https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116696
Bug ID: 116696
Summary: function template not considered constexpr even with
-fimplicit-constexpr
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
I was trying to implement constexpr std::format as cheaply as possible — by
using -fimplicit-constexpr instead of bothering to mark dozens (if not more)
functions constexpr. That mostly worked quite well.
Oddly, there was one function template that did not get marked constexpr
implicitly, which was this one:
https://github.com/gcc-mirror/gcc/blob/8c01976b8e34eaa2483ab37d1bd18ebc5c8ada95/libstdc%2B%2B-v3/include/std/format#L736
You can see this on compiler explorer here: https://godbolt.org/z/xq1zvqPWh
(the function template in question is on line 733)
The example fails with:
<source>:1355:42: error: call to non-'constexpr' function '_Out
std::__format::__write_padded(_Out, std::basic_string_view<_CharT>, _Align,
std::size_t, char32_t) [with _Out = _Sink_iter<char>; _CharT = char;
std::size_t = long unsigned int]'
1355 | return __format::__write_padded(std::move(__out), __str,
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
1356 | __align, __nfill,
__fill_char);
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:734:5: note: '_Out std::__format::__write_padded(_Out,
std::basic_string_view<_CharT>, _Align, std::size_t, char32_t) [with _Out =
_Sink_iter<char>; _CharT = char; std::size_t = long unsigned int]' declared
here
734 | __write_padded(_Out __out, basic_string_view<_CharT> __str,
| ^~~~~~~~~~~~~~
Notably, there are lots of other function templates not marked constexpr in
this header that work just fine with -fimplicit-constexpr. Not sure what's
special about this one.
