From e71afc32ad8cb4d1e48444fb38a0a2da4d39f989 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Tue, 24 Apr 2018 10:02:34 -0700
Subject: [PATCH] Reinstate type simplification for 'keyof T[K]' as target in
 relation

---
 src/compiler/checker.ts | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 75f8715fca7..4e1d71c3d7a 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10482,11 +10482,14 @@ namespace ts {
                         }
                     }
                     // A type S is assignable to keyof T if S is assignable to keyof C, where C is the
-                    // constraint of T.
-                    const constraint = getConstraintForRelation((<IndexType>target).type);
-                    if (constraint) {
-                        if (result = isRelatedTo(source, getIndexType(constraint, (target as IndexType).stringsOnly), reportErrors)) {
-                            return result;
+                    // simplified form of T or, if T doesn't simplify, the constraint of T.
+                    if (relation !== definitelyAssignableRelation) {
+                        const simplified = getSimplifiedType((<IndexType>target).type);
+                        const constraint = simplified !== (<IndexType>target).type ? simplified : getConstraintOfType((<IndexType>target).type);
+                        if (constraint) {
+                            if (result = isRelatedTo(source, getIndexType(constraint, (target as IndexType).stringsOnly), reportErrors)) {
+                                return result;
+                            }
                         }
                     }
                 }