libstdc++: P1976R2 Fixed-size span construction from dynamic range

This includes fixes for first, last, as_bytes and as_writable_bytes
which were missing from the paper.

	* include/std/span (__cpp_lib_span): Update value.
	(span(It, size_type), span(It, End)): Make conditionally explicit. Add
	assertion.
	(span(R&&), span(const span<OType, OExtent>&)): Likewise and relax
	constraints.
	(span::first<Count>(), span::last<Count>()): Use explicit type in
	return statement.
	(as_bytes, as_writable_bytes): Likewise.
	* include/std/version (__cpp_lib_span): Update value.
	* testsuite/23_containers/span/1.cc: Check new value.
	* testsuite/23_containers/span/2.cc: Check new value.
	* testsuite/23_containers/span/explicit.cc: New test.

3 files changed, 50 insertions, 2 deletions

diff --git a/libstdc++-v3/testsuite/23_containers/span/1.cc b/libstdc++-v3/testsuite/23_containers/span/1.cc
index 7c02aade939..6c622f3e33d 100644
--- a/libstdc++-v3/testsuite/23_containers/span/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/1.cc
@@ -22,6 +22,6 @@
 
 #ifndef __cpp_lib_span
 # error "Feature-test macro for span missing in <span>"
-#elif __cpp_lib_span != 201902L
+#elif __cpp_lib_span != 202002L
 # error "Feature-test macro for span has wrong value in <span>"
 #endif
diff --git a/libstdc++-v3/testsuite/23_containers/span/2.cc b/libstdc++-v3/testsuite/23_containers/span/2.cc
index 4a9d1f07325..9e91b5084e0 100644
--- a/libstdc++-v3/testsuite/23_containers/span/2.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/2.cc
@@ -22,6 +22,6 @@
 
 #ifndef __cpp_lib_span
 # error "Feature-test macro for span missing in <version>"
-#elif __cpp_lib_span != 201902L
+#elif __cpp_lib_span != 202002L
 # error "Feature-test macro for span has wrong value in <version>"
 #endif
diff --git a/libstdc++-v3/testsuite/23_containers/span/explicit.cc b/libstdc++-v3/testsuite/23_containers/span/explicit.cc
new file mode 100644
index 00000000000..a763f8045af
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/explicit.cc
@@ -0,0 +1,48 @@
+// 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 <span>
+
+struct Range
+{
+  int* begin();
+  int* end();
+  unsigned size() const;
+} r;
+
+auto first = std::begin(r), last = std::end(r);
+
+// span(It, size_type)
+std::span<int> s1 = {first, 2};
+std::span<int, 2> s2 = {first, 2}; // { dg-error "could not convert" }
+
+// span(It, End)
+std::span<int> s3 = {first, last};
+std::span<int, 2> s4 = {first, last}; // { dg-error "could not convert" }
+
+// span(R&&)
+std::span<int> s5 = r;
+std::span<int, 2> s6 = r; // { dg-error "conversion from" }
+
+// span(const span<OtherElement, OtherExtent>&)
+std::span<const int> s7 = s5;
+std::span<const int> s8 = s6;
+std::span<const int, 1> s9 = s5.first(1);  // { dg-error "conversion from" }
+std::span<const int, 1> s10 = s7.first(1); // { dg-error "conversion from" }