On Wed, Oct 1, 2025 at 2:34 PM Jonathan Wakely <[email protected]> wrote:
> Running the testsuite with warnings enabled gives:
>
> FAIL: 20_util/specialized_algorithms/uninitialized_copy/58982.cc
> -std=gnu++26 (test for excess errors)
> Excess errors:
> .../libstdc++-v3/include/bits/stl_uninitialized.h:293: warning: 'void*
> __builtin_memcpy(void*, const void*, long unsigned int)' writing to an
> object of type 'struct T' with no trivial copy-assignment; use
> copy-initialization instead [-Wclass-memaccess]
>
> This is because -Wclass-memaccess warns about using memcpy on types
> which have a deleted assignment, even though those can be trivially
> copyable and so using memcpy on them is technically valid. Where these
> warnings occur in bits/stl_uninitialized.h we're only using memcpy after
> checking for trivially copyable (and any other relevant conditions) so
> we can be confident that we're using it safely. We also don't actually
> care about assignment here because we're only constructing new objects,
> not copying over existing ones (which is what std::copy does, but not
> std::uninitialized_copy).
>
> Uses of memcpy in the C++98 code don't need to have -Wclass-memaccess
> suppressed, because there are no deleted functions in C++98 so there are
> no types which are trivially copyable but trigger the warning.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/stl_uninitialized.h (uninitialized_copy)
> (uninitialized_fill, uninitialized_fill_n): Use pragmas to
> suppress -Wclass-memaccess warnings.
> ---
>
> Tested powerpc64le-linux.
>
LGTM
>
> libstdc++-v3/include/bits/stl_uninitialized.h | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h
> b/libstdc++-v3/include/bits/stl_uninitialized.h
> index 0398b65fa140..70a564659814 100644
> --- a/libstdc++-v3/include/bits/stl_uninitialized.h
> +++ b/libstdc++-v3/include/bits/stl_uninitialized.h
> @@ -236,6 +236,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> #pragma GCC diagnostic push
> #pragma GCC diagnostic ignored "-Wc++17-extensions"
> +#pragma GCC diagnostic ignored "-Wclass-memaccess"
> /**
> * @brief Copies the range [first,last) into result.
> * @param __first An input iterator.
> @@ -427,6 +428,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #if __cplusplus >= 201103L
> #pragma GCC diagnostic push
> #pragma GCC diagnostic ignored "-Wc++17-extensions"
> +#pragma GCC diagnostic ignored "-Wclass-memaccess"
> #if __glibcxx_raw_memory_algorithms >= 202411L // >= C++26
> if consteval {
> return std::__do_uninit_fill(__first, __last, __x);
> @@ -529,6 +531,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> #pragma GCC diagnostic push
> #pragma GCC diagnostic ignored "-Wc++17-extensions"
> +#pragma GCC diagnostic ignored "-Wclass-memaccess"
> // _GLIBCXX_RESOLVE_LIB_DEFECTS
> // DR 1339. uninitialized_fill_n should return the end of its range
> /**
> --
> 2.51.0
>
>
