[PATCH] libstdc++: Unnecessary type completion in __is_complete_or_unbounded [PR120717]

[Tomasz Kaminski]

On Tue, Jun 24, 2025 at 4:19 AM Patrick Palka <ppalka@redhat.com> wrote:

> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> -- >8 --
>
> When checking __is_complete_or_unbounded on a reference to incomplete
> type, we overeagerly try to instantiate/complete the referenced type
> which besides being unnecessary may also produce a -Wsfinae-incomplete
> warning (added in r16-1527) if the referenced type is later defined.
>
> This patch fixes this by effectively restricting the sizeof check to
> object (except unknown-bound array) types.  In passing simplify the
> implementation by using is_object instead of is_function/reference/void.
>
>         PR libstdc++/120717
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/type_traits (__is_complete_or_unbounded): Don't
>         check sizeof on a reference or unbounded array type.  Simplify
>         using is_object.  Correct formatting.
>         * testsuite/20_util/is_complete_or_unbounded/120717.cc: New test.
> ---
>  libstdc++-v3/include/std/type_traits          | 34 +++++++++----------
>  .../is_complete_or_unbounded/120717.cc        | 20 +++++++++++
>  2 files changed, 37 insertions(+), 17 deletions(-)
>  create mode 100644
> libstdc++-v3/testsuite/20_util/is_complete_or_unbounded/120717.cc
>
LGTM. Thanks.

>
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index abff9f880001..28960befd2c7 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -280,11 +280,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>    // Forward declarations
>    template<typename>
> -    struct is_reference;
> -  template<typename>
> -    struct is_function;
> -  template<typename>
> -    struct is_void;
> +    struct is_object;
>    template<typename>
>      struct remove_cv;
>    template<typename>
> @@ -297,18 +293,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    // Helper functions that return false_type for incomplete classes,
>    // incomplete unions and arrays of known bound from those.
>
> -  template <typename _Tp, size_t = sizeof(_Tp)>
> -    constexpr true_type __is_complete_or_unbounded(__type_identity<_Tp>)
> -    { return {}; }
> -
> -  template <typename _TypeIdentity,
> -      typename _NestedType = typename _TypeIdentity::type>
> -    constexpr typename __or_<
> -      is_reference<_NestedType>,
> -      is_function<_NestedType>,
> -      is_void<_NestedType>,
> -      __is_array_unknown_bounds<_NestedType>
> -    >::type __is_complete_or_unbounded(_TypeIdentity)
> +  // More specialized overload for complete object types.
> +  template<typename _Tp,
> +          typename = __enable_if_t<!__or_<__not_<is_object<_Tp>>,
> +
> __is_array_unknown_bounds<_Tp>>::value>,
> +          size_t = sizeof(_Tp)>
> +    constexpr true_type
> +    __is_complete_or_unbounded(__type_identity<_Tp>)
> +    { return {}; };
> +
> +  // Less specialized overload for reference and unknown-bound array
> types, and
> +  // incomplete types.
> +  template<typename _TypeIdentity,
> +          typename _NestedType = typename _TypeIdentity::type>
> +    constexpr typename __or_<__not_<is_object<_NestedType>>,
> +                            __is_array_unknown_bounds<_NestedType>>::type
> +    __is_complete_or_unbounded(_TypeIdentity)
>      { return {}; }
>
>    // __remove_cv_t (std::remove_cv_t for C++11).
> diff --git
> a/libstdc++-v3/testsuite/20_util/is_complete_or_unbounded/120717.cc
> b/libstdc++-v3/testsuite/20_util/is_complete_or_unbounded/120717.cc
> new file mode 100644
> index 000000000000..31fdf8fe9227
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/is_complete_or_unbounded/120717.cc
> @@ -0,0 +1,20 @@
> +// PR libstdc++/120717
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options "-Wsfinae-incomplete" }
> +
> +#include <type_traits>
> +
> +// Verify __is_complete_or_unbounded doesn't try to instantiate the
> underlying
> +// type of a reference or array of unknown bound.
> +template<class T> struct A { static_assert(false, "do not instantiate");
> };
> +static_assert(std::__is_complete_or_unbounded(std::__type_identity<A<int>&>{}),
> "");
> +static_assert(std::__is_complete_or_unbounded(std::__type_identity<A<int>&&>{}),
> "");
> +static_assert(std::__is_complete_or_unbounded(std::__type_identity<A<int>[]>{}),
> "");
> +
> +// Verify __is_complete_or_unbounded doesn't produce -Wsfinae-incomplete
> +// warnings.
> +struct B;
> +static_assert(std::__is_complete_or_unbounded(std::__type_identity<B&>{}),
> "");
> +static_assert(std::__is_complete_or_unbounded(std::__type_identity<B&&>{}),
> "");
> +static_assert(std::__is_complete_or_unbounded(std::__type_identity<B[]>{}),
> "");
> +struct B { }; // { dg-bogus "-Wsfinae-incomplete" }
> --
> 2.50.0.81.gcb3b40381e
>
>
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