From 1f9317f593dc41a2805a3093e2e1890665485e76 Mon Sep 17 00:00:00 2001
From: bzoracler <50305397+bzoracler@users.noreply.github.com>
Date: Thu, 19 Dec 2024 19:01:21 +1300
Subject: [PATCH] fix: fail check if not enough or too many types provided to
 `TypeAliasType` (#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                        | 6 +++++-
 test-data/unit/check-type-aliases.test | 4 ++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/mypy/semanal.py b/mypy/semanal.py
index e90ab9f160e0a..42803727a9588 100644
--- a/mypy/semanal.py
+++ b/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
diff --git a/test-data/unit/check-type-aliases.test b/test-data/unit/check-type-aliases.test
index c7b9694a91887..4073836dd973b 100644
--- a/test-data/unit/check-type-aliases.test
+++ b/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]