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[].

4 files changed, 30 insertions, 15 deletions

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index f658adb..1cdc058 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 c451ed1..f777edf 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 38f87aa..14e5bc1 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 1e8b2d8..6a3bb88 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" }