On Fri, Nov 12, 2021 at 3:26 AM Jens Maurer via Std-Discussion
<std-discussion_at_[hidden]> wrote:
>
> On 12/11/2021 04.04, Hani Deek via Std-Discussion wrote:
> > The following code is accepted by GCC, Clang and MSVC. I thought it was ill-formed because the variable V0 is not usable in constant expressions. Is this just a bug in all of those compilers? Or is there an explanation in the C++ rules?
> >
> > struct S{ constexpr operator int() const{ return 0; } };
> >
> > void foo(S V0) { constexpr int V1 = V0; }
> >
> > int main()
> > {
> > S V0{};
> > foo(V0);
> > }
>
> There is no lvalue-to-rvalue conversion on any part of V0.
>
> Do you feel any other provision in [expr.const] p5 should
> apply, making this not a constant expression?

I got a bit lost in the details, but my confusion comes from the use
of the name `V0`. C++20 says:

> An object or reference is usable in constant expressions if it is:
> * a variable that is usable in constant expressions, or ...

And:

> A constant-initialized potentially-constant variable is usable in constant expressions ...

This means that the only variables that are "usable in constant
expressions" are those which are constant-initialized and potentially
constant. Neither is true of `V0`.

Are you suggesting that the use of the name `V0` does not trigger one
of the conditions in [expr.const]/5 because it never triggers
lvalue-to-rvalue conversion? If this is true, can you explain why it
does not?