From 1ed8674bba538a280c1846d628069fb9646731ab Mon Sep 17 00:00:00 2001
From: Copilot <198982749+Copilot@users.noreply.github.com>
Date: Wed, 25 Jun 2025 14:29:02 -0700
Subject: [PATCH 1/3] Allow `accessor` properties in ambient contexts when
 targeting ES5 (#61927)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>
---
 src/compiler/checker.ts                       |  2 +-
 .../accessorInAmbientContextES5.errors.txt    | 32 ++++++++
 .../reference/accessorInAmbientContextES5.js  | 81 +++++++++++++++++++
 .../accessorInAmbientContextES5.symbols       | 51 ++++++++++++
 .../accessorInAmbientContextES5.types         | 64 +++++++++++++++
 .../compiler/accessorInAmbientContextES5.ts   | 29 +++++++
 6 files changed, 258 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/accessorInAmbientContextES5.errors.txt
 create mode 100644 tests/baselines/reference/accessorInAmbientContextES5.js
 create mode 100644 tests/baselines/reference/accessorInAmbientContextES5.symbols
 create mode 100644 tests/baselines/reference/accessorInAmbientContextES5.types
 create mode 100644 tests/cases/compiler/accessorInAmbientContextES5.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index ba8ec48a39e..5614da35039 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -53039,7 +53039,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (languageVersion < ScriptTarget.ES2015 && isPrivateIdentifier(node.name)) {
                 return grammarErrorOnNode(node.name, Diagnostics.Private_identifiers_are_only_available_when_targeting_ECMAScript_2015_and_higher);
             }
-            if (languageVersion < ScriptTarget.ES2015 && isAutoAccessorPropertyDeclaration(node)) {
+            if (languageVersion < ScriptTarget.ES2015 && isAutoAccessorPropertyDeclaration(node) && !(node.flags & NodeFlags.Ambient)) {
                 return grammarErrorOnNode(node.name, Diagnostics.Properties_with_the_accessor_modifier_are_only_available_when_targeting_ECMAScript_2015_and_higher);
             }
             if (isAutoAccessorPropertyDeclaration(node) && checkGrammarForInvalidQuestionMark(node.questionToken, Diagnostics.An_accessor_property_cannot_be_declared_optional)) {
diff --git a/tests/baselines/reference/accessorInAmbientContextES5.errors.txt b/tests/baselines/reference/accessorInAmbientContextES5.errors.txt
new file mode 100644
index 00000000000..c9351f69f72
--- /dev/null
+++ b/tests/baselines/reference/accessorInAmbientContextES5.errors.txt
@@ -0,0 +1,32 @@
+accessorInAmbientContextES5.ts(25,14): error TS18045: Properties with the 'accessor' modifier are only available when targeting ECMAScript 2015 and higher.
+
+
+==== accessorInAmbientContextES5.ts (1 errors) ====
+    // Should allow accessor in ambient contexts even when targeting ES5
+    
+    declare class AmbientClass {
+        accessor prop1: string;
+        static accessor prop2: number;
+        private accessor prop3: boolean;
+        private static accessor prop4: symbol;
+    }
+    
+    declare namespace AmbientNamespace {
+        class C {
+            accessor prop: string;
+        }
+    }
+    
+    // Should also work in .d.ts files (simulated with declare)
+    declare module "some-module" {
+        export class ExportedClass {
+            accessor value: any;
+        }
+    }
+    
+    // Regular class should still error when targeting ES5
+    class RegularClass {
+        accessor shouldError: string; // Should still error
+                 ~~~~~~~~~~~
+!!! error TS18045: Properties with the 'accessor' modifier are only available when targeting ECMAScript 2015 and higher.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/accessorInAmbientContextES5.js b/tests/baselines/reference/accessorInAmbientContextES5.js
