From 01dff2440d92cb494a2c672f242d2dadc86a3f14 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 19 Sep 2026 03:10:52 +0000
Subject: [PATCH 1/2] Initial plan


From 011b6cf73c2b5df602d0dec93f45f9629403e996 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 19 Sep 2026 03:29:19 +0000
Subject: [PATCH 2/2] Model Intl.Collator#compare as readonly bound function
 property

Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>
---
 tsc/internal/bundled/libs/lib.es5.d.ts        |  2 +-
 .../compiler/intlCollatorCompare.errors.txt   | 19 ++++++
 .../reference/compiler/intlCollatorCompare.js | 27 ++++++++
 .../compiler/intlCollatorCompare.symbols      | 43 +++++++++++++
 .../compiler/intlCollatorCompare.types        | 62 +++++++++++++++++++
 .../cases/compiler/intlCollatorCompare.ts     | 15 +++++
 6 files changed, 167 insertions(+), 1 deletion(-)
 create mode 100644 tsc/testdata/baselines/reference/compiler/intlCollatorCompare.errors.txt
 create mode 100644 tsc/testdata/baselines/reference/compiler/intlCollatorCompare.js
 create mode 100644 tsc/testdata/baselines/reference/compiler/intlCollatorCompare.symbols
 create mode 100644 tsc/testdata/baselines/reference/compiler/intlCollatorCompare.types
 create mode 100644 tsc/testdata/tests/cases/compiler/intlCollatorCompare.ts

diff --git a/tsc/internal/bundled/libs/lib.es5.d.ts b/tsc/internal/bundled/libs/lib.es5.d.ts
index e394111cb0e6c..ec4d8eaaa25e6 100644
--- a/tsc/internal/bundled/libs/lib.es5.d.ts
+++ b/tsc/internal/bundled/libs/lib.es5.d.ts
@@ -4435,7 +4435,7 @@ declare namespace Intl {
     }
 
     interface Collator {
-        compare(x: string, y: string): number;
+        readonly compare: (this: void, x: string, y: string) => number;
         resolvedOptions(): ResolvedCollatorOptions;
     }
 
