Support Top Level "for await of".

Author: hikarino-my

src/compiler/checker.ts

@@ -38441,18 +38441,34 @@ namespace ts {
 
             if (forInOrOfStatement.kind === SyntaxKind.ForOfStatement && forInOrOfStatement.awaitModifier) {
                 if ((forInOrOfStatement.flags & NodeFlags.AwaitContext) === NodeFlags.None) {
-                    // use of 'for-await-of' in non-async function
                     const sourceFile = getSourceFileOfNode(forInOrOfStatement);
-                    if (!hasParseDiagnostics(sourceFile)) {
-                        const diagnostic = createDiagnosticForNode(forInOrOfStatement.awaitModifier, Diagnostics.A_for_await_of_statement_is_only_allowed_within_an_async_function_or_async_generator);
-                        const func = getContainingFunction(forInOrOfStatement);
-                        if (func && func.kind !== SyntaxKind.Constructor) {
-                            Debug.assert((getFunctionFlags(func) & FunctionFlags.Async) === 0, "Enclosing function should never be an async function.");
-                            const relatedInfo = createDiagnosticForNode(func, Diagnostics.Did_you_mean_to_mark_this_function_as_async);
-                            addRelatedInfo(diagnostic, relatedInfo);
-                        }
-                        diagnostics.add(diagnostic);
-                        return true;
+                    if (isInTopLevelContext(forInOrOfStatement)) {
+                        if (!hasParseDiagnostics(sourceFile)) {
+                            if (!isEffectiveExternalModule(sourceFile, compilerOptions)) {
+                                const diagnostic = createDiagnosticForNode(forInOrOfStatement.awaitModifier,
+                                    Diagnostics.await_expressions_are_only_allowed_at_the_top_level_of_a_file_when_that_file_is_a_module_but_this_file_has_no_imports_or_exports_Consider_adding_an_empty_export_to_make_this_file_a_module);
+                                diagnostics.add(diagnostic);
+                            }
+                            if ((moduleKind !== ModuleKind.ESNext && moduleKind !== ModuleKind.System) || languageVersion < ScriptTarget.ES2017) {
+                                const diagnostic = createDiagnosticForNode(forInOrOfStatement.awaitModifier,
+                                    Diagnostics.Top_level_await_expressions_are_only_allowed_when_the_module_option_is_set_to_esnext_or_system_and_the_target_option_is_set_to_es2017_or_higher);
+                                diagnostics.add(diagnostic);
+                            }
+                        }
+                    }
+                    else {
+                        // use of 'for-await-of' in non-async function
+                        if (!hasParseDiagnostics(sourceFile)) {
+                            const diagnostic = createDiagnosticForNode(forInOrOfStatement.awaitModifier, Diagnostics.A_for_await_of_statement_is_only_allowed_within_an_async_function_or_async_generator);
+                            const func = getContainingFunction(forInOrOfStatement);
+                            if (func && func.kind !== SyntaxKind.Constructor) {
+                                Debug.assert((getFunctionFlags(func) & FunctionFlags.Async) === 0, "Enclosing function should never be an async function.");
+                                const relatedInfo = createDiagnosticForNode(func, Diagnostics.Did_you_mean_to_mark_this_function_as_async);
+                                addRelatedInfo(diagnostic, relatedInfo);
+                            }
+                            diagnostics.add(diagnostic);
+                            return true;
+                        }
                     }
                     return false;
                 }