Static member access in constant expressions

Clang seems to be in the right. When accessing a static member with the member access syntax [class.static/1]:

A static member s of class X may be referred to using the qualified-id
expression X​::​s; it is not necessary to use the class member access
syntax to refer to a static member. A static member may be referred to
using the class member access syntax, in which case the object
expression is evaluated.

So s.v() will cause s to be evaluated. Now, according to [expr.const/2.11], s is not a constant expression:

2 An expression e is a core constant expression unless the evaluation
of e, following the rules of the abstract machine, would evaluate one
of the following expressions:

[…]

an id-expression that refers to a variable or data member of reference
type unless the reference has a preceding initialization and either:
(2.11.1) – it is initialized with a constant expression or
(2.11.2) – its lifetime began within the evaluation of e;

s doesn’t have a preceding initialization with a constant expression, not in the scope of foo.

---

If you want to access the static members based of a function parameter, without hard-coding the type, the way forward is std::remove_reference_t<decltype(s)>. This is accepted by Clang and GCC both:

#include <type_traits>

struct S 
{
    constexpr static auto s_v = 42;    
    constexpr static auto v() { return s_v; }
};

constexpr auto foo(S const& s) 
{
    constexpr auto v = std::remove_reference_t<decltype(s)>::v();
    return v;
}

constexpr auto bar(S const& s)
{
    constexpr auto v = std::remove_reference_t<decltype(s)>::s_v;
    return v;
}

int main() {}