fix false positive

Author: hauntsaninja

mypy/join.py

@@ -671,6 +671,9 @@ def is_better(t: Type, s: Type) -> bool:
     if isinstance(t, Instance):
         if not isinstance(s, Instance):
             return True
+        if t.type.is_protocol != s.type.is_protocol and s.type.fullname != "builtins.object":
+            # mro of protocol is not really relevant
+            return not t.type.is_protocol
         # Use len(mro) as a proxy for the better choice.
         if len(t.type.mro) > len(s.type.mro):
             return True

test-data/unit/check-protocols.test

@@ -4460,3 +4460,26 @@ f2(a4)  # E: Argument 1 to "f2" has incompatible type "A4"; expected "P2" \
         # N:     foo: expected "B1", got "str" \
         # N:     foo: expected setter type "C1", got "str"
 [builtins fixtures/property.pyi]
+
+
+[case testExplicitProtocolJoinPreference]
+from typing import Protocol, TypeVar
+
+T = TypeVar("T")
+
+class Proto1(Protocol):
+    def foo(self) -> int: ...
+class Proto2(Proto1):
+    def bar(self) -> str: ...
+class Proto3(Proto2):
+    def baz(self) -> str: ...
+
+class Base: ...
+
+class A(Base, Proto3): ...
+class B(Base, Proto3): ...
+
+def join(a: T, b: T) -> T: ...
+
+def main(a: A, b: B) -> None:
+    reveal_type(join(a, b))  # N: Revealed type is "__main__.Proto3"