prevent rewriting union types with forward annotations

pyupgrade/_plugins/typing_pep604.py

@@ -126,6 +126,15 @@ def _supported_version(state: State) -> bool:
     )
 
 
+def _any_arg_is_str(node_slice: ast.expr) -> bool:
+    return (
+        isinstance(node_slice, ast.Str) or (
+            isinstance(node_slice, ast.Tuple) and
+            any(isinstance(elt, ast.Str) for elt in node_slice.elts)
+        )
+    )
+
+
 @register(ast.Subscript)
 def visit_Subscript(
         state: State,
@@ -135,6 +144,17 @@ def visit_Subscript(
     if not _supported_version(state):
         return
 
+    # prevent rewriting forward annotations
+    if (
+            (sys.version_info >= (3, 9) and _any_arg_is_str(node.slice)) or
+            (
+                sys.version_info < (3, 9) and
+                isinstance(node.slice, ast.Index) and
+                _any_arg_is_str(node.slice.value)
+            )
+    ):
+        return
+
     if is_name_attr(node.value, state.from_imports, 'typing', ('Optional',)):
         yield ast_to_offset(node), _fix_optional
     elif is_name_attr(node.value, state.from_imports, 'typing', ('Union',)):

tests/features/typing_pep604_test.py

@@ -41,6 +41,19 @@
             (3, 10),
             id='3.10+ empty Union',
         ),
+        # https://github.com/asottile/pyupgrade/issues/567
+        pytest.param(
+            'from typing import Optional\n'
+            'def f() -> Optional["str"]: ...\n',
+            (3, 10),
+            id='3.10+ Optional of forward reference',
+        ),
+        pytest.param(
+            'from typing import Union\n'
+            'def f() -> Union[int, "str"]: ...\n',
+            (3, 10),
+            id='3.10+ Union of forward reference',
+        ),
     ),
 )
 def test_fix_pep604_types_noop(s, version):