add note

Author: hauntsaninja

mypy/checkexpr.py

@@ -1669,10 +1669,17 @@ def check_callable_call(
             return callee.ret_type, callee
 
         if callee.is_type_obj() and callee.type_object().is_protocol:
+            protocol_name = callee.type_object().name
             self.chk.fail(
-                message_registry.CANNOT_INSTANTIATE_PROTOCOL.format(callee.type_object().name),
+                message_registry.CANNOT_INSTANTIATE_PROTOCOL.format(protocol_name),
                 context,
             )
+            if callee.from_type_type:
+                self.chk.note(
+                    f'Consider using "Callable[..., {protocol_name}]" '
+                    f'instead of "type[{protocol_name}]"',
+                    context,
+                )
         elif (
             callee.is_type_obj()
             and callee.type_object().is_abstract

test-data/unit/check-protocols.test

@@ -1603,7 +1603,8 @@ class C:
         pass
 
 def f(cls: Type[P]) -> P:
-    return cls()  # E: Cannot instantiate protocol class "P"
+    return cls()  # E: Cannot instantiate protocol class "P" \
+                  # N: Consider using "Callable[..., P]" instead of "type[P]"
 def g() -> P:
     return P()  # E: Cannot instantiate protocol class "P"
 
@@ -1625,7 +1626,8 @@ class C:
         pass
 
 def f(cls: Type[P]) -> P:
-    return cls()  # E: Cannot instantiate protocol class "P"
+    return cls()  # E: Cannot instantiate protocol class "P" \
+                  # N: Consider using "Callable[..., P]" instead of "type[P]"
 
 Alias = P
 GoodAlias = C
@@ -1646,14 +1648,16 @@ class C:
         pass
 
 var: Type[P]
-var()  # E: Cannot instantiate protocol class "P"
+var()  # E: Cannot instantiate protocol class "P" \
+       # N: Consider using "Callable[..., P]" instead of "type[P]"
 if int():
     var = P # E: Can only assign concrete classes to a variable of type "Type[P]"
     var = B # OK
     var = C # OK
 
 var_old = None # type: Type[P] # Old syntax for variable annotations
-var_old()  # E: Cannot instantiate protocol class "P"
+var_old()  # E: Cannot instantiate protocol class "P" \
+           # N: Consider using "Callable[..., P]" instead of "type[P]"
 if int():
     var_old = P # E: Can only assign concrete classes to a variable of type "Type[P]"
     var_old = B # OK
@@ -1669,7 +1673,8 @@ class Logger:
 class C(Protocol):
     @classmethod
     def action(cls) -> None:
-        cls()  # E: Cannot instantiate protocol class "C"
+        cls()  # E: Cannot instantiate protocol class "C" \
+               # N: Consider using "Callable[..., C]" instead of "type[C]"
         Logger.log(cls)  #OK for classmethods
 [builtins fixtures/classmethod.pyi]
 

test-data/unit/check-selftype.test

@@ -1023,7 +1023,8 @@ T = TypeVar('T', bound=HasX)
 class Meta(type):
     def do_x(cls: Type[T]) -> T:
         cls.x
-        return cls()  # E: Cannot instantiate protocol class "HasX"
+        return cls()  # E: Cannot instantiate protocol class "HasX" \
+                      # N: Consider using "Callable[..., HasX]" instead of "type[HasX]"
 
 class Good(metaclass=Meta):
     x: int