new file mode 100644
index 00000000000..caea4933da8
--- /dev/null
+++ b/tests/baselines/reference/accessorInAmbientContextES5.js
@@ -0,0 +1,81 @@
+//// [tests/cases/compiler/accessorInAmbientContextES5.ts] ////
+
+//// [accessorInAmbientContextES5.ts]
+// Should allow accessor in ambient contexts even when targeting ES5
+
+declare class AmbientClass {
+    accessor prop1: string;
+    static accessor prop2: number;
+    private accessor prop3: boolean;
+    private static accessor prop4: symbol;
+}
+
+declare namespace AmbientNamespace {
+    class C {
+        accessor prop: string;
+    }
+}
+
+// Should also work in .d.ts files (simulated with declare)
+declare module "some-module" {
+    export class ExportedClass {
+        accessor value: any;
+    }
+}
+
+// Regular class should still error when targeting ES5
+class RegularClass {
+    accessor shouldError: string; // Should still error
+}
+
+//// [accessorInAmbientContextES5.js]
+// Should allow accessor in ambient contexts even when targeting ES5
+var __classPrivateFieldGet = (this && this.__classPrivateFieldGet) || function (receiver, state, kind, f) {
+    if (kind === "a" && !f) throw new TypeError("Private accessor was defined without a getter");
+    if (typeof state === "function" ? receiver !== state || !f : !state.has(receiver)) throw new TypeError("Cannot read private member from an object whose class did not declare it");
+    return kind === "m" ? f : kind === "a" ? f.call(receiver) : f ? f.value : state.get(receiver);
+};
+var __classPrivateFieldSet = (this && this.__classPrivateFieldSet) || function (receiver, state, value, kind, f) {
+    if (kind === "m") throw new TypeError("Private method is not writable");
+    if (kind === "a" && !f) throw new TypeError("Private accessor was defined without a setter");
+    if (typeof state === "function" ? receiver !== state || !f : !state.has(receiver)) throw new TypeError("Cannot write private member to an object whose class did not declare it");
+    return (kind === "a" ? f.call(receiver, value) : f ? f.value = value : state.set(receiver, value)), value;
+};
+var _RegularClass_shouldError_accessor_storage;
+// Regular class should still error when targeting ES5
+var RegularClass = /** @class */ (function () {
+    function RegularClass() {
+        _RegularClass_shouldError_accessor_storage.set(this, void 0);
+    }
+    Object.defineProperty(RegularClass.prototype, "shouldError", {
+        get: function () { return __classPrivateFieldGet(this, _RegularClass_shouldError_accessor_storage, "f"); } // Should still error
+        ,
+        set: function (value) { __classPrivateFieldSet(this, _RegularClass_shouldError_accessor_storage, value, "f"); },
+        enumerable: false,
+        configurable: true
+    });
+    return RegularClass;
+}());
+_RegularClass_shouldError_accessor_storage = new WeakMap();
+
+
+//// [accessorInAmbientContextES5.d.ts]
+declare class AmbientClass {
+    accessor prop1: string;
+    static accessor prop2: number;
+    private accessor prop3;
+    private static accessor prop4;
+}
+declare namespace AmbientNamespace {
+    class C {
+        accessor prop: string;
+    }
+}
+declare module "some-module" {
+    class ExportedClass {
+        accessor value: any;
+    }
+}
+declare class RegularClass {
+    accessor shouldError: string;
+}
diff --git a/tests/baselines/reference/accessorInAmbientContextES5.symbols b/tests/baselines/reference/accessorInAmbientContextES5.symbols
new file mode 100644
index 00000000000..c37f91917d2
--- /dev/null
