[Bug libstdc++/93978] A snippet using views::join fails to compile with -O1, but succeeds with -O0

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93978

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:6d082cd90131a9c0ce3142217e84194a5bf0de27

commit r10-7066-g6d082cd90131a9c0ce3142217e84194a5bf0de27
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Mar 5 10:04:06 2020 -0500

    libstdc++: Give ranges::empty() a concrete return type (PR 93978)

    This works around PR 93978 by avoiding having to instantiate the body of
    ranges::empty() when checking the constraints of view_interface::operator
    bool().  When ranges::empty() has an auto return type, then we must
instantiate
    its body in order to determine whether the requires expression {
    ranges::empty(_M_derived()); } is well-formed.  But this means
instantiating
    view_interface::empty() and hence view_interface::_M_derived(), all before
we've
    yet deduced the return type of join_view::end().  (The reason
    view_interface::operator bool() is needed in join_view::end() in the first
place
    is because in this function we perform direct initialization of
    join_view::_Sentinel from a join_view, and so we try to find a conversion
    sequence from the latter to the former that goes through this conversion
    operator.)

    Giving ranges::empty() a concrete return type of bool should be safe
according
    to [range.prim.empty]/4 which says "whenever ranges::empty(E) is a valid
    expression, it has type bool."

    This fixes the test case in PR 93978 when compiling without -Wall, but with
-Wall
    the test case still fails due to the issue described in PR c++/94038, I
think.
    I still don't quite understand why the test case doesn't fail without -O.

    libstdc++-v3/ChangeLog:

        PR libstdc++/93978
        * include/bits/range_access.h (__cust_access::_Empty::operator()):
        Declare return type to be bool instead of auto.
        * testsuite/std/ranges/adaptors/93978.cc: New test.