From 7740822d02d8072f6360090ab10a953775fbe1a3 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Tue, 21 Aug 2018 15:45:29 -0700
Subject: [PATCH] Simplify or get constraint in distributive conditional type,
 but not both

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

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5d2b0187eb..8f043d51cd 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6921,7 +6921,8 @@ namespace ts {
             // over the conditional type and possibly reduced. For example, 'T extends undefined ? never : T'
             // removes 'undefined' from T.
             if (type.root.isDistributive) {
-                const constraint = getConstraintOfType(getSimplifiedType(type.checkType));
+                const simplified = getSimplifiedType(type.checkType);
+                const constraint = simplified === type.checkType ? getConstraintOfType(simplified) : simplified;
                 if (constraint) {
                     const mapper = makeUnaryTypeMapper(type.root.checkType, constraint);
                     const instantiated = getConditionalTypeInstantiation(type, combineTypeMappers(mapper, type.mapper));