https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82840
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC changed behaviour for C++1z mode at r240889:
Further P0135 refinement.
* call.c (build_user_type_conversion_1): Consider conversions from
a single element in an initializer-list.
(build_temp): Undo early_elide_copy change.
(build_over_call): Check that we don't try to copy a TARGET_EXPR
in C++17 mode. Set user_conv_p here.
(convert_like_real): Not here.
(check_self_delegation): Split out from...
(build_special_member_call): ...here. Handle C++17 copy elision.
* cvt.c (early_elide_copy): Remove.
(ocp_convert): Undo early_elide_copy change.
* except.c (build_throw): Likewise.
* init.c (expand_default_init): Likewise.
* typeck.c (cp_build_modify_expr): Likewise.
That refers to the guaranteed copy elision paper: https://wg21.link/p0135r1
