[Bug c++/90858] New: Pointer to member is treated as non-constexpr

[m.cencora at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90858

            Bug ID: 90858
           Summary: Pointer to member is treated as non-constexpr
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.cencora at gmail dot com
  Target Milestone: ---

Hi,

this is a minimal reproducer I could come up with. Compiles fine with clang.

It compiles fine if Foo1 is not nested inside Foo2.

#include <array>
#include <variant>


template <typename T>
struct type{};

constexpr bool validate(unsigned i)
{
    return true;
}

template <typename T,
          decltype(getMembers(type<T>{}), false) = false>
constexpr bool validate(const T& object)
{
    for (auto member: getMembers(type<T>{}))
    {
        std::visit([&] (auto memPtr) {
            validate(object.*memPtr);
        }, member);
    }
    return true;
}


struct Foo1
{
   unsigned a;
};

struct Foo2
{
   unsigned f1;
   Foo1 f2;
};


template <typename Class, typename T>
using MemPtr = T (Class::*);


constexpr std::array<std::variant<MemPtr<Foo1, unsigned>>, 1>
getMembers(type<Foo1>)
{
   return {
      &Foo1::a,
   };
};

constexpr std::array<
    std::variant<MemPtr<Foo2, unsigned>,
                 MemPtr<Foo2, Foo1>>, 2> getMembers(type<Foo2>)
{
   return {
      &Foo2::f1,
      &Foo2::f2
   };
};

static_assert(validate(Foo2{}), "");