Suggest importing from typing_extensions (#14591)

mypy/semanal.py

@@ -2597,6 +2597,16 @@ def report_missing_module_attribute(
             ):
                 # Yes. Generate a helpful note.
                 self.msg.add_fixture_note(fullname, context)
+            else:
+                typing_extensions = self.modules.get("typing_extensions")
+                if typing_extensions and source_id in typing_extensions.names:
+                    self.msg.note(
+                        f"Use `from typing_extensions import {source_id}` instead", context
+                    )
+                    self.msg.note(
+                        "See https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-new-additions-to-the-typing-module",
+                        context,
+                    )
 
     def process_import_over_existing_name(
         self,

test-data/unit/check-statements.test

@@ -2198,3 +2198,17 @@ def foo(x: int) -> Union[Generator[A, None, None], Generator[B, None, None]]:
 [case testNoCrashOnStarRightHandSide]
 x = *(1, 2, 3)  # E: Can use starred expression only as assignment target
 [builtins fixtures/tuple.pyi]
+
+
+[case testTypingExtensionsSuggestion]
+from typing import _FutureFeatureFixture
+
+# This import is only needed in tests. In real life, mypy will always have typing_extensions in its
+# build due to its pervasive use in typeshed. This assumption may one day prove False, but when
+# that day comes this suggestion will also be less helpful than it is today.
+import typing_extensions
+[out]
+main:1: error: Module "typing" has no attribute "_FutureFeatureFixture"
+main:1: note: Use `from typing_extensions import _FutureFeatureFixture` instead
+main:1: note: See https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-new-additions-to-the-typing-module
+[builtins fixtures/tuple.pyi]

test-data/unit/lib-stub/typing_extensions.pyi

@@ -59,3 +59,5 @@ def TypedDict(typename: str, fields: Dict[str, Type[_T]], *, total: Any = ...) -
 def reveal_type(__obj: T) -> T: pass
 
 def dataclass_transform() -> Callable[[T], T]: ...
+
+_FutureFeatureFixture = 0