optionalTupleElements1.ts(25,5): error TS2322: Type 'T4' is not assignable to type 'T3'.
-  Property '0' is optional in type '[(number | undefined)?, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, (string | undefined)?, (boolean | undefined)?]'.
+  Source provides no match for required element at position 0 in target.
 
 
 ==== tests/cases/conformance/types/tuple/optionalTupleElements1.ts (7 errors) ====
@@ -33,32 +33,32 @@ tests/cases/conformance/types/tuple/optionalTupleElements1.ts(25,5): error TS232
         t1 = t2;  // Error
         ~~
 !!! error TS2322: Type 'T2' is not assignable to type 'T1'.
-!!! error TS2322:   Property '2' is optional in type '[number, string, (boolean | undefined)?]' but required in type '[number, string, boolean]'.
+!!! error TS2322:   Source provides no match for required element at position 2 in target.
         t1 = t3;  // Error
         ~~
 !!! error TS2322: Type 'T3' is not assignable to type 'T1'.
-!!! error TS2322:   Property '1' is optional in type '[number, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, boolean]'.
+!!! error TS2322:   Source provides no match for required element at position 1 in target.
         t1 = t4;  // Error
         ~~
 !!! error TS2322: Type 'T4' is not assignable to type 'T1'.
-!!! error TS2322:   Property '0' is optional in type '[(number | undefined)?, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, boolean]'.
+!!! error TS2322:   Source provides no match for required element at position 0 in target.
         t2 = t1;
         t2 = t2;
         t2 = t3;  // Error
         ~~
 !!! error TS2322: Type 'T3' is not assignable to type 'T2'.
-!!! error TS2322:   Property '1' is optional in type '[number, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, (boolean | undefined)?]'.
+!!! error TS2322:   Source provides no match for required element at position 1 in target.
         t2 = t4;  // Error
         ~~
 !!! error TS2322: Type 'T4' is not assignable to type 'T2'.
-!!! error TS2322:   Property '0' is optional in type '[(number | undefined)?, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, string, (boolean | undefined)?]'.
+!!! error TS2322:   Source provides no match for required element at position 0 in target.
         t3 = t1;
         t3 = t2;
         t3 = t3;
         t3 = t4;  // Error
         ~~
 !!! error TS2322: Type 'T4' is not assignable to type 'T3'.
-!!! error TS2322:   Property '0' is optional in type '[(number | undefined)?, (string | undefined)?, (boolean | undefined)?]' but required in type '[number, (string | undefined)?, (boolean | undefined)?]'.
+!!! error TS2322:   Source provides no match for required element at position 0 in target.
         t4 = t1;
         t4 = t2;
         t4 = t3;

tests/baselines/reference/restTupleElements1.errors.txt

@@ -1,10 +1,9 @@
 tests/cases/conformance/types/tuple/restTupleElements1.ts(3,22): error TS1257: A required element cannot follow an optional element.
-tests/cases/conformance/types/tuple/restTupleElements1.ts(8,13): error TS1256: A rest element must be last in a tuple type.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(9,13): error TS2574: A rest element type must be an array type.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(10,13): error TS2574: A rest element type must be an array type.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(10,16): error TS8020: JSDoc types can only be used inside documentation comments.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(23,31): error TS2344: Type 'number[]' does not satisfy the constraint '[number, ...number[]]'.
-  Property '0' is optional in type 'number[]' but required in type '[number, ...number[]]'.
+  Source provides no match for required element at position 0 in target.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(24,31): error TS2344: Type '[]' does not satisfy the constraint '[number, ...number[]]'.
   Source has 0 element(s) but target requires 1.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(29,18): error TS2344: Type 'number[]' does not satisfy the constraint '[number]'.
@@ -16,16 +15,18 @@ tests/cases/conformance/types/tuple/restTupleElements1.ts(32,31): error TS2344:
 tests/cases/conformance/types/tuple/restTupleElements1.ts(33,31): error TS2344: Type '[string, ...number[]]' does not satisfy the constraint '[number, ...number[]]'.
   Type 'string' is not assignable to type 'number'.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(34,31): error TS2344: Type '[number, number, string]' does not satisfy the constraint '[number, ...number[]]'.
-  Types of property '2' are incompatible.
-    Type 'string' is not assignable to type 'number'.
+  Type at positions 1 through 2 in source is not compatible with type at position 1 in target.
+    Type 'string | number' is not assignable to type 'number'.
+      Type 'string' is not assignable to type 'number'.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(35,31): error TS2344: Type '[number, number, number, string]' does not satisfy the constraint '[number, ...number[]]'.
-  Types of property '3' are incompatible.
-    Type 'string' is not assignable to type 'number'.
+  Type at positions 1 through 3 in source is not compatible with type at position 1 in target.
+    Type 'string | number' is not assignable to type 'number'.
+      Type 'string' is not assignable to type 'number'.
 tests/cases/conformance/types/tuple/restTupleElements1.ts(59,4): error TS2345: Argument of type '[]' is not assignable to parameter of type '[unknown, ...unknown[]]'.
   Source has 0 element(s) but target requires 1.
 
