SubtypeVisitor: treat ParamSpec as if it was a covariant TypeVar

Author: Akuli

mypy/subtypes.py

@@ -277,6 +277,9 @@ def visit_instance(self, left: Instance) -> bool:
                     if isinstance(tvar, TypeVarType):
                         if not self.check_type_parameter(lefta, righta, tvar.variance):
                             nominal = False
+                    elif isinstance(tvar, ParamSpecType):
+                        if not self.check_type_parameter(lefta, righta, COVARIANT):
+                            nominal = False
                     else:
                         if not is_equivalent(lefta, righta):
                             nominal = False

test-data/unit/check-parameter-specification.test

@@ -416,3 +416,26 @@ with f() as x:
     pass
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testDecoratingClassesThatUseParamSpec]
+from typing import Generic, TypeVar, Callable, Any
+from typing_extensions import ParamSpec
+
+_P = ParamSpec("_P")
+_T = TypeVar("_T")
+_F = TypeVar("_F", bound=Callable[..., Any])
+
+def f(x: _F) -> _F: ...
+
+@f  # Should be ok
+class OnlyParamSpec(Generic[_P]):
+    pass
+
+@f  # Should be ok
+class MixedWithTypeVar1(Generic[_P, _T]):
+    pass
+
+@f  # Should be ok
+class MixedWithTypeVar2(Generic[_T, _P]):
+    pass
+[builtins fixtures/dict.pyi]