From 74341a01d72a10024eaa62f9343d9abc36dcb1aa Mon Sep 17 00:00:00 2001
From: Orta <git@orta.io>
Date: Tue, 12 Jan 2021 19:24:10 +0000
Subject: [PATCH] A second attempt at readonly array support persisting through
 isArray

---
 lib/lib.es5.d.ts        |  2 +-
 src/compiler/checker.ts | 15 ++++++++++-----
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/lib/lib.es5.d.ts b/lib/lib.es5.d.ts
index 221882e80f6d5..ab6a5d0fb3806 100644
--- a/lib/lib.es5.d.ts
+++ b/lib/lib.es5.d.ts
@@ -1408,7 +1408,7 @@ interface ArrayConstructor {
     (arrayLength?: number): any[];
     <T>(arrayLength: number): T[];
     <T>(...items: T[]): T[];
-    isArray(arg: any): arg is any[];
+    isArray(arg: any): arg is readonly unknown[];
     readonly prototype: any[];
 }
 
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 10f8dc923b88f..fab3f26ea4d40 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -22999,11 +22999,16 @@ namespace ts {
                     }
                 }
 
-                // If the candidate type is a subtype of the target type, narrow to the candidate type,
-                // if the target type is a subtype of the candidate type, narrow to the target type,
-                // otherwise, narrow to an intersection of the two types.
-                return isTypeSubtypeOf(candidate, type) ? candidate : isTypeSubtypeOf(type, candidate) ? type : getIntersectionType([type, candidate]);
-            }
+                 // If the candidate type is a subtype of the target type, narrow to the candidate type.
+                 // Otherwise, if the target type is assignable to the candidate type, keep the target type.
+                 // Otherwise, if the candidate type is assignable to the target type, narrow to the candidate
+                 // type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
+                 // two types.
+                 return isTypeSubtypeOf(candidate, type) ? candidate :
+                     isTypeAssignableTo(type, candidate) ? type :
+                     isTypeAssignableTo(candidate, type) ? candidate :
+                     getIntersectionType([type, candidate]);
+                }
 
             function narrowTypeByCallExpression(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
                 if (hasMatchingArgument(callExpression, reference)) {