constexpr operator==(variant, variant)

[Tim Shen]

Hi,

I realized that operator== on variant is required to be constexpr by
p0088r3, which is not the case in our implementation (it's probably my
oversight).

Currently, operator== is implemented in terms of an array of function pointers:
  static constexpr Func_ptr vtable[] = { &handle_types<Types...> };
  return vtable[v1.index()](v1.raw_storage(), v2.raw_storage());

When v1.index() == v2.index(). The problem with this approach is that,
in handle_types we have to cast the input parameters as void* to T*,
which isn't allowed in constexpr functions.

If we take another approach, that is constructing an if-else chain
using meta-programming:
  if (v1.index() == 0) { return get<0>(v1) == get<0>(v2); } else
  if (v1.index() == 1) { return get<1>(v1) == get<1>(v2); } else
  ...

This satisfies constexpr requirements. However, GCC generates
inefficient code for if-else chain: https://godbolt.org/g/AEYMwC
Although Clang generates perfect local jump tables.

My question is: what do we do now? I tend to leave it as is (not
constexpr) until GCC catches up, then we switch to the if-else chain
solution.


-- 
Regards,
Tim Shen