Fix panic in checker.GetTypeArguments for non-reference types (#4338)

_packages/native-preview/test/async/api.test.ts

@@ -2332,6 +2332,25 @@ describe("Checker - getTypeArguments", () => {
             await api.close();
         }
     });
+
+    test("does not panic for a non-reference type (issue #4338)", async () => {
+        const api = spawnAPI({
+            "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
+            "/src/main.ts": `export const s: string = "";`,
+        });
+        try {
+            const snapshot = await api.updateSnapshot({ openProject: "/tsconfig.json" });
+            const project = snapshot.getProject("/tsconfig.json")!;
+            const symbol = await project.checker.getSymbolAtPosition("/src/main.ts", `export const `.length);
+            assert.ok(symbol);
+            const type = await project.checker.getTypeOfSymbol(symbol);
+            assert.ok(type);
+            assert.deepEqual(await project.checker.getTypeArguments(type), []);
+        }
+        finally {
+            await api.close();
+        }
+    });
 });
 
 describe("TypeParameter - isThisType", () => {

_packages/native-preview/test/sync/api.test.ts

@@ -2340,6 +2340,25 @@ describe("Checker - getTypeArguments", () => {
             api.close();
         }
     });
+
+    test("does not panic for a non-reference type (issue #4338)", () => {
+        const api = spawnAPI({
+            "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
+            "/src/main.ts": `export const s: string = "";`,
+        });
+        try {
+            const snapshot = api.updateSnapshot({ openProject: "/tsconfig.json" });
+            const project = snapshot.getProject("/tsconfig.json")!;
+            const symbol = project.checker.getSymbolAtPosition("/src/main.ts", `export const `.length);
+            assert.ok(symbol);
+            const type = project.checker.getTypeOfSymbol(symbol);
+            assert.ok(type);
+            assert.deepEqual(project.checker.getTypeArguments(type), []);
+        }
+        finally {
+            api.close();
+        }
+    });
 });
 
 describe("TypeParameter - isThisType", () => {

internal/checker/exports.go

@@ -275,6 +275,12 @@ func (c *Checker) GetRestTypeOfSignature(sig *Signature) *Type {
 }
 
 func (c *Checker) GetTypeArguments(t *Type) []*Type {
+	// getTypeArguments assumes a type reference; non-reference types (e.g. the
+	// intrinsic `string`) would nil-deref through the public API. Match tsc's
+	// behavior of having no type arguments for them.
+	if t.objectFlags&ObjectFlagsReference == 0 {
+		return nil
+	}
 	return c.getTypeArguments(t)
 }