[Bug c++/124352] New: Forced inlining and constexpr combined change diagnostics of ill-formed code

[ivan.lazaric.gcc at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124352

            Bug ID: 124352
           Summary: Forced inlining and constexpr combined change
                    diagnostics of ill-formed code
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ivan.lazaric.gcc at gmail dot com
  Target Milestone: ---

```
template <typename>
struct arg_store {
  int data;

  static constexpr int create() {
    static_assert(false);
    return 0;
  }

#ifdef CONSTEXPR
  constexpr 
#endif
  arg_store(auto&&) : data(create()) {}
};

#ifdef INLINE
[[__gnu__::__always_inline__]]
#endif
constexpr auto make_format_args(auto& arg) {
  return arg_store<int>(arg);
}

struct S {};

void fn() {
  S s;
  (void)make_format_args(s);
}
```

This code is ill-formed, regardless whether `INLINE` or `CONSTEXPR` are
defined.

Consider 3 compilations, with flags:
1) "-std=c++26 -DCONSTEXPR -fdiagnostics-plain-output"
2) "-std=c++26 -DINLINE -fdiagnostics-plain-output"
3) "-std=c++26 -DINLINE -DCONSTEXPR -fdiagnostics-plain-output"

1 and 2 produce (near) identical diagnostics:
```
<source>: In instantiation of 'static constexpr int arg_store<
<template-parameter-1-1> >::create() [with <template-parameter-1-1> = int]':
<source>:13:34:   required from '**constexpr** arg_store<
<template-parameter-1-1> >::arg_store(auto:1&&) [with auto:1 = S&;
<template-parameter-1-1> = int]'
<source>:20:10:   required from 'constexpr auto make_format_args(auto:2&) [with
auto:2 = S]'
<source>:27:25:   required from here
<source>:6:19: error: static assertion failed
<source>:6:19: note: 'false' evaluates to false
```

They only differ in the **constexpr** part.

3 produces significantly different diagnostic:
```
<source>: In instantiation of 'static constexpr int arg_store<
<template-parameter-1-1> >::create() [with <template-parameter-1-1> = int]':
<source>:13:34:   required from here
<source>:27:25:   in 'constexpr' expansion of 'make_format_args<S>(s)'
<source>:20:10:   in 'constexpr' expansion of 'arg_store<int>((* & arg))'
<source>:6:19: error: static assertion failed
<source>:6:19: note: 'false' evaluates to false
```

The "required from here" is now pointing at a different line,
arg_store constructor instead of the preferred make_format_args function call.

Godbolt link showcasing the 3 compilations: https://godbolt.org/z/Tcvx4oWfo