Fix: Consult referenced project options for synthetic default export eligibility (#63038)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: andrewbranch <[email protected]>
Co-authored-by: Andrew Branch <[email protected]>

Author: Copilot

src/compiler/checker.ts

@@ -3794,6 +3794,21 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 // are ESM, there cannot be a synthetic default.
                 return false;
             }
+            // For other files (not node16/nodenext with impliedNodeFormat), check if we can determine
+            // the module format from project references
+            if (!targetMode && file.isDeclarationFile) {
+                // Try to get the project reference - try both source file mapping and output file mapping
+                // since declaration files can be mapped either way depending on how they're resolved
+                const redirect = host.getRedirectFromSourceFile(file.path) || host.getRedirectFromOutput(file.path);
+                if (redirect) {
+                    // This is a declaration file from a project reference, so we can determine
+                    // its module format from the referenced project's options
+                    const targetModuleKind = host.getEmitModuleFormatOfFile(file);
+                    if (usageMode === ModuleKind.ESNext && ModuleKind.ES2015 <= targetModuleKind && targetModuleKind <= ModuleKind.ESNext) {
+                        return false;
+                    }
+                }
+            }
         }
         if (!allowSyntheticDefaultImports) {
             return false;

src/testRunner/unittests/tsc/projectReferences.ts

@@ -90,6 +90,42 @@ describe("unittests:: tsc:: projectReferences::", () => {
         commandLineArgs: ["--p", "app", "--pretty", "false"],
     });
 
+    verifyTsc({
+        scenario: "projectReferences",
+        subScenario: "referenced project with esnext module disallows synthetic default imports",
+        sys: () =>
+            TestServerHost.createWatchedSystem({
+                "/home/src/workspaces/project/lib/tsconfig.json": jsonToReadableText({
+                    compilerOptions: {
+                        composite: true,
+                        declaration: true,
+                        module: "esnext",
+                        moduleResolution: "bundler",
+                        rootDir: "src",
+                        outDir: "dist",
+                    },
+                    include: ["src"],
+                }),
+                "/home/src/workspaces/project/lib/src/utils.ts": "export const test = () => 'test';",
+                "/home/src/workspaces/project/lib/dist/utils.d.ts": "export declare const test: () => string;",
+                "/home/src/workspaces/project/app/tsconfig.json": jsonToReadableText({
+                    compilerOptions: {
+                        module: "esnext",
+                        moduleResolution: "bundler",
+                    },
+                    references: [
+                        { path: "../lib" },
+                    ],
+                }),
+                "/home/src/workspaces/project/app/index.ts": `
+                    import TestSrc from '../lib/src/utils'; // Error
+                    import TestDecl from '../lib/dist/utils'; // Error
+                    console.log(TestSrc.test());
+                    console.log(TestDecl.test());`,
+            }),
+        commandLineArgs: ["--p", "app", "--pretty", "false"],
+    });
+
     verifyTsc({
         scenario: "projectReferences",
         subScenario: "referencing ambient const enum from referenced project with preserveConstEnums",

tests/baselines/reference/tsc/projectReferences/default-import-interop-uses-referenced-project-settings.js

@@ -86,6 +86,8 @@ declare const console: { log(msg: any): void; };
 Output::
 app/src/index.ts(2,28): error TS2613: Module '"/home/src/workspaces/project/app/src/local"' has no default export. Did you mean to use 'import { local } from "/home/src/workspaces/project/app/src/local"' instead?
 app/src/index.ts(3,28): error TS2613: Module '"/home/src/workspaces/project/node_modules/esm-package/index"' has no default export. Did you mean to use 'import { esm } from "/home/src/workspaces/project/node_modules/esm-package/index"' instead?
+app/src/index.ts(4,28): error TS1192: Module '"/home/src/workspaces/project/lib/dist/a"' has no default export.
+app/src/index.ts(5,28): error TS1192: Module '"/home/src/workspaces/project/lib/dist/a"' has no default export.
 
 
 //// [/home/src/workspaces/project/app/dist/local.js]

tests/baselines/reference/tsc/projectReferences/referenced-project-with-esnext-module-disallows-synthetic-default-imports.js

@@ -0,0 +1,73 @@
+currentDirectory:: /home/src/workspaces/project useCaseSensitiveFileNames:: false
+Input::
+//// [/home/src/workspaces/project/lib/tsconfig.json]
+{
+  "compilerOptions": {
+    "composite": true,
+    "declaration": true,
+    "module": "esnext",
+    "moduleResolution": "bundler",
+    "rootDir": "src",
+    "outDir": "dist"
+  },
+  "include": [
+    "src"
+  ]
+}
+
+//// [/home/src/workspaces/project/lib/src/utils.ts]
+export const test = () => 'test';
+
+//// [/home/src/workspaces/project/lib/dist/utils.d.ts]
+export declare const test: () => string;
+
+//// [/home/src/workspaces/project/app/tsconfig.json]
+{
+  "compilerOptions": {
+    "module": "esnext",
+    "moduleResolution": "bundler"
+  },
+  "references": [
+    {
+      "path": "../lib"
+    }
+  ]
+}
+
+//// [/home/src/workspaces/project/app/index.ts]
+
+                    import TestSrc from '../lib/src/utils'; // Error
+                    import TestDecl from '../lib/dist/utils'; // Error
+                    console.log(TestSrc.test());
+                    console.log(TestDecl.test());
+
+//// [/home/src/tslibs/TS/Lib/lib.d.ts]
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+declare const console: { log(msg: any): void; };
+
+
+/home/src/tslibs/TS/Lib/tsc.js --p app --pretty false
+Output::
+app/index.ts(2,28): error TS1192: Module '"/home/src/workspaces/project/lib/dist/utils"' has no default export.
+app/index.ts(3,28): error TS1192: Module '"/home/src/workspaces/project/lib/dist/utils"' has no default export.
+
+
+//// [/home/src/workspaces/project/app/index.js]
+import TestSrc from '../lib/src/utils'; // Error
+import TestDecl from '../lib/dist/utils'; // Error
+console.log(TestSrc.test());
+console.log(TestDecl.test());
+
+
+
+exitCode:: ExitStatus.DiagnosticsPresent_OutputsGenerated