Fix --strict-equality crash for instances of a class generic over a…

… `ParamSpec` (#14792)

Fixes #14783.

Running mypy on this snippet of code currently causes a crash if you
have the `--strict-equality` option enabled:

```python
from typing import Generic, ParamSpec

P = ParamSpec("P")

class Foo(Generic[P]): ...

def checker(foo1: Foo[[int]], foo2: Foo[[str]]) -> None:
    foo1 == foo2
```

This is because the overlapping-equality logic in `meet.py` currently
does not account for the fact that `left` and `right` might both be
instances of `mypy.types.Parameters`, leading to this assertion being
tripped:


https://github.com/python/mypy/blob/800e8ffdf17de9fc641fefff46389a940f147eef/mypy/meet.py#L519

This PR attempts to add the necessary logic to `meet.py` to handle
instances of `mypy.types.Parameters`.

mypy/meet.py

@@ -342,7 +342,22 @@ def _is_overlapping_types(left: Type, right: Type) -> bool:
     left_possible = get_possible_variants(left)
     right_possible = get_possible_variants(right)
 
-    # We start by checking multi-variant types like Unions first. We also perform
+    # First handle special cases relating to PEP 612:
+    # - comparing a `Parameters` to a `Parameters`
+    # - comparing a `Parameters` to a `ParamSpecType`
+    # - comparing a `ParamSpecType` to a `ParamSpecType`
+    #
+    # These should all always be considered overlapping equality checks.
+    # These need to be done before we move on to other TypeVarLike comparisons.
+    if isinstance(left, (Parameters, ParamSpecType)) and isinstance(
+        right, (Parameters, ParamSpecType)
+    ):
+        return True
+    # A `Parameters` does not overlap with anything else, however
+    if isinstance(left, Parameters) or isinstance(right, Parameters):
+        return False
+
+    # Now move on to checking multi-variant types like Unions. We also perform
     # the same logic if either type happens to be a TypeVar/ParamSpec/TypeVarTuple.
     #
     # Handling the TypeVarLikes now lets us simulate having them bind to the corresponding

test-data/unit/pythoneval.test

@@ -1924,3 +1924,46 @@ _testStarUnpackNestedUnderscore.py:10: error: List item 0 has incompatible type
 _testStarUnpackNestedUnderscore.py:10: error: List item 1 has incompatible type "int"; expected "str"
 _testStarUnpackNestedUnderscore.py:11: note: Revealed type is "builtins.list[builtins.str]"
 _testStarUnpackNestedUnderscore.py:16: note: Revealed type is "builtins.list[builtins.object]"
+
+[case testStrictEqualitywithParamSpec]
+# flags: --strict-equality
+from typing import Generic
+from typing_extensions import Concatenate, ParamSpec
+
+P = ParamSpec("P")
+
+class Foo(Generic[P]): ...
+class Bar(Generic[P]): ...
+
+def bad(foo: Foo[[int]], bar: Bar[[int]]) -> bool:
+    return foo == bar
+
+def good1(foo1: Foo[[int]], foo2: Foo[[str]]) -> bool:
+    return foo1 == foo2
+
+def good2(foo1: Foo[[int, str]], foo2: Foo[[int, bytes]]) -> bool:
+    return foo1 == foo2
+
+def good3(foo1: Foo[[int]], foo2: Foo[[int, int]]) -> bool:
+    return foo1 == foo2
+
+def good4(foo1: Foo[[int]], foo2: Foo[[int]]) -> bool:
+    return foo1 == foo2
+
+def good5(foo1: Foo[[int]], foo2: Foo[[bool]]) -> bool:
+    return foo1 == foo2
+
+def good6(foo1: Foo[[int, int]], foo2: Foo[[bool, bool]]) -> bool:
+    return foo1 == foo2
+
+def good7(foo1: Foo[[int]], foo2: Foo[P], *args: P.args, **kwargs: P.kwargs) -> bool:
+    return foo1 == foo2
+
+def good8(foo1: Foo[P], foo2: Foo[[int, str, bytes]], *args: P.args, **kwargs: P.kwargs) -> bool:
+    return foo1 == foo2
+
+def good9(foo1: Foo[Concatenate[int, P]], foo2: Foo[[int, str, bytes]], *args: P.args, **kwargs: P.kwargs) -> bool:
+    return foo1 == foo2
+
+[out]
+_testStrictEqualitywithParamSpec.py:11: error: Non-overlapping equality check (left operand type: "Foo[[int]]", right operand type: "Bar[[int]]")