diff --git a/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.errors.txt b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.errors.txt
new file mode 100644
index 0000000000000..9ab6ef7477587
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.errors.txt
@@ -0,0 +1,19 @@
+intlCollatorCompare.ts(12,10): error TS2540: Cannot assign to 'compare' because it is a read-only property.
+
+
+==== intlCollatorCompare.ts (1 errors) ====
+    const collator = new Intl.Collator("en");
+    
+    const compare = collator.compare;
+    compare("a", "b");
+    
+    ["b", "a"].sort(collator.compare);
+    ["b", "a"].sort(compare);
+    
+    collator.compare("a", "b");
+    
+    // error: compare is readonly
+    collator.compare = (x: string, y: string) => 0;
+             ~~~~~~~
+!!! error TS2540: Cannot assign to 'compare' because it is a read-only property.
+    
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.js b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.js
new file mode 100644
index 0000000000000..de353420f6337
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.js
@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/intlCollatorCompare.ts] ////
+
+//// [intlCollatorCompare.ts]
+const collator = new Intl.Collator("en");
+
+const compare = collator.compare;
+compare("a", "b");
+
+["b", "a"].sort(collator.compare);
+["b", "a"].sort(compare);
+
+collator.compare("a", "b");
+
+// error: compare is readonly
+collator.compare = (x: string, y: string) => 0;
+
+
+//// [intlCollatorCompare.js]
+"use strict";
+const collator = new Intl.Collator("en");
+const compare = collator.compare;
+compare("a", "b");
+["b", "a"].sort(collator.compare);
+["b", "a"].sort(compare);
+collator.compare("a", "b");
+// error: compare is readonly
+collator.compare = (x, y) => 0;
diff --git a/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.symbols b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.symbols
new file mode 100644
index 0000000000000..73f7719e09880
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.symbols
@@ -0,0 +1,43 @@
+//// [tests/cases/compiler/intlCollatorCompare.ts] ////
+
+=== intlCollatorCompare.ts ===
+const collator = new Intl.Collator("en");
+>collator : Symbol(collator, Decl(intlCollatorCompare.ts, 0, 5))
+>Intl.Collator : Symbol(Intl.Collator, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Intl : Symbol(Intl, Decl(lib.es5.d.ts, --, --))
+>Collator : Symbol(Intl.Collator, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+const compare = collator.compare;
+>compare : Symbol(compare, Decl(intlCollatorCompare.ts, 2, 5))
+>collator.compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+>collator : Symbol(collator, Decl(intlCollatorCompare.ts, 0, 5))
+>compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+
+compare("a", "b");
+>compare : Symbol(compare, Decl(intlCollatorCompare.ts, 2, 5))
+
+["b", "a"].sort(collator.compare);
+>["b", "a"].sort : Symbol(Array.sort, Decl(lib.es5.d.ts, --, --))
+>sort : Symbol(Array.sort, Decl(lib.es5.d.ts, --, --))
+>collator.compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+>collator : Symbol(collator, Decl(intlCollatorCompare.ts, 0, 5))
+>compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+
+["b", "a"].sort(compare);
+>["b", "a"].sort : Symbol(Array.sort, Decl(lib.es5.d.ts, --, --))
+>sort : Symbol(Array.sort, Decl(lib.es5.d.ts, --, --))
+>compare : Symbol(compare, Decl(intlCollatorCompare.ts, 2, 5))
+
+collator.compare("a", "b");
+>collator.compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+>collator : Symbol(collator, Decl(intlCollatorCompare.ts, 0, 5))
+>compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+
+// error: compare is readonly
+collator.compare = (x: string, y: string) => 0;
+>collator.compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+>collator : Symbol(collator, Decl(intlCollatorCompare.ts, 0, 5))
+>compare : Symbol(Intl.Collator.compare, Decl(lib.es5.d.ts, --, --))
+>x : Symbol(x, Decl(intlCollatorCompare.ts, 11, 20))
+>y : Symbol(y, Decl(intlCollatorCompare.ts, 11, 30))
+
diff --git a/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.types b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.types
new file mode 100644
index 0000000000000..a6a3228a0dbb5
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/intlCollatorCompare.types
@@ -0,0 +1,62 @@
+//// [tests/cases/compiler/intlCollatorCompare.ts] ////
+
+=== intlCollatorCompare.ts ===
+const collator = new Intl.Collator("en");
+>collator : Intl.Collator
+>new Intl.Collator("en") : Intl.Collator
+>Intl.Collator : Intl.CollatorConstructor
+>Intl : typeof Intl
+>Collator : Intl.CollatorConstructor
+>"en" : "en"
+
+const compare = collator.compare;
+>compare : (this: void, x: string, y: string) => number
+>collator.compare : (this: void, x: string, y: string) => number
+>collator : Intl.Collator
+>compare : (this: void, x: string, y: string) => number
+
+compare("a", "b");
+>compare("a", "b") : number
+>compare : (this: void, x: string, y: string) => number
+>"a" : "a"
+>"b" : "b"
+
+["b", "a"].sort(collator.compare);
+>["b", "a"].sort(collator.compare) : string[]
+>["b", "a"].sort : (compareFn?: ((a: string, b: string) => number) | undefined) => string[]
+>["b", "a"] : string[]
+>"b" : "b"
+>"a" : "a"
+>sort : (compareFn?: ((a: string, b: string) => number) | undefined) => string[]
+>collator.compare : (this: void, x: string, y: string) => number
+>collator : Intl.Collator
+>compare : (this: void, x: string, y: string) => number
+
+["b", "a"].sort(compare);
+>["b", "a"].sort(compare) : string[]
+>["b", "a"].sort : (compareFn?: ((a: string, b: string) => number) | undefined) => string[]
+>["b", "a"] : string[]
+>"b" : "b"
+>"a" : "a"
+>sort : (compareFn?: ((a: string, b: string) => number) | undefined) => string[]
+>compare : (this: void, x: string, y: string) => number
+
+collator.compare("a", "b");
+>collator.compare("a", "b") : number
+>collator.compare : (this: void, x: string, y: string) => number
+>collator : Intl.Collator
+>compare : (this: void, x: string, y: string) => number
+>"a" : "a"
+>"b" : "b"
+
+// error: compare is readonly
+collator.compare = (x: string, y: string) => 0;
+>collator.compare = (x: string, y: string) => 0 : (x: string, y: string) => number
+>collator.compare : any
+>collator : Intl.Collator
+>compare : any
+>(x: string, y: string) => 0 : (x: string, y: string) => number
+>x : string
+>y : string
+>0 : 0
+
diff --git a/tsc/testdata/tests/cases/compiler/intlCollatorCompare.ts b/tsc/testdata/tests/cases/compiler/intlCollatorCompare.ts
new file mode 100644
index 0000000000000..77679b1f25802
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/intlCollatorCompare.ts
@@ -0,0 +1,15 @@
+// @strict: true
+// @target: es2015
+
+const collator = new Intl.Collator("en");
+
+const compare = collator.compare;
+compare("a", "b");
+
+["b", "a"].sort(collator.compare);
+["b", "a"].sort(compare);
+
+collator.compare("a", "b");
+
+// error: compare is readonly
+collator.compare = (x: string, y: string) => 0;