[PATCH] libstdc++: Remove overzealous static_asserts from std::span

[Patrick Palka <ppalka@redhat.com>]

For a span with empty static extent, we currently model the
preconditions of front(), back(), and operator[] as if they were
mandates, by using a static_assert to verify that extent != 0.  This
causes us to incorrectly reject valid programs that instantiate these
member functions but never call them.

libstdc++-v3/ChangeLog:

	* include/std/span (span::front): Remove static_assert.
	(span::back): Likewise.
	(span::operator[]): Likewise.
	* testsuite/23_containers/span/back_neg.cc: Remove.
	* testsuite/23_containers/span/front_neg.cc: Remove.
	* testsuite/23_containers/span/index_op_neg.cc: Remove.
---
 libstdc++-v3/include/std/span                 |  3 --
 .../testsuite/23_containers/span/back_neg.cc  | 29 -------------------
 .../testsuite/23_containers/span/front_neg.cc | 29 -------------------
 .../23_containers/span/index_op_neg.cc        | 29 -------------------
 4 files changed, 90 deletions(-)
 delete mode 100644 libstdc++-v3/testsuite/23_containers/span/back_neg.cc
 delete mode 100644 libstdc++-v3/testsuite/23_containers/span/front_neg.cc
 delete mode 100644 libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index f658adb04cf..1cdc0589ddb 100644
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -264,7 +264,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr reference
       front() const noexcept
       {
-	static_assert(extent != 0);
 	__glibcxx_assert(!empty());
 	return *this->_M_ptr;
       }
@@ -272,7 +271,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr reference
       back() const noexcept
       {
-	static_assert(extent != 0);
 	__glibcxx_assert(!empty());
 	return *(this->_M_ptr + (size() - 1));
       }
@@ -280,7 +278,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr reference
       operator[](size_type __idx) const noexcept
       {
-	static_assert(extent != 0);
 	__glibcxx_assert(__idx < size());
 	return *(this->_M_ptr + __idx);
       }
diff --git a/libstdc++-v3/testsuite/23_containers/span/back_neg.cc b/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
deleted file mode 100644
index c451ed10df8..00000000000
--- a/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
+++ /dev/null
@@ -1,29 +0,0 @@
-// Copyright (C) 2019-2020 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>
-
-void
-test01()
-{
-  std::span<int, 0> s;
-  s.back(); // { dg-error "here" }
-}
-// { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/front_neg.cc b/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
deleted file mode 100644
index 38f87aa2cd5..00000000000
--- a/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
+++ /dev/null
@@ -1,29 +0,0 @@
-// Copyright (C) 2019-2020 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>
-
-void
-test01()
-{
-  std::span<int, 0> s;
-  s.front(); // { dg-error "here" }
-}
-// { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc b/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
deleted file mode 100644
index 1e8b2d8724e..00000000000
--- a/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
+++ /dev/null
@@ -1,29 +0,0 @@
-// Copyright (C) 2019-2020 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>
-
-void
-test01()
-{
-  std::span<int, 0> s;
-  s[99]; // { dg-error "here" }
-}
-// { dg-error "static assertion failed" "" { target *-*-* } 0 }
-- 
2.28.0.497.g54e85e7af1