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FMT_COMPILE fails to format std::array #4949
Description
On main (e4751cc), this fails to compile with GCC 15.2 and -std=c++20. It compiles and runs correctly with fmt 12.2.0.
Compiler Explorer: https://godbolt.org/z/hxMdoozx8
#include <array>
#include <fmt/compile.h>
#include <fmt/ranges.h>
int main() {
std::array<int, 3> values{1, 2, 3};
auto s = fmt::format(FMT_COMPILE("array={}"), values);
fmt::print("{}\n", s);
}Expected output:
array=[1, 2, 3]
It works without FMT_COMPILE.
This issue seems to have been introduced in 61e3b92.
A branch in nested_format_specs::write attempts to construct an iterator-specific FormatContext from a basic_appender<char>, which cannot be converted to a std::back_insert_iterator<std::string> as required by the generic_context<std::back_insert_iterator<std::string>, char> constructor. This example never actually tries to construct the FormatContext (as the branch is for padding, and we don't specify a width), but it still needs to be instantiated.
Activity
HarshXAI commentedon Sep 29, 2026
I can take this.
FMT_COMPILEfailing onstd::arraylooks like a missing formatter / compile-time path for contiguous fixed-size ranges. I will reproduce against currentmain, check howstd::span/ other ranges are handled in the compile-time formatter, and add a failing test then the fix so both runtime andFMT_COMPILEformatstd::arraythe same way.Will open a PR with a focused test case once I have the repro locked down.
mayankjaiswal12 commentedon Oct 5, 2026
I opened #4974 with a fix and the reproducer from this issue as a test in compile-test.cc. It keeps the outer context's arguments, so dynamic element specs like {:*>20:{}} also work.