Allow T[N] where N is numeric and T has apparent numeric index signature

Author: ahejlsberg

src/compiler/checker.ts

@@ -16032,14 +16032,24 @@ 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;
         }