[PATCH] PR libstdc++/80939 Remove unmeetable constexpr specifiers

[Tim Song]

On Fri, Jun 2, 2017 at 3:19 PM, Tim Shen <timshen@google.com> wrote:
> On Fri, Jun 2, 2017 at 6:07 AM, Jonathan Wakely wrote:
>> As the PR points out, we aren't qualifying calls to __ref_cast, and
>> have 'constexpr' on function templates that can never be usable in
>> constant expressions.
>
> Apology for the constexpr trolling, but that was not intentional. :)
>
> I'm curious why no tests break. Is it because constexpr in a template
> function is a no-op instead of a hard error, when the function
> definition is not constexpr?
>
> The patch looks good.
>

A non-template, non-default constexpr function that can never be used
in a constant expression is ill-formed NDR. ([dcl.constexpr]/5)

A constexpr function template for which no specialization that
satisfies the requirements for a constexpr function (when considered
as a non-template) can be generated is ill-formed NDR.
([dcl.constexpr]/6)

It's not really clear to me whether the second rule incorporates the
first or if it's just talking about the requirements in
[dcl.constexpr]/3, but regardless it's not required to break.