Disallow calling type on Protocol

Fixes python#16919, fixes python#16890

Author: hauntsaninja

mypy/checkexpr.py

@@ -1802,10 +1802,15 @@ def check_callable_call(
 
         if (
             callee.is_type_obj()
-            and (len(arg_types) == 1)
+            and len(arg_types) == 1
             and is_equivalent(callee.ret_type, self.named_type("builtins.type"))
         ):
-            callee = callee.copy_modified(ret_type=TypeType.make_normalized(arg_types[0]))
+            proper_arg = get_proper_type(arg_types[0])
+            if isinstance(proper_arg, Instance) and proper_arg.type.is_protocol:
+                self.msg.fail("Calling type() on a protocol class is unsound", context)
+                callee = callee.copy_modified(ret_type=self.named_type("builtins.object"))
+            else:
+                callee = callee.copy_modified(ret_type=TypeType.make_normalized(arg_types[0]))
 
         if callable_node:
             # Store the inferred callable type.

test-data/unit/check-protocols.test

@@ -4215,3 +4215,22 @@ def g4(a: Input[bytes], b: Output[str]) -> None:
     f(a, b)  # E: Cannot infer type argument 1 of "f"
 
 [builtins fixtures/tuple.pyi]
+
+[case testTypeCallOnProtocol]
+from typing import Protocol
+
+import mod
+
+class P(Protocol):
+    def foo(self) -> None: ...
+
+a: P = mod
+value = type(a)  # E: Calling type() on a protocol class is unsound
+reveal_type(value)  # N: Revealed type is "builtins.object"
+reveal_type(value.foo)  # E: "object" has no attribute "foo" \
+                        # N: Revealed type is "Any"
+
+[file mod.py]
+def foo() -> None: ...
+
+[builtins fixtures/tuple.pyi]