Suggest to use a protocol instead of a module (#13861)

Author: sobolevn

mypy/typeanal.py

@@ -761,8 +761,8 @@ def analyze_unbound_type_without_type_info(
             else:
                 notes.append('Perhaps you need "Callable[...]" or a callback protocol?')
         elif isinstance(sym.node, MypyFile):
-            # TODO: suggest a protocol when supported.
             message = 'Module "{}" is not valid as a type'
+            notes.append("Perhaps you meant to use a protocol matching the module structure?")
         elif unbound_tvar:
             message = 'Type variable "{}" is unbound'
             short = name.split(".")[-1]

test-data/unit/check-basic.test

@@ -350,7 +350,8 @@ from typing import Union
 class A: ...
 class B: ...
 
-x: Union[mock, A]  # E: Module "mock" is not valid as a type
+x: Union[mock, A]  # E: Module "mock" is not valid as a type \
+                   # N: Perhaps you meant to use a protocol matching the module structure?
 
 if isinstance(x, B):
     pass
@@ -366,7 +367,8 @@ from typing import overload, Any, Union
 @overload
 def f(x: int) -> int: ...
 @overload
-def f(x: str) -> Union[mock, str]: ...  # E: Module "mock" is not valid as a type
+def f(x: str) -> Union[mock, str]: ...  # E: Module "mock" is not valid as a type \
+                                        # N: Perhaps you meant to use a protocol matching the module structure?
 def f(x):
     pass
 

test-data/unit/check-errorcodes.test

@@ -245,7 +245,8 @@ x: f  # E: Function "__main__.f" is not valid as a type  [valid-type] \
       # N: Perhaps you need "Callable[...]" or a callback protocol?
 
 import sys
-y: sys  # E: Module "sys" is not valid as a type  [valid-type]
+y: sys  # E: Module "sys" is not valid as a type  [valid-type] \
+        # N: Perhaps you meant to use a protocol matching the module structure?
 z: y  # E: Variable "__main__.y" is not valid as a type  [valid-type] \
       # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
 [builtins fixtures/tuple.pyi]

test-data/unit/fine-grained.test

@@ -5576,9 +5576,12 @@ import T
 ==
 main:4: error: "C" expects no type arguments, but 1 given
 main:4: error: Module "T" is not valid as a type
+main:4: note: Perhaps you meant to use a protocol matching the module structure?
 main:6: error: Free type variable expected in Generic[...]
 main:7: error: Module "T" is not valid as a type
+main:7: note: Perhaps you meant to use a protocol matching the module structure?
 main:10: error: Module "T" is not valid as a type
+main:10: note: Perhaps you meant to use a protocol matching the module structure?
 main:10: error: Bad number of arguments for type alias, expected: 0, given: 1
 
 [case testChangeClassToModule]
@@ -5602,8 +5605,10 @@ import C
 ==
 ==
 main:3: error: Module "C" is not valid as a type
+main:3: note: Perhaps you meant to use a protocol matching the module structure?
 main:5: error: Module not callable
 main:8: error: Module "C" is not valid as a type
+main:8: note: Perhaps you meant to use a protocol matching the module structure?
 
 [case testChangeTypeVarToTypeAlias]
 
@@ -5653,8 +5658,10 @@ import D
 ==
 ==
 main:3: error: Module "D" is not valid as a type
+main:3: note: Perhaps you meant to use a protocol matching the module structure?
 main:5: error: Module not callable
 main:8: error: Module "D" is not valid as a type
+main:8: note: Perhaps you meant to use a protocol matching the module structure?
 
 [case testChangeTypeAliasToModuleUnqualified]
 
@@ -5680,8 +5687,10 @@ import D
 ==
 ==
 main:3: error: Module "D" is not valid as a type
+main:3: note: Perhaps you meant to use a protocol matching the module structure?
 main:5: error: Module not callable
 main:8: error: Module "D" is not valid as a type
+main:8: note: Perhaps you meant to use a protocol matching the module structure?
 
 [case testChangeFunctionToVariableAndRefreshUsingStaleDependency]
 import a