[Bug c++/114393] New: over eager "invalid use of void expression" ?

["prlw1 at cam dot ac.uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114393

            Bug ID: 114393
           Summary: over eager "invalid use of void expression" ?
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: prlw1 at cam dot ac.uk
  Target Milestone: ---

When trying to use the reference implementation of the `std::execution`
proposal over at https://github.com/nvidia/stdexec, I get errors of the form

/usr/src/local/stdexec/include/exec/async_scope.hpp:735:31: error: invalid use
of void expression
  735 |         return when_empty(just());

with `g++ (GCC) 14.0.1 20240318 (experimental)`

In https://github.com/NVIDIA/stdexec/issues/1285 Eric Niebler traces this to
a__make_sexpr invocations which can't ever have type void.

The easiest way to reproduce this is to see the standard example on goldbolt:
https://godbolt.org/z/3cseorf7M

It compiles fine with gcc 12.3 and 13.2 (even with `-Wall -Wextra
--pendantic-errors`, but not with gcc trunk, which possibly suggests a
regression.

I have been using stdexec successfully for a while with trunk. Old checkouts of
stdexec which used to work (with maybe gcc trunk from about 2 months ago?) now
fail, which also points towards a change in gcc rather than in stdexec.