Allow super() for mixin protocols (#14082)

Fixes #12344 

FWIW this is unsafe (since we don't know where the mixin will appear in
the MRO of the actual implementation), but the alternative is having
annoying false positives like this issue and e.g.
#4335

Author: ilevkivskyi

mypy/checkexpr.py

@@ -4321,8 +4321,18 @@ def visit_super_expr(self, e: SuperExpr) -> Type:
                     mro = e.info.mro
                     index = mro.index(type_info)
         if index is None:
-            self.chk.fail(message_registry.SUPER_ARG_2_NOT_INSTANCE_OF_ARG_1, e)
-            return AnyType(TypeOfAny.from_error)
+            if (
+                instance_info.is_protocol
+                and instance_info != type_info
+                and not type_info.is_protocol
+            ):
+                # A special case for mixins, in this case super() should point
+                # directly to the host protocol, this is not safe, since the real MRO
+                # is not known yet for mixin, but this feature is more like an escape hatch.
+                index = -1
+            else:
+                self.chk.fail(message_registry.SUPER_ARG_2_NOT_INSTANCE_OF_ARG_1, e)
+                return AnyType(TypeOfAny.from_error)
 
         if len(mro) == index + 1:
             self.chk.fail(message_registry.TARGET_CLASS_HAS_NO_BASE_CLASS, e)

test-data/unit/check-selftype.test

@@ -792,6 +792,26 @@ reveal_type(f.copy())  # N: Revealed type is "__main__.File"
 b.copy()  # E: Invalid self argument "Bad" to attribute function "copy" with type "Callable[[T], T]"
 [builtins fixtures/tuple.pyi]
 
+[case testMixinProtocolSuper]
+from typing import Protocol
+
+class Base(Protocol):
+    def func(self) -> int:
+        ...
+
+class TweakFunc:
+    def func(self: Base) -> int:
+        return reveal_type(super().func())  # N: Revealed type is "builtins.int"
+
+class Good:
+    def func(self) -> int: ...
+class C(TweakFunc, Good): pass
+C().func()  # OK
+
+class Bad:
+    def func(self) -> str: ...
+class CC(TweakFunc, Bad): pass  # E: Definition of "func" in base class "TweakFunc" is incompatible with definition in base class "Bad"
+
 [case testBadClassLevelDecoratorHack]
 from typing_extensions import Protocol
 from typing import TypeVar, Any