On Mon, 21 Sep 2020, Patrick Palka wrote:
> 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.
Here's a version that rewrites rather than removes the testcases:
-- >8 --
Subject: [PATCH] libstdc++: Remove overzealous static_asserts from std::span
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 would never call them at runtime.
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[].
---
libstdc++-v3/include/std/span | 3 ---
.../testsuite/23_containers/span/back_neg.cc | 14 ++++++++++----
.../testsuite/23_containers/span/front_neg.cc | 14 ++++++++++----
.../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" }
--
2.28.0.497.g54e85e7af1
