From 3dd11e48a9fda2e4ee8d4efb2698eb9f92814980 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 9 Nov 2016 09:59:15 -0800
Subject: [PATCH] Properly implement type relationship for '[P in S]: X' and
 '[P in T]: Y'

---
 src/compiler/checker.ts | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 8ed8df17c68..cfabcd3f5ba 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7047,11 +7047,12 @@ namespace ts {
                         }
                     }
                     if (isGenericMappedType(target)) {
-                        // A type [P in S]: X is related to a type [P in T]: X if T is related to S.
-                        if (isGenericMappedType(source) &&
-                            isRelatedTo(getConstraintTypeFromMappedType(<MappedType>target), getConstraintTypeFromMappedType(<MappedType>source), /*reportErrors*/ false) &&
-                            isTypeIdenticalTo(getTemplateTypeFromMappedType(<MappedType>source), getTemplateTypeFromMappedType(<MappedType>target))) {
-                            return Ternary.True;
+                        // A type [P in S]: X is related to a type [P in T]: Y if T is related to S and X is related to Y.
+                        if (isGenericMappedType(source)) {
+                            if ((result = isRelatedTo(getConstraintTypeFromMappedType(<MappedType>target), getConstraintTypeFromMappedType(<MappedType>source), reportErrors)) &&
+                                (result = isRelatedTo(getTemplateTypeFromMappedType(<MappedType>source), getTemplateTypeFromMappedType(<MappedType>target), reportErrors))) {
+                                return result;
+                            }
                         }
                     }
                     else {