Properly check T[K] constraints in type relationships

Author: ahejlsberg

src/compiler/checker.ts

@@ -7206,7 +7206,7 @@ namespace ts {
                     return related === RelationComparisonResult.Succeeded;
                 }
             }
-            if (source.flags & TypeFlags.StructuredOrTypeParameter || target.flags & TypeFlags.StructuredOrTypeParameter) {
+            if (source.flags & TypeFlags.StructuredOrTypeVariable || target.flags & TypeFlags.StructuredOrTypeVariable) {
                 return checkTypeRelatedTo(source, target, relation, undefined, undefined, undefined);
             }
             return false;

src/compiler/types.ts

@@ -2803,7 +2803,7 @@ namespace ts {
         EnumLike = Enum | EnumLiteral,
         UnionOrIntersection = Union | Intersection,
         StructuredType = Object | Union | Intersection,
-        StructuredOrTypeParameter = StructuredType | TypeParameter | Index,
+        StructuredOrTypeVariable = StructuredType | TypeParameter | Index | IndexedAccess,
         TypeVariable = TypeParameter | IndexedAccess,
 
         // 'Narrowable' types are types where narrowing actually narrows.