From cc5d0f2de88791737f3312f6c2dfee5d804ceaa0 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Tue, 29 Dec 2020 08:51:55 -1000
Subject: [PATCH] Preserve aliases during relationship checks

---
 src/compiler/checker.ts | 9 +--------
 src/compiler/types.ts   | 2 --
 2 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5e91d1cc96e..135ee4775f2 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11445,13 +11445,6 @@ namespace ts {
             return reduced;
         }
 
-        function getUnaliasedUnionType(type: Type) {
-            if (type.flags & TypeFlags.Union && !(type.flags & TypeFlags.EnumLiteral) && ((<UnionType>type).aliasSymbol || (<UnionType>type).origin)) {
-                return (<UnionType>type).unaliasedType || ((<UnionType>type).unaliasedType = getUnionTypeFromSortedList((<UnionType>type).types, (<UnionType>type).objectFlags));
-            }
-            return type;
-        }
-
         function isNeverReducedProperty(prop: Symbol) {
             return isDiscriminantWithNeverType(prop) || isConflictingPrivateProperty(prop);
         }
@@ -16599,7 +16592,7 @@ namespace ts {
             while (true) {
                 const t = isFreshLiteralType(type) ? (<FreshableType>type).regularType :
                     getObjectFlags(type) & ObjectFlags.Reference && (<TypeReference>type).node ? createTypeReference((<TypeReference>type).target, getTypeArguments(<TypeReference>type)) :
-                    type.flags & TypeFlags.UnionOrIntersection ? getUnaliasedUnionType(getReducedType(type)) :
+                    type.flags & TypeFlags.UnionOrIntersection ? getReducedType(type) :
                     type.flags & TypeFlags.Substitution ? writing ? (<SubstitutionType>type).baseType : (<SubstitutionType>type).substitute :
                     type.flags & TypeFlags.Simplifiable ? getSimplifiedType(type, writing) :
                     type;
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index dab3f115372..f4ffd616899 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -5296,8 +5296,6 @@ namespace ts {
         /* @internal */
         regularType?: UnionType;
         /* @internal */
-        unaliasedType?: Type;
-        /* @internal */
         origin?: UnionOrigin;
     }