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). 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