tweak behavior of tuples with rest elements

src/compiler/checker.ts

@@ -13664,6 +13664,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
     function forEachMappedTypePropertyKeyTypeAndIndexSignatureKeyType(type: Type, include: TypeFlags, stringsOnly: boolean, cb: (keyType: Type) => void) {
         if (isTupleType(type)) {
+            if (type.target.hasRestElement) {
+                // key of any rest/variadic element is number
+                // it subsumes all potential leading fixed elements
+                cb(numberType);
+                return    
+            }
             forEachType(getUnionType(getElementTypes(type).map((_, i) => getStringLiteralType("" + i))), cb);
             return;
         }

tests/baselines/reference/mappedTypeTupleConstraintTypeParameterInNameType.errors.txt

@@ -0,0 +1,66 @@
+mappedTypeTupleConstraintTypeParameterInNameType.ts(45,7): error TS2322: Type 'string | number | bigint' is not assignable to type 'number'.
+  Type 'string' is not assignable to type 'number'.
+mappedTypeTupleConstraintTypeParameterInNameType.ts(46,7): error TS2322: Type 'string | number | bigint' is not assignable to type 'string'.
+  Type 'number' is not assignable to type 'string'.
+mappedTypeTupleConstraintTypeParameterInNameType.ts(47,7): error TS2322: Type 'string | number | bigint' is not assignable to type 'bigint'.
+  Type 'string' is not assignable to type 'bigint'.
+
+
+==== mappedTypeTupleConstraintTypeParameterInNameType.ts (3 errors) ====
+    // based on https://github.com/microsoft/TypeScript/issues/55762
+
+    declare class Decoder<T> {
+      decode(arrayBuffer: ArrayBuffer): T;
+    }
+
+    type ValueTypeOf<T extends Decoder<any>> = T extends Decoder<infer R>
+      ? R
+      : never;
+
+    type StructDescriptor = ReadonlyArray<
+      readonly [key: string, type: Decoder<any>]
+    >;
+
+    type StructTypeFor<Descriptor extends StructDescriptor> = {
+      [K in keyof Descriptor as Descriptor[K][0]]: ValueTypeOf<Descriptor[K][1]>;
+    };
+
+    class StructDecoder<const Descriptor extends StructDescriptor> extends Decoder<
+      StructTypeFor<Descriptor>
+    > {
+      constructor(descriptor: Descriptor) {
+        super();
+      }
+    }
+
+    declare const i32Decoder: Decoder<number>;
+    declare const i64Decoder: Decoder<bigint>;
+
+    const structDecoder1 = new StructDecoder([
+      ["a", i32Decoder],
+      ["b", i64Decoder],
+    ]);
+
+    const struct1 = structDecoder1.decode(new ArrayBuffer(100));
+
+    const v1_1: number = struct1.a;
+    const v1_2: bigint = struct1.b;
+
+    declare const descriptor2: [["a", Decoder<number>], ["b", Decoder<string>], ...["c", Decoder<bigint>][]]
+    const structDecoder2 = new StructDecoder(descriptor2);
+
+    const struct2 = structDecoder2.decode(new ArrayBuffer(100));
+
+    const v2_1: number = struct2.a; // error, rest element expands to index signature access
+          ~~~~
+!!! error TS2322: Type 'string | number | bigint' is not assignable to type 'number'.
+!!! error TS2322:   Type 'string' is not assignable to type 'number'.
+    const v2_2: string = struct2.b; // error, rest element expands to index signature access
+          ~~~~
+!!! error TS2322: Type 'string | number | bigint' is not assignable to type 'string'.
+!!! error TS2322:   Type 'number' is not assignable to type 'string'.
+    const v2_3: bigint = struct2.c; // error, rest element expands to index signature access
+          ~~~~
+!!! error TS2322: Type 'string | number | bigint' is not assignable to type 'bigint'.
+!!! error TS2322:   Type 'string' is not assignable to type 'bigint'.
+

tests/baselines/reference/mappedTypeTupleConstraintTypeParameterInNameType.symbols

