[Bug c++/126420] Rejects valid? deducing return type with expansion statement (empty)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126420

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The difference between
auto
foo ()
{
  template for (auto x : {})
    return 42;
  return 42L;
}
and
template <bool X>
auto
foo ()
{
  if (X)
    return 42;
  else
    return 42L;
}
is in the check_return_expr condition:
11591         if ((WILDCARD_TYPE_P (TREE_TYPE (DECL_RESULT
(current_function_decl)))
11592              && !FNDECL_USED_AUTO (current_function_decl))
11593             || (retval != NULL_TREE
11594                 && (TREE_TYPE (retval) == NULL_TREE
11595                     || WILDCARD_TYPE_P (TREE_TYPE (retval)))))
11596           goto dependent;
Now, in neither cases is retval dependent here, WILDCARD_TYPE_P is true in both
cases the first time we run into this.  But the difference is in in
FNDECL_USED_AUTO flag,
in the first testcase it is set because of start_preparsed_function:
20019     if (!processing_template_decl && type_uses_auto (restype))
20020       {
20021         FNDECL_USED_AUTO (decl1) = true;
20022         DECL_SAVED_AUTO_RETURN_TYPE (decl1) = restype;
20023       }
while in the second case it is not.  Perhaps we should just goto dependent;
also whenever in_expansion_stmt ?