Fix temp cleanup in async functions when the return value is in a temp variable.

Closes #3337.

CHANGES.rst

@@ -5,6 +5,9 @@ Cython Changelog
 0.29.15 (20??-??-??)
 ====================
 
+* Crash when returning a temporary Python object from an async-def function.
+  (Github issue #3337)
+
 * Crash when using ``**kwargs`` in generators.
   Patch by David Woods.  (Github issue #3265)
 

Cython/Compiler/Nodes.py

@@ -5996,6 +5996,7 @@ class ReturnStatNode(StatNode):
                     rhs=value,
                     code=code,
                     have_gil=self.in_nogil_context)
+                value.generate_post_assignment_code(code)
             elif self.in_generator:
                 # return value == raise StopIteration(value), but uncatchable
                 code.globalstate.use_utility_code(

tests/run/async_def.pyx

+# cython: language_level=3, binding=True
+# mode: run
+# tag: pep492, await, gh3337
+
+"""
+Cython specific tests in addition to "test_coroutines_pep492.pyx"
+(which is copied from CPython).
+"""
+
+import sys
+
+
+def run_async(coro):
+    #assert coro.__class__ is types.GeneratorType
+    assert coro.__class__.__name__ in ('coroutine', '_GeneratorWrapper'), coro.__class__.__name__
+
+    buffer = []
+    result = None
+    while True:
+        try:
+            buffer.append(coro.send(None))
+        except StopIteration as ex:
+            result = ex.value if sys.version_info >= (3, 5) else ex.args[0] if ex.args else None
+            break
+    return buffer, result
+
+
+async def test_async_temp_gh3337(x, y):
+    """
+    >>> run_async(test_async_temp_gh3337(2, 3))
+    ([], -1)
+    >>> run_async(test_async_temp_gh3337(3, 2))
+    ([], 0)
+    """
+    return min(x - y, 0)