[PATCH] libstdc++: Fix laziness of __and/or/not_

[Patrick Palka <ppalka@redhat.com>]

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