From b7439c1baf62f9ce18cde8ae8ec455f64b29ec15 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 19 Jun 2025 08:08:58 -0700
Subject: [PATCH 1/4] Use strict subtype relation in getCommonSupertype

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

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 76e81a959790b..a327c0ed49962 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -25340,7 +25340,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // right is a supertype.
         const superTypeOrUnion = literalTypesWithSameBaseType(primaryTypes) ?
             getUnionType(primaryTypes) :
-            reduceLeft(primaryTypes, (s, t) => isTypeSubtypeOf(s, t) ? t : s)!;
+            reduceLeft(primaryTypes, (s, t) => isTypeStrictSubtypeOf(s, t) ? t : s)!;
         // Add any nullable types that occurred in the candidates back to the result.
         return primaryTypes === types ? superTypeOrUnion : getNullableType(superTypeOrUnion, getCombinedTypeFlags(types) & TypeFlags.Nullable);
     }

From a6d3453117662e1eac62f5975f25fb2169f17595 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 20 Jun 2025 15:25:02 -0400
Subject: [PATCH 2/4] Find single strict supertype, otherwise use assignability
 relation

---
 src/compiler/checker.ts | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a327c0ed49962..77974932889cd 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -25340,11 +25340,18 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // right is a supertype.
         const superTypeOrUnion = literalTypesWithSameBaseType(primaryTypes) ?
             getUnionType(primaryTypes) :
-            reduceLeft(primaryTypes, (s, t) => isTypeStrictSubtypeOf(s, t) ? t : s)!;
+            getSingleCommonSupertype(primaryTypes)
         // Add any nullable types that occurred in the candidates back to the result.
         return primaryTypes === types ? superTypeOrUnion : getNullableType(superTypeOrUnion, getCombinedTypeFlags(types) & TypeFlags.Nullable);
     }
 
+    function getSingleCommonSupertype(types: Type[]) {
+        const candidate = reduceLeft(types, (s, t) => isTypeStrictSubtypeOf(s, t) ? t : s)!;
+        return every(types, t => t === candidate || isTypeStrictSubtypeOf(t, candidate)) ?
+            candidate :
+            reduceLeft(types, (s, t) => isTypeAssignableTo(s, t) ? t : s)!;
+    }
+
     // Return the leftmost type for which no type to the right is a subtype.
     function getCommonSubtype(types: Type[]) {
         return reduceLeft(types, (s, t) => isTypeSubtypeOf(t, s) ? t : s)!;

From 544f4d7b39d936cb57297c6149802205426a0bdf Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 20 Jun 2025 16:49:02 -0400
Subject: [PATCH 3/4] Try to find single strict supertype, otherwise do
 previous behavior

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

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 77974932889cd..9dc463d3cc9f1 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -25349,7 +25349,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const candidate = reduceLeft(types, (s, t) => isTypeStrictSubtypeOf(s, t) ? t : s)!;
         return every(types, t => t === candidate || isTypeStrictSubtypeOf(t, candidate)) ?
             candidate :
-            reduceLeft(types, (s, t) => isTypeAssignableTo(s, t) ? t : s)!;
+            reduceLeft(types, (s, t) => isTypeSubtypeOf(s, t) ? t : s)!;
     }
 
     // Return the leftmost type for which no type to the right is a subtype.

From cb554a834ee17236acb2d34f545b14ab43b68f8e Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 20 Jun 2025 19:19:18 -0400
Subject: [PATCH 4/4] Fix lint issue and add comment

---
 src/compiler/checker.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9dc463d3cc9f1..ba8ec48a39eff 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -25340,12 +25340,15 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // right is a supertype.
         const superTypeOrUnion = literalTypesWithSameBaseType(primaryTypes) ?
             getUnionType(primaryTypes) :
-            getSingleCommonSupertype(primaryTypes)
+            getSingleCommonSupertype(primaryTypes);
         // Add any nullable types that occurred in the candidates back to the result.
         return primaryTypes === types ? superTypeOrUnion : getNullableType(superTypeOrUnion, getCombinedTypeFlags(types) & TypeFlags.Nullable);
     }
 
     function getSingleCommonSupertype(types: Type[]) {
+        // First, find the leftmost type for which no type to the right is a strict supertype, and if that
+        // type is a strict supertype of all other candidates, return it. Otherwise, return the leftmost type
+        // for which no type to the right is a (regular) supertype.
         const candidate = reduceLeft(types, (s, t) => isTypeStrictSubtypeOf(s, t) ? t : s)!;
         return every(types, t => t === candidate || isTypeStrictSubtypeOf(t, candidate)) ?
             candidate :