Fix handling of named tuples in class match pattern (python#18663)

Fixes python#15299

The fix is straightforward, named tuples should be properly represented
as tuples with fallback, not as instances.

Author: ilevkivskyi

mypy/checkpattern.py

@@ -54,7 +54,7 @@
     get_proper_type,
     split_with_prefix_and_suffix,
 )
-from mypy.typevars import fill_typevars
+from mypy.typevars import fill_typevars, fill_typevars_with_any
 from mypy.visitor import PatternVisitor
 
 self_match_type_names: Final = [
@@ -544,16 +544,7 @@ def visit_class_pattern(self, o: ClassPattern) -> PatternType:
             self.msg.fail(message_registry.CLASS_PATTERN_GENERIC_TYPE_ALIAS, o)
             return self.early_non_match()
         if isinstance(type_info, TypeInfo):
-            any_type = AnyType(TypeOfAny.implementation_artifact)
-            args: list[Type] = []
-            for tv in type_info.defn.type_vars:
-                if isinstance(tv, TypeVarTupleType):
-                    args.append(
-                        UnpackType(self.chk.named_generic_type("builtins.tuple", [any_type]))
-                    )
-                else:
-                    args.append(any_type)
-            typ: Type = Instance(type_info, args)
+            typ: Type = fill_typevars_with_any(type_info)
         elif isinstance(type_info, TypeAlias):
             typ = type_info.target
         elif (
@@ -703,6 +694,8 @@ def visit_class_pattern(self, o: ClassPattern) -> PatternType:
 
     def should_self_match(self, typ: Type) -> bool:
         typ = get_proper_type(typ)
+        if isinstance(typ, TupleType):
+            typ = typ.partial_fallback
         if isinstance(typ, Instance) and typ.type.get("__match_args__") is not None:
             # Named tuples and other subtypes of builtins that define __match_args__
             # should not self match.

test-data/unit/check-python310.test

@@ -770,6 +770,21 @@ match m:
         reveal_type(j)  # N: Revealed type is "builtins.int"
 [builtins fixtures/tuple.pyi]
 
+[case testMatchSequencePatternCaptureNamedTuple]
+from typing import NamedTuple
+
+class N(NamedTuple):
+    x: int
+    y: str
+
+a = N(1, "a")
+
+match a:
+    case [x, y]:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+        reveal_type(y)  # N: Revealed type is "builtins.str"
+[builtins fixtures/tuple.pyi]
+
 [case testMatchClassPatternCaptureGeneric]
 from typing import Generic, TypeVar
 
@@ -2522,3 +2537,18 @@ def fn2(x: Some | int | str) -> None:
         case Some(value):  # E: Incompatible types in capture pattern (pattern captures type "Union[int, str]", variable has type "Callable[[], str]")
             pass
 [builtins fixtures/dict.pyi]
+
+[case testMatchNamedTupleSequence]
+from typing import Any, NamedTuple
+
+class T(NamedTuple):
+    t: list[Any]
+
+class K(NamedTuple):
+    k: int
+
+def f(t: T) -> None:
+    match t:
+        case T([K() as k]):
+            reveal_type(k)  # N: Revealed type is "Tuple[builtins.int, fallback=__main__.K]"
+[builtins fixtures/tuple.pyi]