Fix crash in match statement with unmatchable ClassPattern (#13731)

Fixes #12886

`contract_starred_pattern_types` and `expand_starred_pattern_types` are
sort of like inverses of each other, so you get crashes if
`contracted_inner_types` and `contracted_new_inner_types` don't
correspond (in the case that `can_match` gets set to `False`).
I'm not sure `can_match` is doing anything useful, so this
PR now just gets rid of it.

Author: hauntsaninja

mypy/checkpattern.py

@@ -257,16 +257,13 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
         contracted_inner_types = self.contract_starred_pattern_types(
             inner_types, star_position, required_patterns
         )
-        can_match = True
         for p, t in zip(o.patterns, contracted_inner_types):
             pattern_type = self.accept(p, t)
             typ, rest, type_map = pattern_type
-            if is_uninhabited(typ):
-                can_match = False
-            else:
-                contracted_new_inner_types.append(typ)
-                contracted_rest_inner_types.append(rest)
+            contracted_new_inner_types.append(typ)
+            contracted_rest_inner_types.append(rest)
             self.update_type_map(captures, type_map)
+
         new_inner_types = self.expand_starred_pattern_types(
             contracted_new_inner_types, star_position, len(inner_types)
         )
@@ -279,9 +276,7 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
         #
         new_type: Type
         rest_type: Type = current_type
-        if not can_match:
-            new_type = UninhabitedType()
-        elif isinstance(current_type, TupleType):
+        if isinstance(current_type, TupleType):
             narrowed_inner_types = []
             inner_rest_types = []
             for inner_type, new_inner_type in zip(inner_types, new_inner_types):

test-data/unit/check-python310.test

@@ -317,6 +317,21 @@ match x:
     case [str()]:
         pass
 
+[case testMatchSequencePatternWithInvalidClassPattern]
+class Example:
+    __match_args__ = ("value",)
+    def __init__(self, value: str) -> None:
+        self.value = value
+
+SubClass: type[Example]
+
+match [SubClass("a"), SubClass("b")]:
+    case [SubClass(value), *rest]:  # E: Expected type in class pattern; found "Type[__main__.Example]"
+        reveal_type(value)  # E: Cannot determine type of "value" \
+                            # N: Revealed type is "Any"
+        reveal_type(rest)  # N: Revealed type is "builtins.list[__main__.Example]"
+[builtins fixtures/tuple.pyi]
+
 [case testMatchSequenceUnion-skip]
 from typing import List, Union
 m: Union[List[List[str]], str]