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From: Ville Voutilainen <ville.voutilainen@gmail.com>
Date: Wed, 4 Nov 2020 10:52:55 +0200
Message-ID: <CAFk2RUaYyjjGekZ0m75HAvoK5Jb2ndbyVW3QzAkGQ++F=sbRoA@mail.gmail.com>
Subject: Re: [committed] libstdc++: Allow Lemire's algorithm to be used in
 more cases
To: Stephan Bergmann <sbergman@redhat.com>
Cc: Jonathan Wakely <jwakely@redhat.com>, "libstdc++" <libstdc++@gcc.gnu.org>, 
 gcc-patches List <gcc-patches@gcc.gnu.org>
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On Wed, 4 Nov 2020 at 10:46, Stephan Bergmann via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
> To me it looks like it boils down to disagreement between g++ and
> clang++ over
>
> > struct S { static constexpr int f() { return 0; } };
> > void f(S & s) { static_assert(s.f(), ""); }
>
> where I think Clang might be right in rejecting it based on [expr.const]
> "An expression e is a core constant expression unless [...] an
> id-expression that refers to a variable or data member of reference type
> unless the reference has a preceding initialization [...]"

There's more to it than that. It's a disagreement over [expr.ref]/1.
For a static
member call, gcc just plain doesn't evaluate the s in s.f(). But [expr.ref]/1
says it's evaluated, and since it's not a constant expression, clang rejects
it, and gcc accepts it. That's why your fix works; it removes the use
of the otherwise-mostly-ignored
object expression for a call to a static member function.

So, I think gcc is accepting-invalid here, and we should just apply
the fix you suggested.