When optimizing, g++ seems to try calling immediate functions inside "if consteval" and then reject what should be valid code.  Here is a simple example (which I place in the public domain):

```
// g++ -std=c++23 -c -O2 consteval.cc

consteval int
plusone(int n)
{
  return n + 1;
}

constexpr int
maybeplusone(int n)
{
  if consteval {
    return plusone(n);
  }
  else {
    return n;
  }
}

int
not_consteval()
{
  return maybeplusone(1);
}
```

It gives the following error:

```
$ g++ -std=c++23 -c -O2 consteval.cc
consteval.cc: In function 'int not_consteval()':
consteval.cc:23:22:   in 'constexpr' expansion of 'maybeplusone(1)'
consteval.cc:13:19: error: call to consteval function 'plusone(n)' is not a constant expression
   13 |     return plusone(n);
      |            ~~~~~~~^~~
consteval.cc:13:20: error: 'n' is not a constant expression
   13 |     return plusone(n);
      |                    ^
$ 
```

The code compiles fine without optimization, and also works with clang 18.