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author | Patrick Palka <ppalka@redhat.com> | 2022-09-02 11:19:51 -0400 |
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committer | Patrick Palka <ppalka@redhat.com> | 2022-09-02 11:19:51 -0400 |
commit | 51c42b38e43b5283b116882529d232719b099bfc (patch) | |
tree | a6f7798576181332772fd734822dd47473ec880b | |
parent | 67b6d1be0623de1a8aa32fe249bfa0129c55b11a (diff) | |
download | gcc-51c42b38e43b5283b116882529d232719b099bfc.zip gcc-51c42b38e43b5283b116882529d232719b099bfc.tar.gz gcc-51c42b38e43b5283b116882529d232719b099bfc.tar.bz2 |
libstdc++: Fix laziness of __and/or/not_
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.
So 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 also compute the inner negation and thus instantiate B
(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 straightforward: redefine these traits
as class templates instead of as alias templates so that computation of
the logical result is triggered by completion, not by specialization.
libstdc++-v3/ChangeLog:
* include/std/type_traits (__or_, __and_, __not_): Redefine as a
class template instead of as 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_.
-rw-r--r-- | libstdc++-v3/include/std/type_traits | 12 | ||||
-rw-r--r-- | libstdc++-v3/testsuite/20_util/logical_traits/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 8b11f31..be9f295 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 86996b2..ff90f8a 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 |