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c++ - Compare std::variant with int using C++20 <=> is not a constant expression

问题描述

Since the std::variant is not allowed to compare with one of its alternative types in the standard library, I am implementing the compare function using C++20 <=> operator:

template <typename... Args, typename T>
constexpr auto operator<=>(const std::variant<Args...>& v, const T& t) {
  return std::visit([&t](const auto& u) -> std::partial_ordering {
    if constexpr (requires { u <=> t; }) return u <=> t;
    else return std::partial_ordering::unordered;
  }, v);
}

But when I testing the above function with my own defined std::variant:

using Variant = std::variant<double, int, std::string_view>;
constexpr Variant v1{1.0};
constexpr Variant v2{1};
constexpr Variant v3{"hello"};
static_assert(v1 < 2);
// compile error
static_assert(v2 < 2);
static_assert(!(v3 > 2) && !(v3 < 2) && !std::is_eq(v3 <=> 2));

The second assertion couldn't compile, GCC says:

<source>:19:17: error: non-constant condition for static assertion
   19 |   static_assert(v2 < 2);
      |                 ^~~~~~~~~
<source>:19:24:   in 'constexpr' expansion of 'operator<=><double, int, std::basic_string_view<char, std::char_traits<char> >, int>(v2, 2)'
<source>:19:17: error: '<anonymous>' is not a constant expression

Why is v2 < 2 not a constant expression? or it's just a GCC bug? Weirder, when I change the second assertion to compare with double, this can compile:

static_assert(v2 < 2.0);

Update:

Clang can pass three assertions, but MSVC can only pass the third assertion, it seems MSVC also has a bug.

标签： c++c++17c++20std-variant

解决方案

The first example reduces to:

#include <compare>

// this one is okay
static_assert(std::partial_ordering(std::strong_ordering::less) < 0);

// this one fails with non-constant condition
static_assert(std::partial_ordering(1 <=> 2) < 0);

Everything here is obviously a constant expression. The fact that strong_ordering::less can be converted to partial_ordering::less just fine while 1 <=> 2 can't be (on gcc) suggests that this is a bug in the compiler, rather than the library.

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