From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 22 Sep 2020 00:53:17 +0000 (-0400)
Subject: libstdc++: Remove overzealous static_asserts from std::span
X-Git-Tag: basepoints/gcc-12~4911
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=37edf28c24b7bd198c27d266af9aefad417635fd;p=thirdparty%2Fgcc.git

libstdc++: Remove overzealous static_asserts from std::span

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

Since they are already followed by more general runtime asserts, this
patch just removes these static_asserts altogether,

libstdc++-v3/ChangeLog:

	* include/std/span (span::front): Remove static_assert.
	(span::back): Likewise.
	(span::operator[]): Likewise.
	* testsuite/23_containers/span/back_neg.cc: Rewrite to verify
	that we check the preconditions of back() only when it's called.
	* testsuite/23_containers/span/front_neg.cc: Likewise for
	front().
	* testsuite/23_containers/span/index_op_neg.cc: Likewise for
	operator[].
---

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index f658adb04cf0..1cdc0589ddb1 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
index c451ed10df87..f777edfa20c5 100644
--- a/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
@@ -20,10 +20,16 @@
 
 #include <span>
 
-void
-test01()
+constexpr bool
+test01(bool b)
 {
   std::span<int, 0> s;
-  s.back(); // { dg-error "here" }
+  if (b || !s.empty())
+    s.back();
+  return true;
 }
-// { dg-error "static assertion failed" "" { target *-*-* } 0 }
+
+static_assert(test01(false));
+static_assert(test01(true)); // { dg-error "non-constant" }
+// { dg-error "assert" "" { target *-*-* } 0 }
+// { dg-prune-output "in 'constexpr' expansion" }
diff --git a/libstdc++-v3/testsuite/23_containers/span/front_neg.cc b/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
index 38f87aa2cd51..14e5bc1e1009 100644
--- a/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
@@ -20,10 +20,16 @@
 
 #include <span>
 
-void
-test01()
+constexpr bool
+test01(bool b)
 {
   std::span<int, 0> s;
-  s.front(); // { dg-error "here" }
+  if (b || !s.empty())
+    s.front();
+  return true;
 }
-// { dg-error "static assertion failed" "" { target *-*-* } 0 }
+
+static_assert(test01(false));
+static_assert(test01(true)); // { dg-error "non-constant" }
+// { dg-error "assert" "" { target *-*-* } 0 }
+// { dg-prune-output "in 'constexpr' expansion" }
diff --git a/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc b/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
index 1e8b2d8724eb..6a3bb8834b43 100644
--- a/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
@@ -20,10 +20,16 @@
 
 #include <span>
 
-void
-test01()
+constexpr bool
+test01(bool b)
 {
   std::span<int, 0> s;
-  s[99]; // { dg-error "here" }
+  if (b || !s.empty())
+    s[99];
+  return true;
 }
-// { dg-error "static assertion failed" "" { target *-*-* } 0 }
+
+static_assert(test01(false));
+static_assert(test01(true)); // { dg-error "non-constant" }
+// { dg-error "assert" "" { target *-*-* } 0 }
+// { dg-prune-output "in 'constexpr' expansion" }