https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87150
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
r251035 clearly has code that handles that rule:
+
+ if (flags & LOOKUP_PREFER_RVALUE)
+ {
+ /* The implicit move specified in 15.8.3/3 fails "...if the type of
+ the first parameter of the selected constructor is not an rvalue
+ reference to the object’s type (possibly cv-qualified)...." */
+ gcc_assert (!(complain & tf_error));
+ tree ptype = convs[0]->type;
+ if (TREE_CODE (ptype) != REFERENCE_TYPE
+ || !TYPE_REF_IS_RVALUE (ptype)
+ || CONVERSION_RANK (convs[0]) > cr_exact)
+ return error_mark_node;
+ }
The thing is that LOOKUP_PREFER_RVALUE isn't set in this case, as we trigger
first:
else if (MAYBE_CLASS_TYPE_P (to) && MAYBE_CLASS_TYPE_P (from)
&& is_properly_derived_from (from, to))
{
if (conv->kind == ck_rvalue)
conv = next_conversion (conv);
conv = build_conv (ck_base, to, conv);
/* The derived-to-base conversion indicates the initialization
of a parameter with base type from an object of a derived
type. A temporary object is created to hold the result of
the conversion unless we're binding directly to a reference. */
conv->need_temporary_p = !(flags & LOOKUP_NO_TEMP_BIND);
}
else
return NULL;
in standard_conversion, where
*conv
$17 = {kind = ck_rvalue, rank = cr_identity, user_conv_p = 0, ellipsis_p = 0,
this_p = 0, bad_p = 0, need_temporary_p = 0, base_p = 0,
rvaluedness_matches_p = 1, check_narrowing = 0, check_narrowing_const_only =
0, type = <record_type 0x7fffefdd32a0 S2>, u = {next = 0x2f92e80,
expr = <error_mark 0x2f92e80>, list = 0x2f92e80}, cand = 0x0}
and
*next_conversion (conv)
$18 = {kind = ck_identity, rank = cr_identity, user_conv_p = 0, ellipsis_p = 0,
this_p = 0, bad_p = 0, need_temporary_p = 0, base_p = 0,
rvaluedness_matches_p = 0, check_narrowing = 0, check_narrowing_const_only =
0, type = <record_type 0x7fffefdd32a0 S2>, u = {
next = 0x7fffefdbd6c0, expr = <indirect_ref 0x7fffefdbd6c0>, list =
0x7fffefdbd6c0}, cand = 0x0}
so we are no longer using ck_rvalue conversion with rvaluedness_matches_p, but
ck_base conversion instead.
