Fix PEP 695 type alias with mix of type args (PEP 696) (#18919)

Fix an issue where TypeVar defaults wouldn't be applied to PEP 695 type
aliases.
Fixes #18921

Author: cdce8p

mypy/semanal.py

@@ -5591,7 +5591,7 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
                 self.msg.unimported_type_becomes_any("Type alias target", res, s)
                 res = make_any_non_unimported(res)
             eager = self.is_func_scope()
-            if isinstance(res, ProperType) and isinstance(res, Instance) and not res.args:
+            if isinstance(res, ProperType) and isinstance(res, Instance):
                 fix_instance(res, self.fail, self.note, disallow_any=False, options=self.options)
             alias_node = TypeAlias(
                 res,

test-data/unit/check-python313.test

@@ -219,7 +219,7 @@ def func_a1(
     reveal_type(b)  # N: Revealed type is "builtins.dict[builtins.float, builtins.str]"
     reveal_type(c)  # N: Revealed type is "builtins.dict[builtins.float, builtins.float]"
     reveal_type(d)  # N: Revealed type is "builtins.dict[builtins.int, builtins.str]"
-[builtins fixtures/tuple.pyi]
+[builtins fixtures/dict.pyi]
 [typing fixtures/typing-full.pyi]
 
 [case testPEP695TypeParameterDefaultTypeAlias2]
@@ -255,3 +255,22 @@ def func_c1(
 
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testPEP695TypeParameterDefaultTypeAlias4]
+# flags: --disallow-any-generics
+class A[L = int, M = str]: ...
+TD1 = A[float]
+type TD2 = A[float]
+
+def func_d1(
+    a: TD1,
+    b: TD1[float],  # E: Bad number of arguments for type alias, expected 0, given 1
+    c: TD2,
+    d: TD2[float],  # E: Bad number of arguments for type alias, expected 0, given 1
+) -> None:
+    reveal_type(a)  # N: Revealed type is "__main__.A[builtins.float, builtins.str]"
+    reveal_type(b)  # N: Revealed type is "__main__.A[builtins.float, builtins.str]"
+    reveal_type(c)  # N: Revealed type is "__main__.A[builtins.float, builtins.str]"
+    reveal_type(d)  # N: Revealed type is "__main__.A[builtins.float, builtins.str]"
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]