Maybe this?

Author: sterliakov

mypy/semanal_typeargs.py

@@ -102,6 +102,7 @@ def visit_type_alias_type(self, t: TypeAliasType) -> None:
             # 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)
+            self.visit_optional_type(t.alias)
 
     def visit_tuple_type(self, t: TupleType) -> None:
         t.items = flatten_nested_tuples(t.items)
@@ -254,6 +255,10 @@ def visit_unpack_type(self, typ: UnpackType) -> None:
     def check_type_var_values(
         self, name: str, actuals: list[Type], arg_name: str, valids: list[Type], context: Context
     ) -> bool:
+        if self.in_type_alias_expr:
+            # See testValidTypeAliasValues - we do not enforce typevar compatibility
+            # at the definition site. We check instantiation validity later.
+            return False
         is_error = False
         for actual in get_proper_types(actuals):
             # We skip UnboundType here, since they may appear in defn.bases,

test-data/unit/check-typevar-tuple.test

@@ -2667,10 +2667,17 @@ class S(tuple[Unpack[Ts]], Generic[T, Unpack[Ts]]):
     def f(self, x: T, /) -> T: ...
 [builtins fixtures/tuple.pyi]
 
-[case testNoCrashSubclassingTUpleWithTrivialUnpack]
+[case testNoCrashSubclassingTupleWithTrivialUnpack]
 # https://github.com/python/mypy/issues/19105
 from typing import Unpack
 
 class A(tuple[Unpack[tuple[int]]]): ...
 class B(tuple[Unpack[tuple[()]]]): ...
+
+a: A
+tuple(a)
+(x,) = a
+
+b: B
+(_,) = b  # E: Need more than 0 values to unpack (1 expected)
 [builtins fixtures/tuple.pyi]