From 056c87f289d54c52b00b9ae6d5727c36ab2e6081 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 22 Aug 2018 07:08:35 -0700
Subject: [PATCH] Ignore constraints of generic mapped types in
 definitelyAssignableRelation

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

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 36cf9aa947..0bc9cd7600 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11778,6 +11778,9 @@ namespace ts {
                         }
                         return Ternary.False;
                     }
+                    if (relation === definitelyAssignableRelation && isGenericMappedType(source)) {
+                        return Ternary.False;
+                    }
                     const sourceIsPrimitive = !!(source.flags & TypeFlags.Primitive);
                     if (relation !== identityRelation) {
                         source = getApparentType(source);