Fix annotation clobbering __all__ (#606)

pyflakes/checker.py

@@ -1140,7 +1140,10 @@ def addBinding(self, node, value):
             # then assume the rebound name is used as a global or within a loop
             value.used = self.scope[value.name].used
 
-        self.scope[value.name] = value
+        # don't treat annotations as assignments if there is an existing value
+        # in scope
+        if value.name not in self.scope or not isinstance(value, Annotation):
+            self.scope[value.name] = value
 
     def _unknown_handler(self, node):
         # this environment variable configures whether to error on unknown

pyflakes/test/test_type_annotations.py

@@ -335,6 +335,19 @@ def f(t: T): pass
         def g(t: 'T'): pass
         ''')
 
+    @skipIf(version_info < (3, 6), 'new in Python 3.6')
+    def test_type_annotation_clobbers_all(self):
+        self.flakes('''\
+        from typing import TYPE_CHECKING, List
+
+        from y import z
+
+        if not TYPE_CHECKING:
+            __all__ = ("z",)
+        else:
+            __all__: List[str]
+        ''')
+
     def test_typeCommentsMarkImportsAsUsed(self):
         self.flakes("""
         from mod import A, B, C, D, E, F, G