+++ b/tests/baselines/reference/accessorInAmbientContextES5.symbols
@@ -0,0 +1,51 @@
+//// [tests/cases/compiler/accessorInAmbientContextES5.ts] ////
+
+=== accessorInAmbientContextES5.ts ===
+// Should allow accessor in ambient contexts even when targeting ES5
+
+declare class AmbientClass {
+>AmbientClass : Symbol(AmbientClass, Decl(accessorInAmbientContextES5.ts, 0, 0))
+
+    accessor prop1: string;
+>prop1 : Symbol(AmbientClass.prop1, Decl(accessorInAmbientContextES5.ts, 2, 28))
+
+    static accessor prop2: number;
+>prop2 : Symbol(AmbientClass.prop2, Decl(accessorInAmbientContextES5.ts, 3, 27))
+
+    private accessor prop3: boolean;
+>prop3 : Symbol(AmbientClass.prop3, Decl(accessorInAmbientContextES5.ts, 4, 34))
+
+    private static accessor prop4: symbol;
+>prop4 : Symbol(AmbientClass.prop4, Decl(accessorInAmbientContextES5.ts, 5, 36))
+}
+
+declare namespace AmbientNamespace {
+>AmbientNamespace : Symbol(AmbientNamespace, Decl(accessorInAmbientContextES5.ts, 7, 1))
+
+    class C {
+>C : Symbol(C, Decl(accessorInAmbientContextES5.ts, 9, 36))
+
+        accessor prop: string;
+>prop : Symbol(C.prop, Decl(accessorInAmbientContextES5.ts, 10, 13))
+    }
+}
+
+// Should also work in .d.ts files (simulated with declare)
+declare module "some-module" {
+>"some-module" : Symbol("some-module", Decl(accessorInAmbientContextES5.ts, 13, 1))
+
+    export class ExportedClass {
+>ExportedClass : Symbol(ExportedClass, Decl(accessorInAmbientContextES5.ts, 16, 30))
+
+        accessor value: any;
+>value : Symbol(ExportedClass.value, Decl(accessorInAmbientContextES5.ts, 17, 32))
+    }
+}
+
+// Regular class should still error when targeting ES5
+class RegularClass {
+>RegularClass : Symbol(RegularClass, Decl(accessorInAmbientContextES5.ts, 20, 1))
+
+    accessor shouldError: string; // Should still error
+>shouldError : Symbol(RegularClass.shouldError, Decl(accessorInAmbientContextES5.ts, 23, 20))
+}
diff --git a/tests/baselines/reference/accessorInAmbientContextES5.types b/tests/baselines/reference/accessorInAmbientContextES5.types
new file mode 100644
index 00000000000..eefedf0cf13
--- /dev/null
+++ b/tests/baselines/reference/accessorInAmbientContextES5.types
@@ -0,0 +1,64 @@
+//// [tests/cases/compiler/accessorInAmbientContextES5.ts] ////
+
+=== accessorInAmbientContextES5.ts ===
+// Should allow accessor in ambient contexts even when targeting ES5
+
+declare class AmbientClass {
+>AmbientClass : AmbientClass
+>             : ^^^^^^^^^^^^
+
+    accessor prop1: string;
+>prop1 : string
+>      : ^^^^^^
+
+    static accessor prop2: number;
+>prop2 : number
+>      : ^^^^^^
+
+    private accessor prop3: boolean;
+>prop3 : boolean
+>      : ^^^^^^^
+
+    private static accessor prop4: symbol;
+>prop4 : symbol
+>      : ^^^^^^
+}
+
+declare namespace AmbientNamespace {
+>AmbientNamespace : typeof AmbientNamespace
+>                 : ^^^^^^^^^^^^^^^^^^^^^^^
+
+    class C {
+>C : C
+>  : ^
+
+        accessor prop: string;
+>prop : string
+>     : ^^^^^^
+    }
+}
+
+// Should also work in .d.ts files (simulated with declare)
+declare module "some-module" {
+>"some-module" : typeof import("some-module")
+>              : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    export class ExportedClass {
+>ExportedClass : ExportedClass
+>              : ^^^^^^^^^^^^^
+
+        accessor value: any;
+>value : any
+>      : ^^^
+    }
+}
+
+// Regular class should still error when targeting ES5
+class RegularClass {
+>RegularClass : RegularClass
+>             : ^^^^^^^^^^^^
+
+    accessor shouldError: string; // Should still error
+>shouldError : string
+>            : ^^^^^^
+}
