Do not call get_proper_type for traversing type alias: that creates a copy preventing union items update.

Author: sterliakov

mypy/semanal_typeargs.py

@@ -101,7 +101,7 @@ def visit_type_alias_type(self, t: TypeAliasType) -> None:
         if not is_error:
             # If there was already an error for the alias itself, there is no point in checking
             # the expansion, most likely it will result in the same kind of error.
-            get_proper_type(t).accept(self)
+            t.alias.target.accept(self)
 
     def visit_tuple_type(self, t: TupleType) -> None:
         t.items = flatten_nested_tuples(t.items)

test-data/unit/check-python312.test

@@ -1972,3 +1972,19 @@ class D:
             class G[Q]:
                 def g(self, x: Q): ...
             d: G[str]
+
+[case testTypeAliasNormalization]
+from collections.abc import Callable
+from typing import Unpack
+from typing_extensions import TypeAlias
+
+type RK_function_args = tuple[float, int]
+type RK_functionBIS = Callable[[Unpack[RK_function_args], int], int]
+
+def ff(a: float, b: int, c: int) -> int:
+    return 2
+
+bis: RK_functionBIS = ff
+res: int = bis(1.0, 2, 3)
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]

test-data/unit/check-type-aliases.test

@@ -977,13 +977,18 @@ class C(Generic[T]): ...
 class D(C[S]): ...  # E: Invalid type argument value for "C"
 
 U = TypeVar("U")
-A = List[C[U]]
-x: A[bytes]  # E: Value of type variable "T" of "C" cannot be "bytes"
+A = List[C[U]]  # E: Type variable "U" not valid as type argument value for "C"
+A2 = List[C[T]]
+x: A[bytes]
+x2: A2[bytes]  # E: Value of type variable "T" of "A2" cannot be "bytes"
 
 V = TypeVar("V", bound=int)
+V2 = TypeVar("V2", bound=int)
 class E(Generic[V]): ...
-B = List[E[U]]
-y: B[str]  # E: Type argument "str" of "E" must be a subtype of "int"
+B = List[E[U]]  # E: Type argument "U" of "E" must be a subtype of "int"
+B2 = List[E[V2]]
+y: B[str]
+y2: B2[str]  # E: Type argument "str" of "B2" must be a subtype of "int"
 
 [case testValidTypeAliasValuesMoreRestrictive]
 from typing import TypeVar, Generic, List