Allow protocols with untyped "__slots__" (#9314)

Closes #7290.

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Author: sobolevn

mypy/semanal.py

@@ -2320,8 +2320,8 @@ def process_type_annotation(self, s: AssignmentStmt) -> None:
                 if isinstance(lvalue.node, Var):
                     lvalue.node.is_abstract_var = True
         else:
-            if (any(isinstance(lv, NameExpr) and lv.is_inferred_def for lv in s.lvalues) and
-                    self.type and self.type.is_protocol and not self.is_func_scope()):
+            if (self.type and self.type.is_protocol and
+                    self.is_annotated_protocol_member(s) and not self.is_func_scope()):
                 self.fail('All protocol members must have explicitly declared types', s)
             # Set the type if the rvalue is a simple literal (even if the above error occurred).
             if len(s.lvalues) == 1 and isinstance(s.lvalues[0], RefExpr):
@@ -2332,6 +2332,19 @@ def process_type_annotation(self, s: AssignmentStmt) -> None:
             for lvalue in s.lvalues:
                 self.store_declared_types(lvalue, s.type)
 
+    def is_annotated_protocol_member(self, s: AssignmentStmt) -> bool:
+        """Check whether a protocol member is annotated.
+
+        There are some exceptions that can be left unannotated, like ``__slots__``."""
+        return any(
+            (
+                isinstance(lv, NameExpr)
+                and lv.name != '__slots__'
+                and lv.is_inferred_def
+            )
+            for lv in s.lvalues
+        )
+
     def analyze_simple_literal_type(self, rvalue: Expression, is_final: bool) -> Optional[Type]:
         """Return builtins.int if rvalue is an int literal, etc.
 

test-data/unit/check-protocols.test

@@ -2499,3 +2499,11 @@ reveal_type(abs(3))  # N: Revealed type is 'builtins.int*'
 reveal_type(abs(ALL))  # N: Revealed type is 'builtins.int*'
 [builtins fixtures/float.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testProtocolWithSlots]
+from typing import Protocol
+
+class A(Protocol):
+    __slots__ = ()
+
+[builtins fixtures/tuple.pyi]