Fix BigInt literal error in ambient contexts when targeting < ES2020 (#61935)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: RyanCavanaugh <[email protected]>

Author: Copilot

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;
                 }

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.

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;

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))
+

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
+>     : ^^^^
+

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;