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 > >