Fix declaration emit for JS default re-exports that resolve to modules through synthesized default exports (#56340)

src/compiler/checker.ts

@@ -9744,6 +9744,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         // does not use localName because the symbol name in this case refers to the name in the exports table,
                         // which we must exactly preserve
                         const specifier = (node.parent.parent as ExportDeclaration).moduleSpecifier;
+                        if (specifier && (node as ExportSpecifier).propertyName?.escapedText === InternalSymbolName.Default) {
+                            verbatimTargetName = InternalSymbolName.Default;
+                        }
                         // targetName is only used when the target is local, as otherwise the target is an alias that points at
                         // another file
                         serializeExportSpecifier(

tests/baselines/reference/declarationEmitJsReExportDefault.js

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/declarationEmitJsReExportDefault.ts] ////
+
+//// [package.json]
+{"name": "a", "version": "0.0.0"}
+//// [index.d.ts]
+export const a = 123;
+
+//// [index.mjs]
+export {default as mod} from 'a';
+
+
+
+
+//// [index.d.mts]
+export { default as mod } from "a";

tests/cases/compiler/declarationEmitJsReExportDefault.ts

@@ -0,0 +1,18 @@
+// @noTypesAndSymbols: true
+
+// @filename: /node_modules/a/package.json
+{"name": "a", "version": "0.0.0"}
+// @filename: /node_modules/a/index.d.ts
+export const a = 123;
+
+// @filename: /tsconfig.json
+{
+  "compilerOptions": {
+    "module": "node16",
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "checkJs": true,
+  }
+}
+// @filename: /index.mjs
+export {default as mod} from 'a';