[gcc r11-3346] libstdc++: Remove overzealous static_asserts from std::span

[Patrick Palka]

https://gcc.gnu.org/g:37edf28c24b7bd198c27d266af9aefad417635fd

commit r11-3346-g37edf28c24b7bd198c27d266af9aefad417635fd
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Sep 21 20:53:17 2020 -0400

    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:
---
 libstdc++-v3/include/std/span                             |  3 ---
 libstdc++-v3/testsuite/23_containers/span/back_neg.cc     | 14 ++++++++++----
 libstdc++-v3/testsuite/23_containers/span/front_neg.cc    | 14 ++++++++++----
 libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc | 14 ++++++++++----
 4 files changed, 30 insertions(+), 15 deletions(-)

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
index c451ed10df8..f777edfa20c 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 38f87aa2cd5..14e5bc1e100 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 1e8b2d8724e..6a3bb8834b4 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" }