add related info to error for use of for-await-of in non-async function

Author: uhyo

src/compiler/checker.ts

@@ -31598,7 +31598,20 @@ namespace ts {
 
             if (forInOrOfStatement.kind === SyntaxKind.ForOfStatement && forInOrOfStatement.awaitModifier) {
                 if ((forInOrOfStatement.flags & NodeFlags.AwaitContext) === NodeFlags.None) {
-                    return grammarErrorOnNode(forInOrOfStatement.awaitModifier, Diagnostics.A_for_await_of_statement_is_only_allowed_within_an_async_function_or_async_generator);
+                    // 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) {
+                            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;
                 }
             }