 
-==== tests/cases/conformance/types/tuple/restTupleElements1.ts (14 errors) ====
+==== tests/cases/conformance/types/tuple/restTupleElements1.ts (13 errors) ====
     type T00 = [string?];
     type T01 = [string, string?];
     type T02 = [string?, string];  // Error
@@ -36,8 +37,6 @@ tests/cases/conformance/types/tuple/restTupleElements1.ts(59,4): error TS2345: A
     type T05 = [...[...[...string[]]]];
     type T06 = [string, ...string[]];
     type T07 = [...string[], string];  // Error
-                ~~~~~~~~~~~
-!!! error TS1256: A rest element must be last in a tuple type.
     type T08 = [...string];  // Error
                 ~~~~~~~~~
 !!! error TS2574: A rest element type must be an array type.
@@ -61,7 +60,7 @@ tests/cases/conformance/types/tuple/restTupleElements1.ts(59,4): error TS2345: A
     assign<[number, ...number[]], number[]>();  // Error
                                   ~~~~~~~~
 !!! error TS2344: Type 'number[]' does not satisfy the constraint '[number, ...number[]]'.
-!!! error TS2344:   Property '0' is optional in type 'number[]' but required in type '[number, ...number[]]'.
+!!! error TS2344:   Source provides no match for required element at position 0 in target.
     assign<[number, ...number[]], []>();  // Error
                                   ~~
 !!! error TS2344: Type '[]' does not satisfy the constraint '[number, ...number[]]'.
@@ -90,13 +89,15 @@ tests/cases/conformance/types/tuple/restTupleElements1.ts(59,4): error TS2345: A
     assign<[number, ...number[]], [number, number, string]>();  // Error
                                   ~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2344: Type '[number, number, string]' does not satisfy the constraint '[number, ...number[]]'.
-!!! error TS2344:   Types of property '2' are incompatible.
-!!! error TS2344:     Type 'string' is not assignable to type 'number'.
+!!! error TS2344:   Type at positions 1 through 2 in source is not compatible with type at position 1 in target.
+!!! error TS2344:     Type 'string | number' is not assignable to type 'number'.
+!!! error TS2344:       Type 'string' is not assignable to type 'number'.
     assign<[number, ...number[]], [number, number, number, string]>();  // Error
                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2344: Type '[number, number, number, string]' does not satisfy the constraint '[number, ...number[]]'.
-!!! error TS2344:   Types of property '3' are incompatible.
-!!! error TS2344:     Type 'string' is not assignable to type 'number'.
+!!! error TS2344:   Type at positions 1 through 3 in source is not compatible with type at position 1 in target.
+!!! error TS2344:     Type 'string | number' is not assignable to type 'number'.
+!!! error TS2344:       Type 'string' is not assignable to type 'number'.
 
     type T20 = [number, string, ...boolean[]];
 

tests/baselines/reference/restTupleElements1.types

@@ -107,7 +107,7 @@ type T21 = T20[0];
 >T21 : number
 
 type T22 = T20[0 | 1];
->T22 : T22
+>T22 : string | number
 
 type T23 = T20[0 | 1 | 2];
 >T23 : string | number | boolean