gh-123942: add missing test for docstring-handling code in ast_opt.c (#123943)

Author: iritkatriel

Lib/test/test_compile.py

@@ -870,6 +870,32 @@ def unused_code_at_end():
             'RETURN_CONST',
             list(dis.get_instructions(unused_code_at_end))[-1].opname)
 
+    @support.cpython_only
+    def test_docstring(self):
+        src = textwrap.dedent("""
+            def with_docstring():
+                "docstring"
+
+            def with_fstring():
+                f"not docstring"
+
+            def with_const_expression():
+                "also" + " not docstring"
+            """)
+
+        for opt in [0, 1, 2]:
+            with self.subTest(opt=opt):
+                code = compile(src, "<test>", "exec", optimize=opt)
+                ns = {}
+                exec(code, ns)
+
+                if opt < 2:
+                    self.assertEqual(ns['with_docstring'].__doc__, "docstring")
+                else:
+                    self.assertIsNone(ns['with_docstring'].__doc__)
+                self.assertIsNone(ns['with_fstring'].__doc__)
+                self.assertIsNone(ns['with_const_expression'].__doc__)
+
     @support.cpython_only
     def test_docstring_omitted(self):
         # See gh-115347