On Fri, 2 Sep 2022, Patrick Palka wrote:
> r13-2230-g390f94eee1ae69 redefined the internal logical operator traits
> __and_, __or_ and __not as alias templates that directly resolve to
> true_type or false_type. But it turns out using an alias template here
> causes the traits to be less lazy than before because we now compute
> the logical result immediately upon _specialization_ of the trait, and
> not later upon _completion_ of the specialization.
>
> Thus, for example, in
>
> using type = __and_<A, __not_<B>>;
>
> we now compute the conjunction and thus instantiate A even though we're
> in a context that doesn't require completion of the __and_. What's
> worse is that we now compute the negation and thus instantiate B as well
> (for the same reason), independent of the __and_ and the value of A!
> Thus the traits are now less lazy and composable than before.
>
> Fortunately, the fix is cheap and simple: redefine these traits as class
> templates instead of as alias templates so that completion not
> specialization triggers computation of the logical result. I added
> comprehensive short circuiting tests for these internal logical operator
> traits in short_circuit.cc guarded by __GLIBCXX__, not sure if
> that's the best place for them. (Note that before this fix, assert #5
> and #10 guarded by __GLIBCXX__ would induce a hard error due to this
> bug).
FWIW this change doesn't seem to have a measurable compile time/memory
impact on the stress test from r13-2230. For std/ranges/adaptors/join.cc,
memory usage increases by around 1% and compile time decreases by around
1%.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> libstdc++-v3/ChangeLog:
>
> * include/std/type_traits (__or_, __and_, __not_): Redefine as a
> class template instead of an alias template.
> * testsuite/20_util/logical_traits/requirements/short_circuit.cc:
> Add more tests for conjunction and disjunction. Add corresponding
> tests for __and_ and __or_v.
> ---
> libstdc++-v3/include/std/type_traits | 12 ++++++--
> .../requirements/short_circuit.cc | 29 +++++++++++++++++++
> 2 files changed, 38 insertions(+), 3 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index 615791f29c8..2feb4b145c5 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -168,13 +168,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // to either true_type or false_type which allows for a more efficient
> // implementation that avoids recursive class template instantiation.
> template<typename... _Bn>
> - using __or_ = decltype(__detail::__or_fn<_Bn...>(0));
> + struct __or_
> + : decltype(__detail::__or_fn<_Bn...>(0))
> + { };
>
> template<typename... _Bn>
> - using __and_ = decltype(__detail::__and_fn<_Bn...>(0));
> + struct __and_
> + : decltype(__detail::__and_fn<_Bn...>(0))
> + { };
>
> template<typename _Pp>
> - using __not_ = __bool_constant<!bool(_Pp::value)>;
> + struct __not_
> + : __bool_constant<!bool(_Pp::value)>
> + { };
> /// @endcond
>
> #if __cplusplus >= 201703L
> diff --git
> a/libstdc++-v3/testsuite/20_util/logical_traits/requirements/short_circuit.cc
> b/libstdc++-v3/testsuite/20_util/logical_traits/requirements/short_circuit.cc
> index 86996b27fa5..ff90f8a47c3 100644
> ---
> a/libstdc++-v3/testsuite/20_util/logical_traits/requirements/short_circuit.cc
> +++
> b/libstdc++-v3/testsuite/20_util/logical_traits/requirements/short_circuit.cc
> @@ -14,6 +14,10 @@ static_assert(!std::conjunction_v<std::false_type,
> invalid>);
> static_assert(!std::conjunction_v<std::false_type, invalid, invalid>);
> static_assert(!std::conjunction_v<std::true_type, std::false_type, invalid>);
> static_assert(!std::conjunction_v<std::true_type, std::false_type, invalid,
> invalid>);
> +static_assert(!std::conjunction_v<std::false_type,
> + std::conjunction<invalid>,
> + std::disjunction<invalid>,
> + std::negation<invalid>>);
>
> // [meta.logical]/8: For a specialization disjunction<B_1, ..., B_n>, if
> // there is a template type argument B_i for which bool(B_i::value) is true,
> @@ -24,3 +28,28 @@ static_assert(std::disjunction_v<std::true_type, invalid>);
> static_assert(std::disjunction_v<std::true_type, invalid, invalid>);
> static_assert(std::disjunction_v<std::false_type, std::true_type, invalid>);
> static_assert(std::disjunction_v<std::false_type, std::true_type, invalid,
> invalid>);
> +static_assert(std::disjunction_v<std::true_type,
> + std::conjunction<invalid>,
> + std::disjunction<invalid>,
> + std::negation<invalid>>);
> +
> +#if __GLIBCXX__
> +// Also test the corresponding internal traits __and_, __or_ and __not_.
> +static_assert(!std::__and_v<std::false_type, invalid>);
> +static_assert(!std::__and_v<std::false_type, invalid, invalid>);
> +static_assert(!std::__and_v<std::true_type, std::false_type, invalid>);
> +static_assert(!std::__and_v<std::true_type, std::false_type, invalid,
> invalid>);
> +static_assert(!std::__and_v<std::false_type,
> + std::__and_<invalid>,
> + std::__or_<invalid>,
> + std::__not_<invalid>>);
> +
> +static_assert(std::__or_v<std::true_type, invalid>);
> +static_assert(std::__or_v<std::true_type, invalid, invalid>);
> +static_assert(std::__or_v<std::false_type, std::true_type, invalid>);
> +static_assert(std::__or_v<std::false_type, std::true_type, invalid,
> invalid>);
> +static_assert(std::__or_v<std::true_type,
> + std::__and_<invalid>,
> + std::__or_<invalid>,
> + std::__not_<invalid>>);
> +#endif
> --
> 2.37.2.490.g6c8e4ee870
>
>
