Fix crashes on special forms in protocol bodies (#13526)

Fixes #6801
Fixes #10577
Fixes #12642
Fixes #12337
Fixes #10639 
Fixes #13390 

All these crashes are in a sense duplicates of each other. Fix is trivial, except I decided to ban type aliases in protocol bodies. Already in the examples in issues, I have found two cases where people wrote `foo = list[str]`, where they clearly wanted `foo: list[str]`. This can cause hard to spot false negatives.

Author: ilevkivskyi

mypy/nodes.py

@@ -2906,7 +2906,10 @@ def protocol_members(self) -> list[str]:
         assert self.mro, "This property can be only accessed after MRO is (re-)calculated"
         for base in self.mro[:-1]:  # we skip "object" since everyone implements it
             if base.is_protocol:
-                for name in base.names:
+                for name, node in base.names.items():
+                    if isinstance(node.node, (TypeAlias, TypeVarExpr)):
+                        # These are auxiliary definitions (and type aliases are prohibited).
+                        continue
                     members.add(name)
         return sorted(list(members))
 

mypy/semanal.py

@@ -3269,6 +3269,12 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
         current_node = existing.node if existing else alias_node
         assert isinstance(current_node, TypeAlias)
         self.disable_invalid_recursive_aliases(s, current_node)
+        if self.is_class_scope():
+            assert self.type is not None
+            if self.type.is_protocol:
+                self.fail("Type aliases are prohibited in protocol bodies", s)
+                if not lvalue.name[0].isupper():
+                    self.note("Use variable annotation syntax to define protocol members", s)
         return True
 
     def disable_invalid_recursive_aliases(

test-data/unit/check-protocols.test

@@ -3546,3 +3546,34 @@ S = TypeVar("S")
 def test(arg: P[S]) -> S: ...
 b: Type[B]
 reveal_type(test(b))  # N: Revealed type is "__main__.B"
+
+[case testTypeAliasInProtocolBody]
+from typing import Protocol, List
+
+class P(Protocol):
+    x = List[str]  # E: Type aliases are prohibited in protocol bodies \
+                   # N: Use variable annotation syntax to define protocol members
+
+class C:
+    x: int
+def foo(x: P) -> None: ...
+foo(C())  # No extra error here
+[builtins fixtures/list.pyi]
+
+[case testTypeVarInProtocolBody]
+from typing import Protocol, TypeVar
+
+class C(Protocol):
+    T = TypeVar('T')
+    def __call__(self, t: T) -> T: ...
+
+def f_bad(t: int) -> int:
+    return t
+
+S = TypeVar("S")
+def f_good(t: S) -> S:
+    return t
+
+g: C = f_bad  # E: Incompatible types in assignment (expression has type "Callable[[int], int]", variable has type "C") \
+              # N: "C.__call__" has type "Callable[[Arg(T, 't')], T]"
+g = f_good  # OK