@@ -129,19 +129,19 @@ const struct2 = structDecoder2.decode(new ArrayBuffer(100));
 >decode : Symbol(Decoder.decode, Decl(mappedTypeTupleConstraintTypeParameterInNameType.ts, 2, 26))
 >ArrayBuffer : Symbol(ArrayBuffer, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
 
-const v2_1: number = struct2.a;
+const v2_1: number = struct2.a; // error, rest element expands to index signature access
 >v2_1 : Symbol(v2_1, Decl(mappedTypeTupleConstraintTypeParameterInNameType.ts, 44, 5))
 >struct2.a : Symbol(a)
 >struct2 : Symbol(struct2, Decl(mappedTypeTupleConstraintTypeParameterInNameType.ts, 42, 5))
 >a : Symbol(a)
 
-const v2_2: string = struct2.b;
+const v2_2: string = struct2.b; // error, rest element expands to index signature access
 >v2_2 : Symbol(v2_2, Decl(mappedTypeTupleConstraintTypeParameterInNameType.ts, 45, 5))
 >struct2.b : Symbol(b)
 >struct2 : Symbol(struct2, Decl(mappedTypeTupleConstraintTypeParameterInNameType.ts, 42, 5))
 >b : Symbol(b)
 
-const v2_3: bigint = struct2.c;
+const v2_3: bigint = struct2.c; // error, rest element expands to index signature access
 >v2_3 : Symbol(v2_3, Decl(mappedTypeTupleConstraintTypeParameterInNameType.ts, 46, 5))
 >struct2.c : Symbol(c)
 >struct2 : Symbol(struct2, Decl(mappedTypeTupleConstraintTypeParameterInNameType.ts, 42, 5))

tests/baselines/reference/mappedTypeTupleConstraintTypeParameterInNameType.types

@@ -161,33 +161,33 @@ const struct2 = structDecoder2.decode(new ArrayBuffer(100));
 >100 : 100
 >    : ^^^
 
-const v2_1: number = struct2.a;
+const v2_1: number = struct2.a; // error, rest element expands to index signature access
 >v2_1 : number
 >     : ^^^^^^
->struct2.a : number
->          : ^^^^^^
+>struct2.a : string | number | bigint
+>          : ^^^^^^^^^^^^^^^^^^^^^^^^
 >struct2 : StructTypeFor<[["a", Decoder<number>], ["b", Decoder<string>], ...["c", Decoder<bigint>][]]>
 >        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
->a : number
->  : ^^^^^^
+>a : string | number | bigint
+>  : ^^^^^^^^^^^^^^^^^^^^^^^^
 
-const v2_2: string = struct2.b;
+const v2_2: string = struct2.b; // error, rest element expands to index signature access
 >v2_2 : string
 >     : ^^^^^^
->struct2.b : string
->          : ^^^^^^
+>struct2.b : string | number | bigint
+>          : ^^^^^^^^^^^^^^^^^^^^^^^^
 >struct2 : StructTypeFor<[["a", Decoder<number>], ["b", Decoder<string>], ...["c", Decoder<bigint>][]]>
 >        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
->b : string
->  : ^^^^^^
+>b : string | number | bigint
+>  : ^^^^^^^^^^^^^^^^^^^^^^^^
 
-const v2_3: bigint = struct2.c;
+const v2_3: bigint = struct2.c; // error, rest element expands to index signature access
 >v2_3 : bigint
 >     : ^^^^^^
->struct2.c : bigint
->          : ^^^^^^
+>struct2.c : string | number | bigint
+>          : ^^^^^^^^^^^^^^^^^^^^^^^^
 >struct2 : StructTypeFor<[["a", Decoder<number>], ["b", Decoder<string>], ...["c", Decoder<bigint>][]]>
 >        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
->c : bigint
->  : ^^^^^^
+>c : string | number | bigint
+>  : ^^^^^^^^^^^^^^^^^^^^^^^^
 

tests/cases/compiler/mappedTypeTupleConstraintTypeParameterInNameType.ts

@@ -45,6 +45,6 @@ const structDecoder2 = new StructDecoder(descriptor2);
 
 const struct2 = structDecoder2.decode(new ArrayBuffer(100));
 
-const v2_1: number = struct2.a;
-const v2_2: string = struct2.b;
-const v2_3: bigint = struct2.c;
+const v2_1: number = struct2.a; // error, rest element expands to index signature access
+const v2_2: string = struct2.b; // error, rest element expands to index signature access
+const v2_3: bigint = struct2.c; // error, rest element expands to index signature access