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* [gcc r13-2379] libstdc++: Fix laziness of __and/or/not_
@ 2022-09-02 15:20 Patrick Palka
  0 siblings, 0 replies; only message in thread
From: Patrick Palka @ 2022-09-02 15:20 UTC (permalink / raw)
  To: gcc-cvs, libstdc++-cvs

commit r13-2379-g51c42b38e43b5283b116882529d232719b099bfc
Author: Patrick Palka <>
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.
            * include/std/type_traits (__or_, __and_, __not_): Redefine as a
            class template instead of as an alias template.
            * testsuite/20_util/logical_traits/requirements/
            Add more tests for conjunction and disjunction.  Add corresponding
            tests for __and_ and __or_.

 libstdc++-v3/include/std/type_traits               | 12 ++++++---
 .../logical_traits/requirements/   | 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
   // 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/ b/libstdc++-v3/testsuite/20_util/logical_traits/requirements/
index 86996b27fa5..ff90f8a47c3 100644
--- a/libstdc++-v3/testsuite/20_util/logical_traits/requirements/
+++ b/libstdc++-v3/testsuite/20_util/logical_traits/requirements/
@@ -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>);
+				  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>);
+				 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>);
+			    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>);
+			  std::__and_<invalid>,
+			  std::__or_<invalid>,
+			  std::__not_<invalid>>);

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