Narrow tagged unions in match statements (#18791)

Fixes #16286.

---

This PR was generated by an AI system in collaboration with maintainers:
@hauntsaninja

---------

Signed-off-by: Gene Parmesan Thomas <[email protected]>
Signed-off-by: gopoto <[email protected]>

Author: gopoto

mypy/checker.py

@@ -5527,6 +5527,8 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                         pattern_map, else_map = conditional_types_to_typemaps(
                             named_subject, pattern_type.type, pattern_type.rest_type
                         )
+                        pattern_map = self.propagate_up_typemap_info(pattern_map)
+                        else_map = self.propagate_up_typemap_info(else_map)
                         self.remove_capture_conflicts(pattern_type.captures, inferred_types)
                         self.push_type_map(pattern_map, from_assignment=False)
                         if pattern_map:

test-data/unit/check-python310.test

@@ -332,6 +332,54 @@ match [SubClass("a"), SubClass("b")]:
         reveal_type(rest)  # N: Revealed type is "builtins.list[__main__.Example]"
 [builtins fixtures/tuple.pyi]
 
+# Narrowing union-based values via a literal pattern on an indexed/attribute subject
+# -------------------------------------------------------------------------------
+# Literal patterns against a union of types can be used to narrow the subject
+# itself, not just the expression being matched. Previously, the patterns below
+# failed to narrow the `d` variable, leading to errors for missing members; we
+# now propagate the type information up to the parent.
+
+[case testMatchNarrowingUnionTypedDictViaIndex]
+from typing import Literal, TypedDict
+
+class A(TypedDict):
+    tag: Literal["a"]
+    name: str
+
+class B(TypedDict):
+    tag: Literal["b"]
+    num: int
+
+d: A | B
+match d["tag"]:
+    case "a":
+        reveal_type(d)  # N: Revealed type is "TypedDict('__main__.A', {'tag': Literal['a'], 'name': builtins.str})"
+        reveal_type(d["name"])  # N: Revealed type is "builtins.str"
+    case "b":
+        reveal_type(d)  # N: Revealed type is "TypedDict('__main__.B', {'tag': Literal['b'], 'num': builtins.int})"
+        reveal_type(d["num"])  # N: Revealed type is "builtins.int"
+[typing fixtures/typing-typeddict.pyi]
+
+[case testMatchNarrowingUnionClassViaAttribute]
+from typing import Literal
+
+class A:
+    tag: Literal["a"]
+    name: str
+
+class B:
+    tag: Literal["b"]
+    num: int
+
+d: A | B
+match d.tag:
+    case "a":
+        reveal_type(d)  # N: Revealed type is "__main__.A"
+        reveal_type(d.name)  # N: Revealed type is "builtins.str"
+    case "b":
+        reveal_type(d)  # N: Revealed type is "__main__.B"
+        reveal_type(d.num)  # N: Revealed type is "builtins.int"
+
 [case testMatchSequenceUnion-skip]
 from typing import List, Union
 m: Union[List[List[str]], str]