From 01ee1b9c25ca8201bfae39aa288357e1c9ec91a4 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 28 Nov 2018 14:56:49 -0800
Subject: [PATCH] Exclude T[K] assignability check from
 definitelyAssignableRelation

---
 src/compiler/checker.ts | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7a1364c7a1f..bc3bd0ebd8a 100644
--- a/src/compiler/checker.ts
+++ b/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)) {