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Model Intl.Collator#compare as a readonly bound function property #64344

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Copilot Sep 19, 2026
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Model Intl.Collator#compare as readonly bound function property
Copilot Sep 19, 2026
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2 changes: 1 addition & 1 deletion tsc/internal/bundled/libs/lib.es5.d.ts
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Expand Up @@ -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;
}

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19 changes: 19 additions & 0 deletions tsc/testdata/baselines/reference/compiler/intlCollatorCompare.errors.txt
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@@ -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.

27 changes: 27 additions & 0 deletions tsc/testdata/baselines/reference/compiler/intlCollatorCompare.js
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//// [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;
43 changes: 43 additions & 0 deletions tsc/testdata/baselines/reference/compiler/intlCollatorCompare.symbols
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//// [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))

62 changes: 62 additions & 0 deletions tsc/testdata/baselines/reference/compiler/intlCollatorCompare.types
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//// [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

15 changes: 15 additions & 0 deletions tsc/testdata/tests/cases/compiler/intlCollatorCompare.ts
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// @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;