Hi @vitaut

Among recent changes in v11 you've refactored traits for testing if type is a specialization of std::back_insert_iterator.
As a result, the following static assert no longer works:

#include <fmt/format.h>
#include <iterator>

using It = decltype(std::back_inserter(std::declval<fmt::memory_buffer&>()));
static_assert(fmt::detail::is_back_insert_iterator<It>::value);

This impacts e.g. fmt::format_to() that fails to select an optimized overload for std::back_insert_iterator<fmt::memory_buffer> and becomes horribly slow. I actually discovered this issue while working on a benchmark where I noticed that formatting to a preallocated fmt::memory_buffer is for some reason much slower than fmt::format().

To fix the issue you need to swap the headers order, i.e. make sure <iterator> is included before <fmt/base.h> s.t. ADL can really see the back_inserter(std::declval<Container&>()):

#include <iterator>
#include <fmt/format.h>

using It = decltype(std::back_inserter(std::declval<fmt::memory_buffer&>()));
static_assert(fmt::detail::is_back_insert_iterator<It>::value);

I suppose the easiest way to fix this is either revert to prev version of traits (which IMO is quite fine) or include <iterator> in the fmt/base.h.