From cebaf5cbf8a8bfeb74bd1c82549f1773f84b5474 Mon Sep 17 00:00:00 2001
From: Eric Fiselier <eric@efcs.ca>
Date: Sat, 2 Jul 2016 00:58:17 +0000
Subject: [PATCH] Make tuple_constructible and family lazy again.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@274413 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/__tuple                               |  12 ++-
 .../tuple.cnstr/test_lazy_sfinae.pass.cpp     | 102 ++++++++++++++++++
 2 files changed, 111 insertions(+), 3 deletions(-)
 create mode 100644 test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp

diff --git a/include/__tuple b/include/__tuple
index 2cd528477..0d180c701 100644
--- a/include/__tuple
+++ b/include/__tuple
@@ -373,21 +373,27 @@ using __all = is_same<__all_dummy<_Pred...>, __all_dummy<(_Pred, true)...>>;
 struct __tuple_sfinae_base {
   template <class ..._FromArgs, class ..._ToArgs>
   static auto __test_constructible(__tuple_types<_FromArgs...>, __tuple_types<_ToArgs...>)
-    -> __all<is_constructible<_ToArgs, _FromArgs>::value...>;
+    -> __all<typename enable_if<
+        is_constructible<_ToArgs, _FromArgs>::value
+      , bool>::type{true}...>;
   static auto __test_constructible(...) -> false_type;
   template <class _Types1, class _Types2>
   using __constructible = decltype(__test_constructible(_Types1{}, _Types2{}));
 
   template <class ..._FromArgs, class ..._ToArgs>
   static auto __test_convertible(__tuple_types<_FromArgs...>, __tuple_types<_ToArgs...>)
-    -> __all<is_convertible<_FromArgs, _ToArgs>::value...>;
+    -> __all<typename enable_if<
+          is_convertible<_FromArgs, _ToArgs>::value
+      , bool>::type{true}...>;
   static auto __test_convertible(...) -> false_type;
   template <class _Types1, class _Types2>
   using __convertible = decltype(__test_convertible(_Types1{}, _Types2{}));
 
   template <class ..._FromArgs, class ..._ToArgs>
   static auto __test_assignable(__tuple_types<_FromArgs...>, __tuple_types<_ToArgs...>)
-    -> __all<is_assignable<_ToArgs&, _FromArgs>::value...>;
+    -> __all<typename enable_if<
+          is_assignable<_ToArgs&, _FromArgs>::value
+      , bool>::type{true}...>;
   static auto __test_assignable(...) -> false_type;
   template <class _Types1, class _Types2>
   using __assignable = decltype(__test_assignable(_Types1{}, _Types2{}));
diff --git a/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp b/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp
new file mode 100644
index 000000000..76f7e794a
--- /dev/null
+++ b/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp
@@ -0,0 +1,102 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// UNSUPPORTED: c++98, c++03
+
+#include <tuple>
+#include <utility>
+#include <cassert>
+
+template <class ConstructFrom>
+struct ConstructibleFromT {
+  ConstructibleFromT() = default;
+  ConstructibleFromT(ConstructFrom v) : value(v) {}
+  ConstructFrom value;
+};
+
+template <class AssertOn>
+struct CtorAssertsT {
+  bool defaulted;
+  CtorAssertsT() : defaulted(true) {}
+  template <class T>
+  constexpr CtorAssertsT(T) : defaulted(false) {
+      static_assert(!std::is_same<T, AssertOn>::value, "");
+  }
+};
+
+template <class AllowT, class AssertT>
+struct AllowAssertT {
+  AllowAssertT() = default;
+  AllowAssertT(AllowT) {}
+  template <class U>
+  constexpr AllowAssertT(U) {
+      static_assert(!std::is_same<U, AssertT>::value, "");
+  }
+};
+
+// Construct a tuple<T1, T2> from pair<int, int> where T1 and T2
+// are not constructible from ints but T1 is constructible from std::pair.
+// This considers the following constructors:
+// (1) tuple(TupleLike) -> checks is_constructible<Tn, int>
+// (2) tuple(UTypes...) -> checks is_constructible<T1, pair<int, int>>
+//                            and is_default_constructible<T2>
+// The point of this test is to ensure that the consideration of (1)
+// short circuits before evaluating is_constructible<T2, int>, which
+// will cause a static assertion.
+void test_tuple_like_lazy_sfinae() {
+#if defined(_LIBCPP_VERSION)
+    // This test requires libc++'s reduced arity initialization.
+    using T1 = ConstructibleFromT<std::pair<int, int>>;
+    using T2 = CtorAssertsT<int>;
+    std::pair<int, int> p(42, 100);
+    std::tuple<T1, T2> t(p);
+    assert(std::get<0>(t).value == p);
+    assert(std::get<1>(t).defaulted);
+#endif
+}
+
+
+struct NonConstCopyable {
+  NonConstCopyable() = default;
+  explicit NonConstCopyable(int v) : value(v) {}
+  NonConstCopyable(NonConstCopyable&) = default;
+  NonConstCopyable(NonConstCopyable const&) = delete;
+  int value;
+};
+
+template <class T>
+struct BlowsUpOnConstCopy {
+  BlowsUpOnConstCopy() = default;
+  constexpr BlowsUpOnConstCopy(BlowsUpOnConstCopy const&) {
+      static_assert(!std::is_same<T, T>::value, "");
+  }
+  BlowsUpOnConstCopy(BlowsUpOnConstCopy&) = default;
+};
+
+// Test the following constructors:
+// (1) tuple(Types const&...)
+// (2) tuple(UTypes&&...)
+// Test that (1) short circuits before evaluating the copy constructor of the
+// second argument. Constructor (2) should be selected.
+void test_const_Types_lazy_sfinae()
+{
+    NonConstCopyable v(42);
+    BlowsUpOnConstCopy<int> b;
+    std::tuple<NonConstCopyable, BlowsUpOnConstCopy<int>> t(v, b);
+    assert(std::get<0>(t).value == 42);
+}
+
+int main() {
+    test_tuple_like_lazy_sfinae();
+    test_const_Types_lazy_sfinae();
+}