Disallow calling type on Protocol

Fixes #16919, fixes #16890

mypy/checkexpr.py

@@ -1802,9 +1802,12 @@ 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"))
         ):
+            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=TypeType.make_normalized(arg_types[0]))
 
         if callable_node:

test-data/unit/check-protocols.test

@@ -4215,3 +4215,21 @@ 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 "Type[__main__.P]"
+reveal_type(value.foo)  # N: Revealed type is "def (self: __main__.P)"
+
+[file mod.py]
+def foo() -> None: ...
+
+[builtins fixtures/tuple.pyi]