Disallow no-args generic aliases when using PEP 613 explicit aliases (#18173)

Per the type system conformance tests, [this is
ok](https://github.com/python/typing/blob/46b05a4c10ed3841c9bc5126ba9f31dd8ae061e7/conformance/tests/aliases_implicit.py#L130):

```python
ListAlias = list
x = ListAlias[int]()  # OK
```

While [this is
not](https://github.com/python/typing/blob/46b05a4c10ed3841c9bc5126ba9f31dd8ae061e7/conformance/tests/aliases_explicit.py#L100):
```python
ListAlias: TypeAlias = list
x: ListAlias[int]  # E: already specialized
```

Mypy currently permits both. This PR makes mypy reject the latter case,
improving conformance.

As part of this, no-args PEP 613 explicit aliases are no longer eagerly
expanded.

(Also removed a stale comment referencing `TypeAliasExpr.no_args`, which
was removed in #15924)

Author: brianschubert

mypy/nodes.py

@@ -3593,7 +3593,7 @@ def f(x: B[T]) -> T: ...  # without T, Any would be used here
     type will be initially an instance type with wrong number of type arguments.
     Such instances are all fixed either during or after main semantic analysis passes.
     We therefore store the difference between `List` and `List[Any]` rvalues (targets)
-    using the `no_args` flag. See also TypeAliasExpr.no_args.
+    using the `no_args` flag.
 
     Meaning of other fields:
 

mypy/semanal.py

@@ -2797,15 +2797,15 @@ def get_declared_metaclass(
                 return None, True, False  # defer later in the caller
 
             # Support type aliases, like `_Meta: TypeAlias = type`
+            metaclass_info: Node | None = sym.node
             if (
                 isinstance(sym.node, TypeAlias)
-                and sym.node.no_args
-                and isinstance(sym.node.target, ProperType)
-                and isinstance(sym.node.target, Instance)
+                and not sym.node.python_3_12_type_alias
+                and not sym.node.alias_tvars
             ):
-                metaclass_info: Node | None = sym.node.target.type
-            else:
-                metaclass_info = sym.node
+                target = get_proper_type(sym.node.target)
+                if isinstance(target, Instance):
+                    metaclass_info = target.type
 
             if not isinstance(metaclass_info, TypeInfo) or metaclass_info.tuple_type is not None:
                 self.fail(f'Invalid metaclass "{metaclass_name}"', metaclass_expr)
@@ -4077,6 +4077,7 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
             and not res.args
             and not empty_tuple_index
             and not pep_695
+            and not pep_613
         )
         if isinstance(res, ProperType) and isinstance(res, Instance):
             if not validate_instance(res, self.fail, empty_tuple_index):

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

@@ -1242,3 +1242,56 @@ from typing import List, Union
 A = Union[int, List[A]]
 def func(x: A) -> int: ...
 [builtins fixtures/tuple.pyi]
+
+[case testAliasExplicitNoArgsBasic]
+from typing import Any, List, assert_type
+from typing_extensions import TypeAlias
+
+Implicit = List
+Explicit: TypeAlias = List
+
+x1: Implicit[str]
+x2: Explicit[str]  # E: Bad number of arguments for type alias, expected 0, given 1
+assert_type(x1, List[str])
+assert_type(x2, List[Any])
+[builtins fixtures/tuple.pyi]
+
+[case testAliasExplicitNoArgsGenericClass]
+# flags: --python-version 3.9
+from typing import Any, assert_type
+from typing_extensions import TypeAlias
+
+Implicit = list
+Explicit: TypeAlias = list
+
+x1: Implicit[str]
+x2: Explicit[str]  # E: Bad number of arguments for type alias, expected 0, given 1
+assert_type(x1, list[str])
+assert_type(x2, list[Any])
+[builtins fixtures/tuple.pyi]
+
+[case testAliasExplicitNoArgsTuple]
+from typing import Any, Tuple, assert_type
+from typing_extensions import TypeAlias
+
+Implicit = Tuple
+Explicit: TypeAlias = Tuple
+
+x1: Implicit[str]  # E: Bad number of arguments for type alias, expected 0, given 1
+x2: Explicit[str]  # E: Bad number of arguments for type alias, expected 0, given 1
+assert_type(x1, Tuple[Any, ...])
+assert_type(x2, Tuple[Any, ...])
+[builtins fixtures/tuple.pyi]
+
+[case testAliasExplicitNoArgsCallable]
+from typing import Any, Callable, assert_type
+from typing_extensions import TypeAlias
+
+Implicit = Callable
+Explicit: TypeAlias = Callable
+
+x1: Implicit[str]  # E: Bad number of arguments for type alias, expected 0, given 1
+x2: Explicit[str]  # E: Bad number of arguments for type alias, expected 0, given 1
+assert_type(x1, Callable[..., Any])
+assert_type(x2, Callable[..., Any])
+[builtins fixtures/tuple.pyi]

test-data/unit/diff.test

@@ -1563,7 +1563,6 @@ type H[T] = int
 __main__.A
 __main__.C
 __main__.D
-__main__.E
 __main__.G
 __main__.H