--isolatedModules: Still allow re-export of type in a declaration file (#16399)

* --isolatedModules: Still allow re-export of type in a declaration file

* Use isInAmbientContext

Author: Andy

src/compiler/checker.ts

@@ -21789,7 +21789,10 @@ namespace ts {
                 }
 
                 // Don't allow to re-export something with no value side when `--isolatedModules` is set.
-                if (node.kind === SyntaxKind.ExportSpecifier && compilerOptions.isolatedModules && !(target.flags & SymbolFlags.Value)) {
+                if (compilerOptions.isolatedModules
+                    && node.kind === SyntaxKind.ExportSpecifier
+                    && !(target.flags & SymbolFlags.Value)
+                    && !isInAmbientContext(node)) {
                     error(node, Diagnostics.Cannot_re_export_a_type_when_the_isolatedModules_flag_is_provided);
                 }
             }

tests/baselines/reference/isolatedModulesReExportType.errors.txt

@@ -35,4 +35,14 @@
     declare type T = number;
     export = T;
 
+==== /node_modules/foo/bar.d.ts (0 errors) ====
+    export type T = number;
+    
+==== /node_modules/foo/index.d.ts (0 errors) ====
+    export { T } from "./bar"; // In a declaration file, so not an error.
+    
+==== /node_modules/baz/index.d.ts (0 errors) ====
+    declare module "baz" {
+        export { T } from "foo"; // Also allowed.
+    }
 

tests/baselines/reference/isolatedModulesReExportType.js

@@ -9,7 +9,17 @@ export class C {}
 //// [exportEqualsT.ts]
 declare type T = number;
 export = T;
-
+
+//// [bar.d.ts]
+export type T = number;
+
+//// [index.d.ts]
+export { T } from "./bar"; // In a declaration file, so not an error.
+
+//// [index.d.ts]
+declare module "baz" {
+    export { T } from "foo"; // Also allowed.
+}
 
 //// [user.ts]
 // Error, can't re-export something that's only a type.

tests/cases/compiler/isolatedModulesReExportType.ts

@@ -10,6 +10,16 @@ export class C {}
 declare type T = number;
 export = T;
 
+// @Filename: /node_modules/foo/bar.d.ts
+export type T = number;
+
+// @Filename: /node_modules/foo/index.d.ts
+export { T } from "./bar"; // In a declaration file, so not an error.
+
+// @Filename: /node_modules/baz/index.d.ts
+declare module "baz" {
+    export { T } from "foo"; // Also allowed.
+}
 
 // @Filename: /user.ts
 // Error, can't re-export something that's only a type.