On Thu, Mar 12, 2026 at 11:34 AM Jonathan Wakely <[email protected]> wrote:
> Using requires { sizeof(T); } in __new_allocator::allocate gives a
> better diagnostic when the static_assert fails.
>
> We can also reduce the total number of diagnostics due to invalid
> sizeof(T) and alignof(T) by using the same requires-expression to make
> the body of the function discarded statement when the static_assert
> already failed. This fixes a regression in diagnostic quality due to
> making __new_allocator::allocate constexpr in r16-7271-g7197d0cce70525,
> which caused a cascade of seven errors instead of just one for
> std::allocator<incomplete-type>().allocate(1).
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/allocator.h (allocator::allocate): Use specific
> feature test macro for constexpr allocate and deallocate. Make
> consteval path a discarded statement if sizeof(T) is ill-formed.
> * include/bits/new_allocator.h (__new_allocator::allocate): Use
> requires-expression for static_cast. Make function body a
> discarded stament if sizeof(T) is ill-formed. Use if-constexpr
> for alignment checks.
> ---
>
> Tested x86_64-linux.
>
LGTM
>
> libstdc++-v3/include/bits/allocator.h | 5 ++++-
> libstdc++-v3/include/bits/new_allocator.h | 22 +++++++++++++++++-----
> 2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/allocator.h
> b/libstdc++-v3/include/bits/allocator.h
> index a5e115265911..9c22c805ebe7 100644
> --- a/libstdc++-v3/include/bits/allocator.h
> +++ b/libstdc++-v3/include/bits/allocator.h
> @@ -188,11 +188,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif
> ~allocator() _GLIBCXX_NOTHROW { }
>
> -#if __cplusplus > 201703L
> +#if __cpp_constexpr_dynamic_alloc // >= C++20
> [[nodiscard,__gnu__::__always_inline__]]
> constexpr _Tp*
> allocate(size_t __n)
> {
> +#if __cpp_concepts
> + if constexpr (requires { sizeof(_Tp); })
> +#endif
> if (std::__is_constant_evaluated())
> {
> if (__builtin_mul_overflow(__n, sizeof(_Tp), &__n))
> diff --git a/libstdc++-v3/include/bits/new_allocator.h
> b/libstdc++-v3/include/bits/new_allocator.h
> index 4fe67f99e5c0..fbe03e392aae 100644
> --- a/libstdc++-v3/include/bits/new_allocator.h
> +++ b/libstdc++-v3/include/bits/new_allocator.h
> @@ -120,6 +120,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> # define _GLIBCXX_OPERATOR_DELETE ::operator delete
> #endif
>
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
> +
> // NB: __n is permitted to be 0. The C++ standard says nothing
> // about what the return value is when __n == 0.
> _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR _Tp*
> @@ -128,9 +131,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #if __cplusplus >= 201103L
> // _GLIBCXX_RESOLVE_LIB_DEFECTS
> // 3308. std::allocator<void>().allocate(n)
> +#if ! __cpp_concepts
> static_assert(sizeof(_Tp) != 0, "cannot allocate incomplete
> types");
> -#endif
> +#else
> + static_assert(requires { sizeof(_Tp); },
> + "cannot allocate incomplete types");
>
> + if constexpr (!requires { sizeof(_Tp); })
> + return nullptr; // static_assert already failed
> + else
> +#endif
> +#endif
> if (__builtin_expect(__n > this->_M_max_size(), false))
> {
> // _GLIBCXX_RESOLVE_LIB_DEFECTS
> @@ -139,16 +150,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> std::__throw_bad_array_new_length();
> std::__throw_bad_alloc();
> }
> -
> #if __cpp_aligned_new && __cplusplus >= 201103L
> - if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
> + else if constexpr (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
> {
> std::align_val_t __al = std::align_val_t(alignof(_Tp));
> return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n *
> sizeof(_Tp),
> __al));
> }
> #endif
> - return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n * sizeof(_Tp)));
> + else
> + return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n *
> sizeof(_Tp)));
> }
>
> // __p is not permitted to be a null pointer.
> @@ -162,7 +173,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif
>
> #if __cpp_aligned_new && __cplusplus >= 201103L
> - if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
> + if constexpr (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
> {
> _GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p, __n),
> std::align_val_t(alignof(_Tp)));
> @@ -172,6 +183,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> _GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p, __n));
> }
>
> +#pragma GCC diagnostic pop
> #undef _GLIBCXX_SIZED_DEALLOC
> #undef _GLIBCXX_OPERATOR_DELETE
> #undef _GLIBCXX_OPERATOR_NEW
> --
> 2.53.0
>
>
