From db9837fa240918fa90f2d9a3952314bd3207ed20 Mon Sep 17 00:00:00 2001
From: bzoracler <50305397+bzoracler@users.noreply.github.com>
Date: Wed, 24 Jul 2024 20:10:42 +1200
Subject: [PATCH] fix: Mismatched signature between checker plugin API and
 implementation (#17343)

This PR changes the `fail` method's signature to be positional-only for
the first two parameters, due to a mismatch between the
[`CheckerPluginInterface`
signature](https://github.com/python/mypy/blob/8dd268ffd84ccf549b3aa9105dd35766a899b2bd/mypy/plugin.py#L242-L244)
and the implementation class
([`TypeChecker`](https://github.com/python/mypy/blob/8dd268ffd84ccf549b3aa9105dd35766a899b2bd/mypy/checker.py#L7116-L7118)).

```python
class CheckerPluginInterface:
    ...
    @abstractmethod
    def fail(
        self, msg: str | ErrorMessage, ctx: Context, *, code: ErrorCode | None = None
    ) -> None: ...
```

```python
class TypeChecker(NodeVisitor[None], CheckerPluginInterface):
    ...
    def fail(
        self, msg: str | ErrorMessage, context: Context, *, code: ErrorCode | None = None
    ) -> None:
```

An alternative fix would be to change `TypeChecker.fail`'s parameter
name to `ctx`, but that has a greater disruption potential.

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
---
 mypy/plugin.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mypy/plugin.py b/mypy/plugin.py
index 858795addb7f..a1af7fa76350 100644
--- a/mypy/plugin.py
+++ b/mypy/plugin.py
@@ -240,7 +240,7 @@ def type_context(self) -> list[Type | None]:
 
     @abstractmethod
     def fail(
-        self, msg: str | ErrorMessage, ctx: Context, *, code: ErrorCode | None = None
+        self, msg: str | ErrorMessage, ctx: Context, /, *, code: ErrorCode | None = None
     ) -> None:
         """Emit an error message at given location."""
         raise NotImplementedError