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* [gcc r13-2379] libstdc++: Fix laziness of __and/or/not_
@ 2022-09-02 15:20 Patrick Palka
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From: Patrick Palka @ 2022-09-02 15:20 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:51c42b38e43b5283b116882529d232719b099bfc
commit r13-2379-g51c42b38e43b5283b116882529d232719b099bfc
Author: Patrick Palka <ppalka@redhat.com>
Date: Fri Sep 2 11:19:51 2022 -0400
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_.
Diff:
---
libstdc++-v3/include/std/type_traits | 12 ++++++---
.../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 8b11f31741b..be9f2955539 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
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