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

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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.
This commit is contained in:
Patrick Palka 2020-03-05 10:04:06 -05:00
parent 4cdcb2c92a
commit 6d082cd901
3 changed files with 43 additions and 1 deletions
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7
libstdc++-v3/ChangeLog
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@ -1,3 +1,10 @@
2020-03-06 Patrick Palka <ppalka@redhat.com>
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.
2020-03-06 Jonathan Wakely <jwakely@redhat.com>
PR libstdc++/94069

2
libstdc++-v3/include/bits/range_access.h
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@ -793,7 +793,7 @@ namespace ranges
template<typename _Tp>
requires __member_empty<_Tp> || __size0_empty<_Tp>
|| __eq_iter_empty<_Tp>
constexpr auto
constexpr bool
operator()(_Tp&& __e) const noexcept(_S_noexcept<_Tp>())
{
if constexpr (__member_empty<_Tp>)

35
libstdc++-v3/testsuite/std/ranges/adaptors/93978.cc Normal file
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@ -0,0 +1,35 @@
// Copyright (C) 2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License along
// with this library; see the file COPYING3. If not see
// <http://www.gnu.org/licenses/>.
// { dg-options "-std=gnu++2a" }
// { dg-additional-options "-O" }
// { dg-do compile { target c++2a } }
#include <ranges>
#include <vector>
namespace ranges = std::ranges;
namespace views = std::views;
void
test()
{
std::vector<std::string> x = {""};
auto i = std::counted_iterator(x.begin(), 1);
auto r = ranges::subrange{i, std::default_sentinel};
auto v = r | views::join;
}