Support apparent types for T[K] indexed access types

Author: ahejlsberg

src/compiler/checker.ts

@@ -4666,13 +4666,22 @@ namespace ts {
             return type.resolvedApparentType;
         }
 
+        /**
+         * The apparent type of an indexed access T[K] is the type of T's string index signature, if any.
+         */
+        function getApparentTypeOfIndexedAccess(type: IndexedAccessType) {
+            return getIndexTypeOfType(getApparentType(type.objectType), IndexKind.String) || type;
+        }
+
         /**
          * For a type parameter, return the base constraint of the type parameter. For the string, number,
          * boolean, and symbol primitive types, return the corresponding object types. Otherwise return the
          * type itself. Note that the apparent type of a union type is the union type itself.
          */
         function getApparentType(type: Type): Type {
-            const t = type.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>type) : type;
+            const t = type.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>type) :
+                type.flags & TypeFlags.IndexedAccess ? getApparentTypeOfIndexedAccess(<IndexedAccessType>type) :
+                type;
             return t.flags & TypeFlags.StringLike ? globalStringType :
                 t.flags & TypeFlags.NumberLike ? globalNumberType :
                 t.flags & TypeFlags.BooleanLike ? globalBooleanType :