Merge pull request #13523 from Microsoft/fixNumericIndexedAccess

Fix numeric indexed access types

Author: ahejlsberg

src/compiler/checker.ts

@@ -16091,14 +16091,24 @@ namespace ts {
         }
 
         function checkIndexedAccessIndexType(type: Type, accessNode: ElementAccessExpression | IndexedAccessTypeNode) {
-            if (type.flags & TypeFlags.IndexedAccess) {
-                // Check that the index type is assignable to 'keyof T' for the object type.
-                const objectType = (<IndexedAccessType>type).objectType;
-                const indexType = (<IndexedAccessType>type).indexType;
-                if (!isTypeAssignableTo(indexType, getIndexType(objectType))) {
-                    error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(objectType));
+            if (!(type.flags & TypeFlags.IndexedAccess)) {
+                return type;
+            }
+            // Check if the index type is assignable to 'keyof T' for the object type.
+            const objectType = (<IndexedAccessType>type).objectType;
+            const indexType = (<IndexedAccessType>type).indexType;
+            if (isTypeAssignableTo(indexType, getIndexType(objectType))) {
+                return type;
+            }
+            // Check if we're indexing with a numeric type and the object type is a generic
+            // type with a constraint that has a numeric index signature.
+            if (maybeTypeOfKind(objectType, TypeFlags.TypeVariable) && isTypeOfKind(indexType, TypeFlags.NumberLike)) {
+                const constraint = getBaseConstraintOfType(<TypeVariable | UnionOrIntersectionType>objectType);
+                if (constraint && getIndexInfoOfType(constraint, IndexKind.Number)) {
+                    return type;
                 }
             }
+            error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(objectType));
             return type;
         }
 

tests/baselines/reference/keyofAndIndexedAccess.js

@@ -500,6 +500,10 @@ function updateIds2<T extends { [x: string]: string }, K extends keyof T>(
     var x = obj[key];
     stringMap[x]; // Should be OK.
 }
+
+// Repro from #13514
+
+declare function head<T extends Array<any>>(list: T): T[0];
 
 
 //// [keyofAndIndexedAccess.js]
@@ -1061,3 +1065,4 @@ declare function updateIds2<T extends {
 }, K extends keyof T>(obj: T, key: K, stringMap: {
     [oldId: string]: string;
 }): void;
+declare function head<T extends Array<any>>(list: T): T[0];

tests/baselines/reference/keyofAndIndexedAccess.symbols

@@ -1806,3 +1806,13 @@ function updateIds2<T extends { [x: string]: string }, K extends keyof T>(
 >x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 498, 7))
 }
 
+// Repro from #13514
+
+declare function head<T extends Array<any>>(list: T): T[0];
+>head : Symbol(head, Decl(keyofAndIndexedAccess.ts, 500, 1))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 504, 22))
+>Array : Symbol(Array, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>list : Symbol(list, Decl(keyofAndIndexedAccess.ts, 504, 44))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 504, 22))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 504, 22))
+

tests/baselines/reference/keyofAndIndexedAccess.types

@@ -2128,3 +2128,13 @@ function updateIds2<T extends { [x: string]: string }, K extends keyof T>(
 >x : T[K]
 }
 
+// Repro from #13514
+
+declare function head<T extends Array<any>>(list: T): T[0];
+>head : <T extends any[]>(list: T) => T[0]
+>T : T
+>Array : T[]
+>list : T
+>T : T
+>T : T
+

tests/cases/conformance/types/keyof/keyofAndIndexedAccess.ts

@@ -501,3 +501,7 @@ function updateIds2<T extends { [x: string]: string }, K extends keyof T>(
     var x = obj[key];
     stringMap[x]; // Should be OK.
 }
+
+// Repro from #13514
+
+declare function head<T extends Array<any>>(list: T): T[0];