Fix #1890 (crash on [1, *[2]]).

Author: Guido van Rossum

mypy/checkexpr.py

@@ -661,6 +661,8 @@ def check_argument_types(self, arg_types: List[Type], arg_kinds: List[int],
         for i, actuals in enumerate(formal_to_actual):
             for actual in actuals:
                 arg_type = arg_types[actual]
+                if arg_type is None:
+                    continue  # Some kind of error was already reported.
                 # Check that a *arg is valid as varargs.
                 if (arg_kinds[actual] == nodes.ARG_STAR and
                         not self.is_valid_var_arg(arg_type)):

test-data/unit/check-lists.test

@@ -62,3 +62,7 @@ class C: pass
 [builtins fixtures/list.py]
 [out]
 main: note: In function "f":
+
+[case testListContainingStarExpr]
+a = [1, *[2]]  # E: Can use starred expression only as assignment target
+[builtins fixtures/list.py]

test-data/unit/semanal-errors.test

@@ -487,9 +487,11 @@ c = 1
 d = 1
 a = *b
 a = b, (c, *d)
+a = [1, *[2]]
 [out]
 main:4: error: Can use starred expression only as assignment target
 main:5: error: Can use starred expression only as assignment target
+main:6: error: Can use starred expression only as assignment target
 
 [case testStarExpressionInExp]
 a = 1