[Bug c++/125398] New: [reflection] Use of std::meta::reflect_constant_array across module boundary results in error

[friedkeenan at protonmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125398

            Bug ID: 125398
           Summary: [reflection] Use of std::meta::reflect_constant_array
                    across module boundary results in error
           Product: gcc
           Version: 16.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: friedkeenan at protonmail dot com
  Target Milestone: ---

If one has a module `mod` in a `mod.cpp` file with the following code:

```
module;

#include <meta>

export module mod;

export consteval auto mod_func(auto &&r) -> std::meta::info {
    return std::meta::reflect_constant_array(r);
}
```

And a `main.cpp` file which imports and uses that module:

```
import mod;

int main() {
    constexpr auto s = mod_func("string");
}
```

And with the following `CMakeLists.txt` to connect them together:

```
cmake_minimum_required(VERSION 4.1)

project(example)

add_library(mod)

target_compile_features(mod PUBLIC cxx_std_26)

target_compile_options(mod PUBLIC -freflection)

target_sources(mod PUBLIC
    FILE_SET CXX_MODULES
    FILES
        mod.cpp
)

add_executable(main main.cpp)

target_link_libraries(main PRIVATE mod)
```

Then this will result in the following error when building:

```
main.cpp:4:32:   in 'constexpr' expansion of 'mod_func@mod<const char
(&)[7]>("string")'
    4 |     constexpr auto s = mod_func("string");
      |                        ~~~~~~~~^~~~~~~~~~
mod.cpp:8:45: error: 'std::ranges' is not a namespace
    8 |     return std::meta::reflect_constant_array(r);
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
main.cpp:4:32:   in 'constexpr' expansion of 'mod_func@mod<const char
(&)[7]>("string")'
    4 |     constexpr auto s = mod_func("string");
      |                        ~~~~~~~~^~~~~~~~~~
mod.cpp:8:45: error: 'std::ranges' is not a namespace
    8 |     return std::meta::reflect_constant_array(r);
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
main.cpp:4:32:   in 'constexpr' expansion of 'mod_func@mod<const char
(&)[7]>("string")'
    4 |     constexpr auto s = mod_func("string");
      |                        ~~~~~~~~^~~~~~~~~~
mod.cpp:8:45: error: 'std::ranges' is not a namespace
    8 |     return std::meta::reflect_constant_array(r);
      |
```

This error also occurs when using `std::meta::rwflect_constant_string`, and
`std::define_static_string`, the latter of which is how I came upon the error.

Here is a Godbolt link for the erroring project:
https://godbolt.org/z/b6o9Kd3hv

If one adds `#include <ranges>` to `main.cpp`, then with GCC trunk the error
can be avoided. This doesn't seem to work with my local GCC 16.1.1, however,
which seems to get caught on some separate bug that's been fixed in trunk.