@llvm/issue-subscribers-clang-frontend

This is DR2631 I think this is on @cor3ntin list but maybe not.

On second thought, maybe it is not DR2631: https://godbolt.org/z/5fr9PEW99

constexpr int g();
consteval int f() {
  return g();
}

int k(int x = f()) {  // error: constexpr evaluation of undefined function g
  return x;
}

constexpr int g() {
  return 42;
}

int main() {
  return k();
}

Although it looks related.

@shafik at a glance, it looks like CWG2631 - I apparently forgot to implement the case where the default argument appartain to a template parameter. I'll try to put it on my infinite plate.

Thanks for reporting that @cjdb

@cor3ntin , if you don't have enough time for this bug ATM and don't mind, I can take a look.

@Fznamznon If you have time sure, that sounds great! Thanks

Clang evaluates and checks constant expression in default non-type template argument once it is met by parser, gcc seems to be doing that only if the default value is required. The illustrating example https://godbolt.org/z/vKofvWees
I was able to make clang do the same and the test doesn't error out anymore.
I've noticed there is some amount of work done in https://reviews.llvm.org/D136554 to make std::source_location::current have the value of location where default argument was required. So, Is my understanding correct that something like:

#include <source_location>

template <int sl = std::source_location::current().line()> struct SLS {
  static const int get = sl;
};

static_assert(SLS<>::get == __LINE__);
static_assert(SLS<>::get == __LINE__);
}

Should compile fine? gcc doesn't compile this. https://godbolt.org/z/3o3974fjT

CC @AaronBallman @cor3ntin

@Fznamznon I think GCC is correct here.
http://eel.is/c++draft/support.srcloc.class#support.srcloc.cons-2 does not mention template default argument at all.

@Fznamznon I think GCC is correct here. http://eel.is/c++draft/support.srcloc.class#support.srcloc.cons-2 does not mention template default argument at all.

It may be worth confirming with WG21 though.

Posted https://reviews.llvm.org/D150108