From 8120094c81236651da967c33d7bd5ca185188faf Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 22 May 2019 06:49:49 -0700
Subject: [PATCH] Simplify index and object types when obtaining indexed access
 constraint

---
 src/compiler/checker.ts | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e9dddff64af39..bd8d4c05901c7 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7657,15 +7657,20 @@ namespace ts {
             return hasNonCircularBaseConstraint(type) ? getConstraintFromIndexedAccess(type) : undefined;
         }
 
+        function getSimplifiedTypeOrConstraint(type: Type) {
+            const simplified = getSimplifiedType(type, /*writing*/ false);
+            return simplified !== type ? simplified : getConstraintOfType(type);
+        }
+
         function getConstraintFromIndexedAccess(type: IndexedAccessType) {
-            const indexConstraint = getConstraintOfType(type.indexType);
+            const indexConstraint = getSimplifiedTypeOrConstraint(type.indexType);
             if (indexConstraint && indexConstraint !== type.indexType) {
                 const indexedAccess = getIndexedAccessTypeOrUndefined(type.objectType, indexConstraint);
                 if (indexedAccess) {
                     return indexedAccess;
                 }
             }
-            const objectConstraint = getConstraintOfType(type.objectType);
+            const objectConstraint = getSimplifiedTypeOrConstraint(type.objectType);
             if (objectConstraint && objectConstraint !== type.objectType) {
                 return getIndexedAccessTypeOrUndefined(objectConstraint, type.indexType);
             }