https://gcc.gnu.org/g:bbc0e70b610f19320339e4a311591b42bd436e9b
commit r16-3435-gbbc0e70b610f19320339e4a311591b42bd436e9b
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Sat Jul 12 13:09:03 2025 +0100
libstdc++: Constrain bitset(const CharT*) constructor [PR121046]
Asking std::is_constructible_v<std::bitset<1>, NonTrivial*> gives an
error, rather than answering the query. The problem is that the
constructor for std::bitset("010101") is not constrained to only accept
pointers to char-like types, and for the second parameter (which has a
default argument) std::basic_string_view<CharT> gets instantiated. If
the type is not char-like then that has undefined behaviour, and might
trigger a static_assert to fail in the body of std::basic_string_view.
We can fix it by constraining that constructor using the requirements
for char-like types from [strings.general] p1. I've submitted LWG 4294
and proposed making this change in the standard.
libstdc++-v3/ChangeLog:
PR libstdc++/121046
* include/std/bitset (bitset(const CharT*, ...)): Add
constraints on CharT type.
* testsuite/23_containers/bitset/lwg4294.cc: New test.
Diff:
---
libstdc++-v3/include/std/bitset | 8 +++++++-
libstdc++-v3/testsuite/23_containers/bitset/lwg4294.cc | 11 +++++++++++
2 files changed, 18 insertions(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/std/bitset b/libstdc++-v3/include/std/bitset
index 93a03f6b5d72..92f11f19807a 100644
--- a/libstdc++-v3/include/std/bitset
+++ b/libstdc++-v3/include/std/bitset
@@ -1040,6 +1040,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
#endif
#if __cplusplus >= 201103L
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 4294. bitset(const CharT*) constructor needs to be constrained
/**
* Construct from a character %array.
* @param __str An %array of characters `__zero` and `__one`.
@@ -1049,7 +1051,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
* @throw std::invalid_argument If a character appears in the string
* which is neither `__zero` nor `__one`.
*/
- template<typename _CharT>
+ template<typename _CharT,
+ typename = _Require<is_trivially_copyable<_CharT>,
+ is_standard_layout<_CharT>,
+ is_trivially_default_constructible<_CharT>,
+ __not_<is_array<_CharT>>>>
[[__gnu__::__nonnull__]]
_GLIBCXX23_CONSTEXPR
explicit
diff --git a/libstdc++-v3/testsuite/23_containers/bitset/lwg4294.cc
b/libstdc++-v3/testsuite/23_containers/bitset/lwg4294.cc
new file mode 100644
index 000000000000..977555f9e1fd
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/bitset/lwg4294.cc
@@ -0,0 +1,11 @@
+// { dg-do compile { target c++11 } }
+
+// Bug 121046
+// Asking is_constructible_v<std::bitset<1>, NonTrivial*> is ill-formed
+
+// LWG 4294. bitset(const CharT*) constructor needs to be constrained
+
+#include <bitset>
+struct NonTrivial { ~NonTrivial() { } };
+static_assert( ! std::is_constructible<std::bitset<1>, NonTrivial*>::value,
+ "std::bitset cannot be constructed from this pointer" );
