From c0c215fb668590f7d469c4f5931adc465710ce22 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 1 Oct 2018 16:20:15 -0700
Subject: [PATCH] No unsound assignments to T[K] when T and K are both generic

---
 src/compiler/checker.ts | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 611b6c4d4b0..58e6075198a 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11876,8 +11876,9 @@ namespace ts {
                     }
                 }
                 else if (target.flags & TypeFlags.IndexedAccess) {
-                    // A type S is related to a type T[K] if S is related to C, where C is the base constraint of T[K]
-                    if (relation !== identityRelation) {
+                    // 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))) {
                         const constraint = getBaseConstraintOfType(target);
                         if (constraint && constraint !== target) {
                             if (result = isRelatedTo(source, constraint, reportErrors)) {