Fix crashes with unpacking SyntaxError (#11499)

In general, mypy doesn't promise to be able to check the remainder of
your code in the presence of syntax errors, so just make this a blocking
error.

Fixes #9137
Fixes #3825 (most of the reports in this issue were fixed by #8827)

Co-authored-by: hauntsaninja <>

mypy/semanal.py

@@ -1683,6 +1683,8 @@ class Foo(Bar, Generic[T]): ...
         declared_tvars: TypeVarLikeList = []
         is_protocol = False
         for i, base_expr in enumerate(base_type_exprs):
+            if isinstance(base_expr, StarExpr):
+                base_expr.valid = True
             self.analyze_type_expr(base_expr)
 
             try:
@@ -4539,8 +4541,7 @@ def visit_dict_expr(self, expr: DictExpr) -> None:
 
     def visit_star_expr(self, expr: StarExpr) -> None:
         if not expr.valid:
-            # XXX TODO Change this error message
-            self.fail("Can use starred expression only as assignment target", expr)
+            self.fail("Can use starred expression only as assignment target", expr, blocker=True)
         else:
             expr.expr.accept(self)
 

test-data/unit/check-tuples.test

@@ -972,6 +972,17 @@ b = (1, 'x')
 a = (0, *b, '')
 [builtins fixtures/tuple.pyi]
 
+[case testUnpackSyntaxError]
+*foo  # E: Can use starred expression only as assignment target
+[builtins fixtures/tuple.pyi]
+
+[case testUnpackBases]
+class A: ...
+class B: ...
+bases = (A, B)
+class C(*bases): ...  # E: Invalid base class
+[builtins fixtures/tuple.pyi]
+
 [case testTupleMeetTupleAny]
 from typing import Union, Tuple
 class A: pass