Don't crash on unsupported Python 3.10 match statement (#11738)

Generate a syntax error instead.

Author: JukkaL

mypy/fastparse.py

@@ -1285,6 +1285,10 @@ def visit_Index(self, n: Index) -> Node:
         # cast for mypyc's benefit on Python 3.9
         return self.visit(cast(Any, n).value)
 
+    def visit_Match(self, n: Any) -> None:
+        self.fail("Match statement is not supported",
+                  line=n.lineno, column=n.col_offset, blocker=True)
+
 
 class TypeConverter:
     def __init__(self,

mypy/test/testcheck.py

@@ -103,6 +103,8 @@
     typecheck_files.append('check-python38.test')
 if sys.version_info >= (3, 9):
     typecheck_files.append('check-python39.test')
+if sys.version_info >= (3, 10):
+    typecheck_files.append('check-python310.test')
 
 # Special tests for platforms with case-insensitive filesystems.
 if sys.platform in ('darwin', 'win32'):

test-data/unit/check-python310.test

@@ -0,0 +1,7 @@
+[case testMatchStatementNotSupported]
+# flags: --python-version 3.10
+match str():  # E: Match statement is not supported
+    case 'x':
+        1 + ''
+    case _:
+        1 + b''