Improve match statement union narrowing/inference

mypy/checkpattern.py

@@ -244,6 +244,27 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
  # get inner types of original type
  #
  unpack_index = None
+
+ if isinstance(current_type, UnionType):
+ union_captures: dict[Expression, Type] = {}
+ union_items: list[Type] = []
+ for t in current_type.items:
+ typ, _, capture = self.accept(o, t)
+ if not is_uninhabited(get_proper_type(typ)):
+ union_items.append(typ)
+ union_captures.update(capture)
+
+ rest_items: list[Type] = []
+ for item in current_type.items:
+ if all(used_item != item for used_item in union_items):
+ rest_items.append(item)
+
+ return PatternType(
+ type=UnionType.make_union(items=union_items),
+ rest_type=UnionType.make_union(items=rest_items),
+ captures=union_captures,
+ )
+
  if isinstance(current_type, TupleType):
  inner_types = current_type.items
  unpack_index = find_unpack_in_list(inner_types)

test-data/unit/check-python310.test

@@ -1777,12 +1777,11 @@ match foo:
 [case testMatchUnionTwoTuplesNoCrash]
 var: tuple[int, int] | tuple[str, str]
 
-# TODO: we can infer better here.
 match var:
  case (42, a):
- reveal_type(a) # N: Revealed type is "Union[builtins.int, builtins.str]"
+ reveal_type(a) # N: Revealed type is "builtins.int"
  case ("yes", b):
- reveal_type(b) # N: Revealed type is "Union[builtins.int, builtins.str]"
+ reveal_type(b) # N: Revealed type is "builtins.str"
 [builtins fixtures/tuple.pyi]
 
 [case testMatchNamedAndKeywordsAreTheSame]