[Bug c++/121238] New: `-fmodules` causes invalid runtime exceptions with `std::format`

[rohan at rohanlean dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121238

            Bug ID: 121238
           Summary: `-fmodules` causes invalid runtime exceptions with
                    `std::format`
           Product: gcc
           Version: 15.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rohan at rohanlean dot de
  Target Milestone: ---

The following two files, compiled with `-std=c++23 -fmodules` give a runtime
exception, `format error: invalid width or precision in format-spec`:

```
//--- bug.cppm

module;

#include <format>

export module bug;

void bug(std::format_args);


//--- bug.cpp

#include <print>

import bug;

auto g(std::string_view fmt, std::format_args args) -> void
{
   std::print("{}\n", std::vformat(fmt, args));
}

template<typename... Ts>
auto f(std::format_string<Ts...> fmt, Ts &&... xs) -> void
{
   g(fmt.get(), std::make_format_args(xs...));
}

int main()
{
   f("{:5.1f}", 3.14);
}
```

It took me a while to reduce to this, but the title and reproducer are probably
still far from the heart of problem; sorry about that.