-Wunneeded-internal-declaration warned on needed function

[PiJoules]

Bugzilla Link	43598
Version	trunk
OS	Linux
CC	@kamleshbhalui,@zygoloid

Extended Description

For this code:

#include <type_traits>

namespace {

// Check if Op is convertible to bool.
template <typename Op>
using enable_relop_t = std::enable_if_t<(std::is_convertible<Op, bool>::value), bool>;

// Chek if can do == with T and U.
template <typename T, typename U,
         enable_relop_t<decltype(std::declval<T>() == std::declval<U>())> = true>
constexpr bool is_comparable(T&&, U&&) {
  return true;
}

struct comparable_a {};

constexpr bool operator==(const comparable_a&, const comparable_a&) { return true; }

static_assert(is_comparable(comparable_a{}, comparable_a{}));

}

We get this warning on line 18: warning: function 'operator==' is not needed and will not be emitted [-Wunneeded-internal-declaration]. If the function were truly unneeded, I would assume I should just be able to comment out line 18 and everything should still compile, but I would get an error no matching function for call to 'is_comparable' meaning it was used.

Compiled with bin/clang++ ~/misc/test.cpp -c -std=c++17 -Wall from ToT clang.

I'm noticing that this compiles without warnings also if I don't wrap everything in an unnamed namespace or mark operator== with __attrubute__((used)).

[PiJoules]

assigned to @kamleshbhalui

[zygoloid]

You do not need a definition of the function on line 18; you can replace it with just a function declaration and the program will still compile. However, we apparently still produce this warning even if the function definition is replaced by a function declaration, so this definitely looks like a bug.

[rollbear]

FWIW, I was just bitten by this, with very similar code, on clang-15.

#include <type_traits>

template <typename T>
struct S
{
    auto operator<(const S&) const
    -> decltype(std::declval<T&>() < std::declval<T&>());
};

namespace {
    struct inner
    {
        friend bool operator<(const inner&, const inner&); // Is needed!
    };
}

[[maybe_unused]] S<inner> s;

https://godbolt.org/z/hYd4bnasK

[shafik]

Also see: #25458