support TypeAlias annotations (#679)

For assignments with `TypeAlias` as annotation, handle the value as an annotation also.

Avoids incorrectly detecting `Bar` as unused in `x: TypeAlias = 'Bar'`.

Author: lkadian

pyflakes/checker.py

@@ -128,7 +128,7 @@ def _is_const_non_singleton(node):  # type: (ast.AST) -> bool
     return _is_constant(node) and not _is_singleton(node)
 
 
-def _is_name_or_attr(node, name):  # type: (ast.Ast, str) -> bool
+def _is_name_or_attr(node, name):  # type: (ast.AST, str) -> bool
     return (
         (isinstance(node, ast.Name) and node.id == name) or
         (isinstance(node, ast.Attribute) and node.attr == name)
@@ -2381,9 +2381,13 @@ def EXCEPTHANDLER(self, node):
     def ANNASSIGN(self, node):
         self.handleNode(node.target, node)
         self.handleAnnotation(node.annotation, node)
+        # If the assignment has value, handle the *value* now.
         if node.value:
-            # If the assignment has value, handle the *value* now.
-            self.handleNode(node.value, node)
+            # If the annotation is `TypeAlias`, handle the *value* as an annotation.
+            if _is_typing(node.annotation, 'TypeAlias', self.scopeStack):
+                self.handleAnnotation(node.value, node)
+            else:
+                self.handleNode(node.value, node)
 
     def COMPARE(self, node):
         left = node.left

pyflakes/test/test_type_annotations.py

@@ -300,6 +300,46 @@ def g(t: 'T'): pass
         a: 'a: "A"'
         ''', m.ForwardAnnotationSyntaxError)
 
+    @skipIf(version_info < (3, 6), 'new in Python 3.6')
+    def test_TypeAlias_annotations(self):
+        self.flakes("""
+        from typing_extensions import TypeAlias
+        from foo import Bar
+
+        bar: TypeAlias = Bar
+        """)
+        self.flakes("""
+        from typing_extensions import TypeAlias
+        from foo import Bar
+
+        bar: TypeAlias = 'Bar'
+        """)
+        self.flakes("""
+        from typing_extensions import TypeAlias
+        from foo import Bar
+
+        class A:
+            bar: TypeAlias = Bar
+        """)
+        self.flakes("""
+        from typing_extensions import TypeAlias
+        from foo import Bar
+
+        class A:
+            bar: TypeAlias = 'Bar'
+        """)
+        self.flakes("""
+        from typing_extensions import TypeAlias
+
+        bar: TypeAlias
+        """)
+        self.flakes("""
+        from typing_extensions import TypeAlias
+        from foo import Bar
+
+        bar: TypeAlias
+        """, m.UnusedImport)
+
     @skipIf(version_info < (3, 6), 'new in Python 3.6')
     def test_annotating_an_import(self):
         self.flakes('''