`[[assume(++a)]]` warns that the side-effects will be discarded, but they are not discarded at compile time

[tbaederr]

constexpr int a() {
  int a = 0;
  [[assume(++a)]];
  return a;
}
static_assert(a() == 1);

#include <iostream>

int main() {
  std::cout << a() << "\n";
}

This code prints 0, even though the static_assert doesn't fail. Clang does print a warning about the side-effects of ++a being discarded, but that is not true at compile time.

CC @Sirraide

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Timm Baeder (tbaederr)

```c++ constexpr int a() { int a = 0; [[assume(++a)]]; return a; } static_assert(a() == 1);

#include <iostream>

int main() {
std::cout << a() << "\n";
}

This code prints `0`, even though the `static_assert` doesn't fail. Clang does print a warning about the side-effects of `++a` being discarded, but that is not true at compile time.

CC @<!-- -->Sirraide 
</details>

[Sirraide]

Oh, duh, I forgot to check for side-effects in constant evaluation; that’s an easy fix, fortunately.