async is actually a token that cannot appear in brackets

parso/python/tokenize.py

@@ -258,6 +258,8 @@ def _create_token_collection(version_info):
 
     ALWAYS_BREAK_TOKENS = (';', 'import', 'class', 'def', 'try', 'except',
                            'finally', 'while', 'with', 'return')
+    if version_info >= (3, 5):
+        ALWAYS_BREAK_TOKENS += ('async',)
     pseudo_token_compiled = _compile(PseudoToken)
     return TokenCollection(
         pseudo_token_compiled, single_quoted, triple_quoted, endpats,

test/test_diff_parser.py

@@ -1487,3 +1487,25 @@ def test_invalid_function(differ):
         ''')
     differ.initialize(code1)
     differ.parse(code2, parsers=1, copies=1, expect_error_leaves=True)
+
+
+def test_async_func2(differ):
+    code1 = dedent('''\
+        async def foo():
+            return ''
+        @bla
+        async def foo():
+            x
+        ''')
+    code2 = dedent('''\
+        async def foo():
+            return ''
+
+          {
+        @bla
+        async def foo():
+            x
+          y
+        ''')
+    differ.initialize(code1)
+    differ.parse(code2, parsers=2, copies=1, expect_error_leaves=True)