Exclude T[K] assignability check from definitelyAssignableRelation

Author: ahejlsberg

src/compiler/checker.ts

@@ -12254,7 +12254,8 @@ namespace ts {
                 else if (target.flags & TypeFlags.IndexedAccess) {
                     // A type S is related to a type T[K], where T and K aren't both type variables, if S is related to C,
                     // where C is the base constraint of T[K]
-                    if (relation !== identityRelation && !(isGenericObjectType((<IndexedAccessType>target).objectType) && isGenericIndexType((<IndexedAccessType>target).indexType))) {
+                    if (relation !== identityRelation && relation !== definitelyAssignableRelation &&
+                        !(isGenericObjectType((<IndexedAccessType>target).objectType) && isGenericIndexType((<IndexedAccessType>target).indexType))) {
                         const constraint = getBaseConstraintOfType(target);
                         if (constraint && constraint !== target) {
                             if (result = isRelatedTo(source, constraint, reportErrors)) {