Fail to consider a member access from an object to a static member a constant expression

[Sainan]

#include <iostream>

struct Usertype
{
    [[nodiscard]] static constexpr bool getSomething() noexcept
    {
        return true;
    }
};

void checkSomething(Usertype& ud)
{
    if constexpr (ud.getSomething()) // on linux, only works if ud is not a reference or r-value
    //if constexpr (std::remove_reference_t<decltype(ud)>::getSomething()) // always works
    {
        std::cout << "Alright!\n";
    }
}

int main()
{
    Usertype ud;
    checkSomething(ud);
}

This compiles fine on Windows, but on Linux I get "constexpr if condition is not a constant expression".

Tested against clang versions:

• Windows: 14.0.5, 13.0.0
• Linux: 14.0.0, 13.0.1, 13.0.0, 10.0.0-4ubuntu1, 7.0.1-8+deb10u2

Using C++17 in all instances.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[tbaederr]

Using Usertype::getSomething() works.

[Sainan]

Right, I was looking at this from the perspective of the type being hidden due to a macro that defines a template function. In any case, I have workarounds, it's not a show-stopper of a problem, but just really intriguing that the build only fails on Linux.

[tbaederr]

the type being hidden due to a macro that defines a template function.

Wait, where's a macro involved? And where a template function?

I was thinking it might be due to a different stdlib, but

struct Usertype
{
    static constexpr bool getSomething()
    {
        return true;
    }
};

void checkSomething(Usertype& ud)
{
    if constexpr (ud.getSomething()) {}
}

Also reproduces.

[Sainan]

Wait, where's a macro involved? And where a template function?

Not in the example, but just explaining how I was looking at it that I didn't think of Usertype::getSomething().

[AaronBallman]

This looks very closely related to #56246 -- that one is about resolving to a static data member of this, but this one is through a member object expression other than this.

[llvmbot]

@llvm/issue-subscribers-c-11

[usx95]

struct Foo {
    static constexpr bool Bar() { return true; }
};

void func(Foo& foo) { 
    static_assert(foo.Bar());
}

https://godbolt.org/z/4add9zche
foo.Bar() is not a constant expression because foo's lifetime began outside the expression and it is not initialised with a constant expression.
http://eel.is/c++draft/expr.const#4

Related discussions: #40438
Related Patch: 00068c4#diff-24549b124046a492279fe53ffe2aca86550f281b025a48067c043a1c944547faR2306-R2323

Can you confirm @AaronBallman

[AaronBallman]

Can you confirm @AaronBallman

I think this is the subject of https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html which we don't claim to implement yet. (Thanks @shafik for noticing that connection!)

[cor3ntin]

Should be fixed by #63139

[zygoloid]

Closing as duplicate of #63139.