Improve match statement union narrowing/inference

mypy/checkpattern.py

@@ -244,6 +244,22 @@ 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 isinstance(get_proper_type(typ), UninhabitedType):
+                    union_items.append(typ)
+                    union_captures.update(capture)
+            if len(union_items) == 0:
+                typ = UninhabitedType()
+            elif len(union_items) == 1:
+                typ = union_items[0]
+            else:
+                typ = UnionType(items=union_items)
+            return PatternType(type=typ, rest_type=current_type, 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]