Fix: Consult referenced project options for synthetic default export eligibility

src/compiler/checker.ts

@@ -3812,6 +3812,24 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 // it definitely is a module and does not have a synthetic default
                 return false;
             }
+            // If this is a declaration file from a project reference, check the referenced project's options
+            // to determine if the module format is ESM
+            if (file) {
+                const redirect = host.getRedirectFromSourceFile(file.path);
+                if (redirect?.resolvedRef) {
+                    const referencedOptions = redirect.resolvedRef.commandLine.options;
+                    // If the referenced project has allowSyntheticDefaultImports disabled, respect that
+                    if (!getAllowSyntheticDefaultImports(referencedOptions)) {
+                        return false;
+                    }
+                    // If the referenced project's module format is ESM (ES2015 or later),
+                    // it cannot have a synthetic default
+                    const referencedModuleKind = referencedOptions.module ?? ModuleKind.CommonJS;
+                    if (referencedModuleKind >= ModuleKind.ES2015) {
+                        return false;
+                    }
+                }
+            }
             // There are _many_ declaration files not written with esmodules in mind that still get compiled into a format with __esModule set
             // Meaning there may be no default at runtime - however to be on the permissive side, we allow access to a synthetic default member
             // as there is no marker to indicate if the accompanying JS has `__esModule` or not, or is even native esm

src/testRunner/unittests/tsc/projectReferences.ts

@@ -90,6 +90,35 @@ 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",
+                    },
+                }),
+                "/home/src/workspaces/project/lib/utils.ts": "export const test = () => 'test';",
+                "/home/src/workspaces/project/lib/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 Test from '../lib/utils';\nconsole.log(Test.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 +1,62 @@
+currentDirectory:: /home/src/workspaces/project useCaseSensitiveFileNames:: false
+Input::
+//// [/home/src/workspaces/project/lib/tsconfig.json]
+{
+  "compilerOptions": {
+    "composite": true,
+    "declaration": true,
+    "module": "esnext",
+    "moduleResolution": "bundler"
+  }
+}
+
+//// [/home/src/workspaces/project/lib/utils.ts]
+export const test = () => 'test';
+
+//// [/home/src/workspaces/project/lib/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 Test from '../lib/utils';
+console.log(Test.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(1,8): error TS1192: Module '"/home/src/workspaces/project/lib/utils"' has no default export.
+
+
+//// [/home/src/workspaces/project/app/index.js]
+import Test from '../lib/utils';
+console.log(Test.test());
+
+
+
+exitCode:: ExitStatus.DiagnosticsPresent_OutputsGenerated