patch 8.2.2886: various pieces of code not covered by tests

Problem:    Various pieces of code not covered by tests.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #8255)
diff --git a/src/testdir/test_user_func.vim b/src/testdir/test_user_func.vim
index 4b8ec99..747ec48 100644
--- a/src/testdir/test_user_func.vim
+++ b/src/testdir/test_user_func.vim
@@ -160,6 +160,16 @@
 	\ .. "1    return deepcopy(a:)\n"
 	\ .. "   endfunction",
 	\ execute('func Args2'))
+
+  " Error in default argument expression
+  let l =<< trim END
+    func F1(x = y)
+      return a:x * 2
+    endfunc
+    echo F1()
+  END
+  let @a = l->join("\n")
+  call assert_fails("exe @a", 'E121:')
 endfunc
 
 func s:addFoo(lead)