Reference argument specified as template can't be used in constexpr if

[abellgithub]

Perhaps a duplicate of #63139

The following fails to compile with 19.1.0 and current trunk. It works if the argument is a value type instead of reference. Works with GCC.

class Foo
{
public:
    constexpr static bool foo = true;
};

template <typename T>
int func(T& t)
{
    if constexpr (t.foo)
    {
        return 1;
    }
    else
    {
        return 0;
    }
}

int main()
{
    Foo f;

    int i = func(f);
    return i;
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Andrew Bell (abellgithub)

Perhaps a duplicate of #63139

The following fails to compile with 19.1.0 and current trunk. It works if the argument is a value type instead of reference. Works with GCC.

class Foo
{
public:
    constexpr static bool foo = true;
};

template <typename T>
int func(T& t)
{
    if constexpr (t.foo)
    {
        return 1;
    }
    else
    {
        return 0;
    }
}

int main()
{
    Foo f;

    int i = func(f);
    return i;
}

[frederick-vs-ja]

FWIW, Clang accepts this since ver. 20 but only in C++23 and later modes (Godbolt link).

As P2280R4 is adopted as a defect report, I think Clang should apply it to C++11 and later, at least after some resolving some bugs.

[tbaederr]

at least after some resolving some bugs.

Like what? Any known problems other than those constexpr-unknown values making it into codegen?

[frederick-vs-ja]

at least after some resolving some bugs.

Like what? Any known problems other than those constexpr-unknown values making it into codegen?

I didn't found any. Hopefully that is the only bug, and we will be able to start backporting after resolving it.