Add tests demonstrating common use of TYPE_CHECKING guards (#531)

* Add tests demonstrating common use of TYPE_CHECKING guards

* Add another example using a runtime guard for Protocol

Author: asottile

pyflakes/test/test_type_annotations.py

@@ -552,3 +552,47 @@ def test_partial_string_annotations_with_future_annotations(self):
             def f() -> Optional['Queue[str]']:
                 return None
         """)
+
+    def test_idomiatic_typing_guards(self):
+        # typing.TYPE_CHECKING: python3.5.3+
+        self.flakes("""
+            from typing import TYPE_CHECKING
+
+            if TYPE_CHECKING:
+                from t import T
+
+            def f():  # type: () -> T
+                pass
+        """)
+        # False: the old, more-compatible approach
+        self.flakes("""
+            if False:
+                from t import T
+
+            def f():  # type: () -> T
+                pass
+        """)
+        # some choose to assign a constant and do it that way
+        self.flakes("""
+            MYPY = False
+
+            if MYPY:
+                from t import T
+
+            def f():  # type: () -> T
+                pass
+        """)
+
+    def test_typing_guard_for_protocol(self):
+        self.flakes("""
+            from typing import TYPE_CHECKING
+
+            if TYPE_CHECKING:
+                from typing import Protocol
+            else:
+                Protocol = object
+
+            class C(Protocol):
+                def f():  # type: () -> int
+                    pass
+        """)