diff --git a/tests/cases/compiler/accessorInAmbientContextES5.ts b/tests/cases/compiler/accessorInAmbientContextES5.ts
new file mode 100644
index 00000000000..42fb79e9774
--- /dev/null
+++ b/tests/cases/compiler/accessorInAmbientContextES5.ts
@@ -0,0 +1,29 @@
+// @target: es5
+// @declaration: true
+
+// Should allow accessor in ambient contexts even when targeting ES5
+
+declare class AmbientClass {
+    accessor prop1: string;
+    static accessor prop2: number;
+    private accessor prop3: boolean;
+    private static accessor prop4: symbol;
+}
+
+declare namespace AmbientNamespace {
+    class C {
+        accessor prop: string;
+    }
+}
+
+// Should also work in .d.ts files (simulated with declare)
+declare module "some-module" {
+    export class ExportedClass {
+        accessor value: any;
+    }
+}
+
+// Regular class should still error when targeting ES5
+class RegularClass {
+    accessor shouldError: string; // Should still error
+}
\ No newline at end of file

From 6e519c59c13e4e99da0bf779a1f2bbee232cf7a1 Mon Sep 17 00:00:00 2001
From: Copilot <198982749+Copilot@users.noreply.github.com>
Date: Wed, 25 Jun 2025 14:29:50 -0700
Subject: [PATCH 2/3] Fix BigInt literal error in ambient contexts when
 targeting < ES2020 (#61935)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>
---
 src/compiler/checker.ts                       |  3 +-
 .../reference/bigintAmbientMinimal.errors.txt | 17 +++++++++
 .../reference/bigintAmbientMinimal.js         | 18 ++++++++++
 .../reference/bigintAmbientMinimal.symbols    | 22 ++++++++++++
 .../reference/bigintAmbientMinimal.types      | 35 +++++++++++++++++++
 tests/cases/compiler/bigintAmbientMinimal.ts  | 14 ++++++++
 6 files changed, 108 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/bigintAmbientMinimal.errors.txt
 create mode 100644 tests/baselines/reference/bigintAmbientMinimal.js
 create mode 100644 tests/baselines/reference/bigintAmbientMinimal.symbols
 create mode 100644 tests/baselines/reference/bigintAmbientMinimal.types
 create mode 100644 tests/cases/compiler/bigintAmbientMinimal.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5614da35039..e66a209af81 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -53188,7 +53188,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const literalType = isLiteralTypeNode(node.parent) ||
             isPrefixUnaryExpression(node.parent) && isLiteralTypeNode(node.parent.parent);
         if (!literalType) {
-            if (languageVersion < ScriptTarget.ES2020) {
+            // Don't error on BigInt literals in ambient contexts
+            if (!(node.flags & NodeFlags.Ambient) && languageVersion < ScriptTarget.ES2020) {
                 if (grammarErrorOnNode(node, Diagnostics.BigInt_literals_are_not_available_when_targeting_lower_than_ES2020)) {
                     return true;
                 }
diff --git a/tests/baselines/reference/bigintAmbientMinimal.errors.txt b/tests/baselines/reference/bigintAmbientMinimal.errors.txt
new file mode 100644
index 00000000000..e95b511da01
--- /dev/null
+++ b/tests/baselines/reference/bigintAmbientMinimal.errors.txt
@@ -0,0 +1,17 @@
+/main.ts(5,17): error TS2737: BigInt literals are not available when targeting lower than ES2020.
+
+
+==== /ambient.d.ts (0 errors) ====
+    declare const fromDts = 789n;
+    declare namespace Lib {
+        const value = 999n;
+    }
+    
+==== /main.ts (1 errors) ====
+    // Minimal repro from issue
+    declare const n = 123n;
+    
+    // Non-ambient for comparison
+    const regular = 456n;
+                    ~~~~
+!!! error TS2737: BigInt literals are not available when targeting lower than ES2020.
