From 014fc0e1eb3885bccf64b59abd641bc7c09fce78 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 9 Sep 2022 11:29:50 -0700
Subject: [PATCH] Preserve special intersections in mapped types

---
 src/compiler/checker.ts | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6da3c9e760c3f..862fb23fa361e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11780,6 +11780,12 @@ namespace ts {
                 return mapType(type as UnionType, getLowerBoundOfKeyType);
             }
             if (type.flags & TypeFlags.Intersection) {
+                // Similarly to getTypeFromIntersectionTypeNode, we preserve the special string & {}, number & {},
+                // and bigint & {} intersections that are used to prevent subtype reduction in union types.
+                const types = (type as IntersectionType).types;
+                if (types.length === 2 && !!(types[0].flags & (TypeFlags.String | TypeFlags.Number | TypeFlags.BigInt)) && types[1] === emptyTypeLiteralType) {
+                    return type;
+                }
                 return getIntersectionType(sameMap((type as UnionType).types, getLowerBoundOfKeyType));
             }
             return type;