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.

3 files changed, 43 insertions, 1 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 98faff614da..b2e995b4daf 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -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
diff --git a/libstdc++-v3/include/bits/range_access.h b/libstdc++-v3/include/bits/range_access.h
index 8b276fd6625..4dd48bbc6df 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -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>)
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..a0152ec0532
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/93978.cc
@@ -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;
+}