Getting warning "mangle of arg not implemened"

[correaa]

When compiling a code like this:

template<class Reference>
auto operator+(Reference&& ref)
->decltype(array(std::forward<Reference>(ref))) {
	return array(std::forward<Reference>(ref)); }

https://gitlab.com/correaa/boost-multi/-/blob/master/include/boost/multi/array.hpp?ref_type=heads#L1555

I get this warning from the compiler "mangle of arg not implemened":

[1/140] ccache /home/correaa/bin/circle -DTBB_FOUND=1 -DTBB_USE_DEBUG -I/home/correaa/boost-multi/include -g -Werror -Wall -MD -MT test/CMakeFiles/allocator.cpp.x.dir/allocator.cpp.o -MF test/CMakeFiles/allocator.cpp.x.dir/allocator.cpp.o.d -o test/CMakeFiles/allocator.cpp.x.dir/allocator.cpp.o -c /home/correaa/boost-multi/test/allocator.cpp
../include/boost/multi/array.hpp:1555:17
mangle of arg not implemened../include/boost/multi/array.hpp:1555:17

https://gitlab.com/correaa/boost-multi/-/jobs/8149345102#L644

This happens with circle 221.
The program seems to still compile, run and work though.

At the minimum, the warning message is not well formatted when printed in the screen. (It also has a typo).