\ No newline at end of file
diff --git a/tests/baselines/reference/bigintAmbientMinimal.js b/tests/baselines/reference/bigintAmbientMinimal.js
new file mode 100644
index 00000000000..d2147fb7d30
--- /dev/null
+++ b/tests/baselines/reference/bigintAmbientMinimal.js
@@ -0,0 +1,18 @@
+//// [tests/cases/compiler/bigintAmbientMinimal.ts] ////
+
+//// [ambient.d.ts]
+declare const fromDts = 789n;
+declare namespace Lib {
+    const value = 999n;
+}
+
+//// [main.ts]
+// Minimal repro from issue
+declare const n = 123n;
+
+// Non-ambient for comparison
+const regular = 456n;
+
+//// [main.js]
+// Non-ambient for comparison
+var regular = 456n;
diff --git a/tests/baselines/reference/bigintAmbientMinimal.symbols b/tests/baselines/reference/bigintAmbientMinimal.symbols
new file mode 100644
index 00000000000..d438ba12359
--- /dev/null
+++ b/tests/baselines/reference/bigintAmbientMinimal.symbols
@@ -0,0 +1,22 @@
+//// [tests/cases/compiler/bigintAmbientMinimal.ts] ////
+
+=== /ambient.d.ts ===
+declare const fromDts = 789n;
+>fromDts : Symbol(fromDts, Decl(ambient.d.ts, 0, 13))
+
+declare namespace Lib {
+>Lib : Symbol(Lib, Decl(ambient.d.ts, 0, 29))
+
+    const value = 999n;
+>value : Symbol(value, Decl(ambient.d.ts, 2, 9))
+}
+
+=== /main.ts ===
+// Minimal repro from issue
+declare const n = 123n;
+>n : Symbol(n, Decl(main.ts, 1, 13))
+
+// Non-ambient for comparison
+const regular = 456n;
+>regular : Symbol(regular, Decl(main.ts, 4, 5))
+
diff --git a/tests/baselines/reference/bigintAmbientMinimal.types b/tests/baselines/reference/bigintAmbientMinimal.types
new file mode 100644
index 00000000000..086ac1d5676
--- /dev/null
+++ b/tests/baselines/reference/bigintAmbientMinimal.types
@@ -0,0 +1,35 @@
+//// [tests/cases/compiler/bigintAmbientMinimal.ts] ////
+
+=== /ambient.d.ts ===
+declare const fromDts = 789n;
+>fromDts : 789n
+>        : ^^^^
+>789n : 789n
+>     : ^^^^
+
+declare namespace Lib {
+>Lib : typeof Lib
+>    : ^^^^^^^^^^
+
+    const value = 999n;
+>value : 999n
+>      : ^^^^
+>999n : 999n
+>     : ^^^^
+}
+
+=== /main.ts ===
+// Minimal repro from issue
+declare const n = 123n;
+>n : 123n
+>  : ^^^^
+>123n : 123n
+>     : ^^^^
+
+// Non-ambient for comparison
+const regular = 456n;
+>regular : 456n
+>        : ^^^^
+>456n : 456n
+>     : ^^^^
+
diff --git a/tests/cases/compiler/bigintAmbientMinimal.ts b/tests/cases/compiler/bigintAmbientMinimal.ts
new file mode 100644
index 00000000000..4bad00279bc
--- /dev/null
+++ b/tests/cases/compiler/bigintAmbientMinimal.ts
@@ -0,0 +1,14 @@
+// @target: ES5
+
+// @Filename: /ambient.d.ts
+declare const fromDts = 789n;
+declare namespace Lib {
+    const value = 999n;
+}
+
+// @Filename: /main.ts
+// Minimal repro from issue
+declare const n = 123n;
+
+// Non-ambient for comparison
+const regular = 456n;
\ No newline at end of file

From f3a6d3165ff91289bf8558ce981042d2fff22a36 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Wed, 25 Jun 2025 14:35:09 -0700
Subject: [PATCH 3/3] Update copilot-setup-steps.yml (#61948)

---
 .github/workflows/copilot-setup-steps.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/copilot-setup-steps.yml b/.github/workflows/copilot-setup-steps.yml
index 083512529cd..8fc19fa4368 100644
--- a/.github/workflows/copilot-setup-steps.yml
+++ b/.github/workflows/copilot-setup-steps.yml
@@ -19,4 +19,4 @@ jobs:
       - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
       - run: npm ci
       # pull dprint caches before network access is blocked
-      - run: npx hereby check:format || true
+      - run: npx hereby check-format || true