libstdc++: Make make-unsigned-like-t<__int128> work [PR 96042]

As well as ensuring that numeric_limits<__int128> is defined, we need to
ensure that make-unsigned-like-t and to-unsigned-like work correctly for
128-bit integers in strict mode. This ensures that a subrange created
from an iota_view's iterator and sentinel can represent its size.

Co-authored-by: Patrick Palka  <ppalka@redhat.com>

libstdc++-v3/ChangeLog:

2020-08-19  Jonathan Wakely  <jwakely@redhat.com>
	    Patrick Palka  <ppalka@redhat.com>

	PR libstdc++/96042
	* include/bits/range_access.h (__detail::__to_unsigned_like):
	Do not use make_unsigned_t<T> in the return type, as it can
	result in an error before the integral<T> constraint is checked.
	[__STRICT_ANSI__]: Add overloads for 128-bit integer types.
	(__detail::__make_unsigned_like_t): Define as the return type
	of __to_unsigned_like.
	* testsuite/std/ranges/subrange/96042.cc: New test.

2 files changed, 48 insertions, 4 deletions

diff --git a/libstdc++-v3/include/bits/range_access.h b/libstdc++-v3/include/bits/range_access.h
index 3eb1f2fd272..bafced31ea8 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -364,13 +364,23 @@ namespace ranges
     { return __max_size_type(__t); }
 
     template<integral _Tp>
-      constexpr make_unsigned_t<_Tp>
+      constexpr auto
       __to_unsigned_like(_Tp __t) noexcept
-      { return __t; }
+      { return static_cast<make_unsigned_t<_Tp>>(__t); }
 
-    template<typename _Tp, bool _MaxDiff = same_as<_Tp, __max_diff_type>>
+#if defined __STRICT_ANSI__ && defined __SIZEOF_INT128__
+    constexpr unsigned __int128
+    __to_unsigned_like(__int128 __t) noexcept
+    { return __t; }
+
+    constexpr unsigned __int128
+    __to_unsigned_like(unsigned __int128 __t) noexcept
+    { return __t; }
+#endif
+
+    template<typename _Tp>
       using __make_unsigned_like_t
-	= conditional_t<_MaxDiff, __max_size_type, make_unsigned_t<_Tp>>;
+	= decltype(__detail::__to_unsigned_like(std::declval<_Tp>()));
 
     // Part of the constraints of ranges::borrowed_range
     template<typename _Tp>
diff --git a/libstdc++-v3/testsuite/std/ranges/subrange/96042.cc b/libstdc++-v3/testsuite/std/ranges/subrange/96042.cc
new file mode 100644
index 00000000000..5826203f03c
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/subrange/96042.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=c++20" }
+// { dg-do compile { target c++2a } }
+
+#include <ranges>
+
+constexpr bool
+test01()
+{
+  using I = unsigned long long;
+  // view with a difference type that doesn't fit in long long:
+  std::ranges::iota_view<I, I> v(0, std::numeric_limits<I>::max());
+  // view with a size type that doesn't fit in unsigned long long:
+  std::ranges::subrange sr{v.begin(), v.end()};
+  auto sz = std::ranges::size(sr);
+  return sz == std::numeric_limits<I>::max();
+}
+static_assert( test01() );