libstdc++: subrange converting constructor should disallow slicing (LWG 3282)

	* include/std/ranges (__detail::__convertible_to_non_slicing): New
	helper concept.
	(__detail::__pair_like_convertible_to): Remove.
	(__detail::__pair_like_convertible_from): Add requirements for
	non-slicing conversions.
	(subrange): Constrain constructors with __convertible_to_non_slicing.
	Remove constructors from pair-like types. Add new deduction guide.
	* testsuite/std/ranges/subrange/lwg3282_neg.cc: New test.

libstdc++-v3/ChangeLog

 2020-02-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/ranges (__detail::__convertible_to_non_slicing): New
+	helper concept.
+	(__detail::__pair_like_convertible_to): Remove.
+	(__detail::__pair_like_convertible_from): Add requirements for
+	non-slicing conversions.
+	(subrange): Constrain constructors with __convertible_to_non_slicing.
+	Remove constructors from pair-like types. Add new deduction guide.
+	* testsuite/std/ranges/subrange/lwg3282_neg.cc: New test.
+
 	* include/bits/iterator_concepts.h (iter_move): Add declaration to
 	prevent unqualified lookup finding a suitable declaration (LWG 3247).
 

libstdc++-v3/include/std/ranges

@@ -214,6 +214,12 @@ namespace ranges
 
   namespace __detail
   {
+    template<class _From, class _To>
+      concept __convertible_to_non_slicing = convertible_to<_From, _To>
+	&& !(is_pointer_v<decay_t<_From>> && is_pointer_v<decay_t<_To>>
+	    && __not_same_as<remove_pointer_t<decay_t<_From>>,
+			     remove_pointer_t<decay_t<_To>>>);
+
     template<typename _Tp>
       concept __pair_like
 	= !is_reference_v<_Tp> && requires(_Tp __t)
@@ -227,18 +233,11 @@ namespace ranges
 	};
 
     template<typename _Tp, typename _Up, typename _Vp>
-      concept __pair_like_convertible_to
-	= !range<_Tp> && __pair_like<remove_reference_t<_Tp>>
-	&& requires(_Tp&& __t)
-	{
-	  { get<0>(std::forward<_Tp>(__t)) } -> convertible_to<_Up>;
-	  { get<1>(std::forward<_Tp>(__t)) } -> convertible_to<_Vp>;
-	};
-
-    template<typename _Tp, typename _Up, typename _Vp>
       concept __pair_like_convertible_from
 	= !range<_Tp> && __pair_like<_Tp>
-	&& constructible_from<_Tp, _Up, _Vp>;
+	&& constructible_from<_Tp, _Up, _Vp>
+	&& __convertible_to_non_slicing<_Up, tuple_element_t<0, _Tp>>
+	&& convertible_to<_Vp, tuple_element_t<1, _Tp>>;
 
     template<typename _Tp>
       concept __iterator_sentinel_pair
@@ -277,12 +276,13 @@ namespace ranges
       subrange() = default;
 
       constexpr
-      subrange(_It __i, _Sent __s) requires (!_S_store_size)
+      subrange(__detail::__convertible_to_non_slicing<_It> auto __i, _Sent __s)
+	requires (!_S_store_size)
       : _M_begin(std::move(__i)), _M_end(__s)
       { }
 
       constexpr
-      subrange(_It __i, _Sent __s,
+      subrange(__detail::__convertible_to_non_slicing<_It> auto __i, _Sent __s,
 	       __detail::__make_unsigned_like_t<iter_difference_t<_It>> __n)
 	requires (_Kind == subrange_kind::sized)
       : _M_begin(std::move(__i)), _M_end(__s)
@@ -295,7 +295,7 @@ namespace ranges
 
       template<__detail::__not_same_as<subrange> _Rng>
 	requires borrowed_range<_Rng>
-	  && convertible_to<iterator_t<_Rng>, _It>
+	  && __detail::__convertible_to_non_slicing<iterator_t<_Rng>, _It>
 	  && convertible_to<sentinel_t<_Rng>, _Sent>
 	constexpr
 	subrange(_Rng&& __r) requires (!_S_store_size || sized_range<_Rng>)
@@ -306,7 +306,7 @@ namespace ranges
 	}
 
       template<borrowed_range _Rng>
-	requires convertible_to<iterator_t<_Rng>, _It>
+	requires __detail::__convertible_to_non_slicing<iterator_t<_Rng>, _It>
 	  && convertible_to<sentinel_t<_Rng>, _Sent>
 	constexpr
 	subrange(_Rng&& __r,
@@ -316,23 +316,6 @@ namespace ranges
 	{ }
 
       template<__detail::__not_same_as<subrange> _PairLike>
-	requires __detail::__pair_like_convertible_to<_PairLike, _It, _Sent>
-	constexpr
-	subrange(_PairLike&& __r) requires (!_S_store_size)
-	: subrange{std::get<0>(std::forward<_PairLike>(__r)),
-		   std::get<1>(std::forward<_PairLike>(__r))}
-	{ }
-
-      template<__detail::__pair_like_convertible_to<_It, _Sent> _PairLike>
-	constexpr
-	subrange(_PairLike&& __r,
-		 __detail::__make_unsigned_like_t<iter_difference_t<_It>> __n)
-	requires (_Kind == subrange_kind::sized)
-	: subrange{std::get<0>(std::forward<_PairLike>(__r)),
-		   std::get<1>(std::forward<_PairLike>(__r)), __n}
-	{ }
-
-      template<__detail::__not_same_as<subrange> _PairLike>
 	requires __detail::__pair_like_convertible_from<_PairLike, const _It&,
 							const _Sent&>
       constexpr
@@ -403,6 +386,9 @@ namespace ranges
     };
 
   template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
+    subrange(_It, _Sent) -> subrange<_It, _Sent>;
+
+  template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
     subrange(_It, _Sent,
 	     __detail::__make_unsigned_like_t<iter_difference_t<_It>>)
       -> subrange<_It, _Sent, subrange_kind::sized>;

libstdc++-v3/testsuite/std/ranges/subrange/lwg3282_neg.cc

+// 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-do compile { target c++2a } }
+
+#include <ranges>
+
+using std::ranges::subrange;
+
+// LWG 3282. subrange converting constructor should disallow derived to base
+// conversions
+
+struct Base {};
+struct Derived : Base {};
+subrange<Derived*> sd;
+subrange<Base*> sb = sd; // { dg-error "conversion" }