fix: fail check if not enough or too many types provided to `TypeAlia…

…sType` (#18308)

Fixes #18307 by failing a type alias call check if the number of
positional arguments isn't exactly 2 (one for the type name as a literal
string, one for the target type to alias).

Before:

```python
from typing_extensions import TypeAliasType

T1 = TypeAliasType("T1", int, str)  # Silently passes and uses `int` as the target type, should be an error
T2 = TypeAliasType("T2")  # Crashes
```

After:

```python
T1 = TypeAliasType("T1", int, str)  # E: Too many positional arguments for "TypeAliasType"
T2 = TypeAliasType("T2")  # E: Missing positional argument "value" in call to "TypeAliasType"
```

The error messages above are propagated from a check with the
[`TypeAliasType` constructor definition from the
stubs](https://github.com/python/typeshed/blob/bd728fbfae18395c37d9fd020e31b1d4f30c6136/stdlib/typing.pyi#L1041-L1044),
so no further special-casing is necessary:

```python
    class TypeAliasType:
        def __init__(
            self, name: str, value: Any, *, type_params: tuple[TypeVar | ParamSpec | TypeVarTuple, ...] = ()
        ) -> None: ...
```

mypy/semanal.py

@@ -4157,8 +4157,12 @@ def check_type_alias_type_call(self, rvalue: Expression, *, name: str) -> TypeGu
             names.append("typing.TypeAliasType")
         if not refers_to_fullname(rvalue.callee, tuple(names)):
             return False
+        if not self.check_typevarlike_name(rvalue, name, rvalue):
+            return False
+        if rvalue.arg_kinds.count(ARG_POS) != 2:
+            return False
 
-        return self.check_typevarlike_name(rvalue, name, rvalue)
+        return True
 
     def analyze_type_alias_type_params(
         self, rvalue: CallExpr

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

@@ -1105,6 +1105,10 @@ t1: T1  # E: Variable "__main__.T1" is not valid as a type \
 T3 = TypeAliasType("T3", -1)  # E: Invalid type: try using Literal[-1] instead?
 t3: T3
 reveal_type(t3)  # N: Revealed type is "Any"
+
+T4 = TypeAliasType("T4")  # E: Missing positional argument "value" in call to "TypeAliasType"
+T5 = TypeAliasType("T5", int, str)  # E: Too many positional arguments for "TypeAliasType" \
+                                    # E: Argument 3 to "TypeAliasType" has incompatible type "Type[str]"; expected "Tuple[Union[TypeVar?, ParamSpec?, TypeVarTuple?], ...]"
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]