From 51c42b38e43b5283b116882529d232719b099bfc Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 2 Sep 2022 11:19:51 -0400
Subject: [PATCH] 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_.
---
 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 8b11f31741b7..be9f29555395 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 86996b27fa5d..ff90f8a47c31 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
-- 
2.47.2