From bb9c5c96c80126a0ff78e1d3524b10adcfd1bed1 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sat, 11 May 2019 11:15:37 -0700
Subject: [PATCH] Reuse existing type instantiations

---
 src/compiler/checker.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5d3bc864a504f..f016a8d3723fe 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10379,7 +10379,7 @@ namespace ts {
             // Simplifications for types of the form `T extends U ? T : never` and `T extends U ? never : T`.
             if (root.falseType.flags & TypeFlags.Never && getActualTypeVariable(root.trueType) === getActualTypeVariable(root.checkType)) {
                 if (checkType.flags & TypeFlags.Any || isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
-                    return instantiateType(root.trueType, mapper);
+                    return checkType;
                 }
                 else if (isIntersectionEmpty(checkType, extendsType)) { // Always false
                     return neverType;
@@ -10390,7 +10390,7 @@ namespace ts {
                     return neverType;
                 }
                 else if (checkType.flags & TypeFlags.Any || isIntersectionEmpty(checkType, extendsType)) { // Always false
-                    return instantiateType(root.falseType, mapper);
+                    return checkType;
                 }
             }
             const checkTypeInstantiable = maybeTypeOfKind(checkType, TypeFlags.Instantiable | TypeFlags.GenericMappedType);