Suggest using a newer Python version if possibly needed (#13197)

Fixes #12357

Author: hauntsaninja

mypy/fastparse.py

@@ -272,12 +272,12 @@ def parse(
         options = Options()
     errors.set_file(fnam, module)
     is_stub_file = fnam.endswith(".pyi")
+    if is_stub_file:
+        feature_version = defaults.PYTHON3_VERSION[1]
+    else:
+        assert options.python_version[0] >= 3
+        feature_version = options.python_version[1]
     try:
-        if is_stub_file:
-            feature_version = defaults.PYTHON3_VERSION[1]
-        else:
-            assert options.python_version[0] >= 3
-            feature_version = options.python_version[1]
         # Disable deprecation warnings about \u
         with warnings.catch_warnings():
             warnings.filterwarnings("ignore", category=DeprecationWarning)
@@ -294,10 +294,14 @@ def parse(
             # start of the f-string. This would be misleading, as mypy will report the error as the
             # lineno within the file.
             e.lineno = None
+        message = e.msg
+        if feature_version > sys.version_info.minor and message.startswith("invalid syntax"):
+            python_version_str = f"{options.python_version[0]}.{options.python_version[1]}"
+            message += f"; you likely need to run mypy using Python {python_version_str} or newer"
         errors.report(
             e.lineno if e.lineno is not None else -1,
             e.offset,
-            e.msg,
+            message,
             blocker=True,
             code=codes.SYNTAX,
         )

test-data/unit/check-newsyntax.test

@@ -151,3 +151,9 @@ v = 1
 reveal_type(f'{v}') # N: Revealed type is "builtins.str"
 reveal_type(f'{1}') # N: Revealed type is "builtins.str"
 [builtins fixtures/f_string.pyi]
+
+[case testFeatureVersionSuggestion]
+# flags: --python-version 3.99
+this is what future python looks like public static void main String[] args await goto exit
+[out]
+main:2: error: invalid syntax; you likely need to run mypy using Python 3.99 or newer