This works around PR 93978 by avoiding having to instantiate ranges::empty()
when checking the constraints of view_interface::operator bool(). When
ranges::empty() has an auto return type, then we must instantiate it 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 [ranges.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.
---
libstdc++-v3/include/bits/range_access.h | 2 +-
.../testsuite/std/ranges/adaptors/93978.cc | 34 +++++++++++++++++++
2 files changed, 35 insertions(+), 1 deletion(-)
create mode 100644 libstdc++-v3/testsuite/std/ranges/adaptors/93978.cc
diff --git a/libstdc++-v3/include/bits/range_access.h
b/libstdc++-v3/include/bits/range_access.h
index c814694623c..eab8bb9721b 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -804,7 +804,7 @@ 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>)
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/93978.cc
b/libstdc++-v3/testsuite/std/ranges/adaptors/93978.cc
new file mode 100644
index 00000000000..62b5f5f9dac
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/93978.cc
@@ -0,0 +1,34 @@
+// 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 -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;
+}
--
2.25.1.460.g2f268890c2
