Fix type inference in pattern matching by positional argument

mypy/expandtype.py

@@ -136,7 +136,7 @@ def visit_erased_type(self, t: ErasedType) -> Type:
     def visit_instance(self, t: Instance) -> Type:
         args = self.expand_types_with_unpack(list(t.args))
         if isinstance(args, list):
-            return Instance(t.type, args, t.line, t.column)
+            return t.copy_modified(args=args)
         else:
             return args
 

test-data/unit/check-python310.test

@@ -1628,3 +1628,49 @@ match var:
     case ("yes", b):
         reveal_type(b)  # N: Revealed type is "Union[builtins.int, builtins.str]"
 [builtins fixtures/tuple.pyi]
+
+[case testMatchNamedAndKeywordsAreTheSame]
+from typing import Generic, TypeVar, Union
+from typing_extensions import Final
+
+T = TypeVar("T")
+
+class Regular:
+    x: str
+    y: int
+    __match_args__ = ("x",)
+class ReveresedOrder:
+    x: int
+    y: str
+    __match_args__ = ("y",)
+class GenericRegular(Generic[T]):
+    x: T
+    __match_args__ = ("x",)
+class GenericWithFinal(Generic[T]):
+    x: T
+    __match_args__: Final = ("x",)
+
+input_arg: Union[Regular, ReveresedOrder, GenericRegular[str], GenericWithFinal[str]]
+
+# Positional:
+match input_arg:
+    case Regular(a):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+    case ReveresedOrder(a):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+    case GenericRegular(a):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+    case GenericWithFinal(a):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+
+# Keywords:
+match input_arg:
+    case Regular(x=a):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+    case ReveresedOrder(x=b):  # note, that order is different
+        reveal_type(b)  # N: Revealed type is "builtins.int"
+    case GenericRegular(x=a):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+    case GenericWithFinal(x=a):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+[builtins fixtures/tuple.pyi]