[libc++] Fix PR20855 -- libc++ incorrectly diagnoses illegal reference binding in std::tuple.

Summary:
See https://bugs.llvm.org/show_bug.cgi?id=20855

Libc++ goes out of it's way to diagnose `std::tuple` constructions which are UB due to lifetime bugs caused by reference creation. For example:

```
// The 'const std::string&' is created *inside* the tuple constructor, and its lifetime is over before the end of the constructor call.
std::tuple<int, const std::string&> t(std::make_tuple(42, "abc"));
```

However, we are over-aggressive and we incorrectly diagnose cases such as:

```
void foo(std::tuple<int const&, int const&> const&);
foo(std::make_tuple(42, 42));
```

This patch fixes the incorrectly diagnosed cases, as well as converting the diagnostic to use the newly added Clang trait `__reference_binds_to_temporary`. The new trait allows us to diagnose cases we previously couldn't such as:

```
std::tuple<int, const std::string&> t(42, "abc");
```

Reviewers: rsmith, mclow.lists

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41977

llvm-svn: 323380

1 files changed, 136 insertions, 0 deletions

diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
new file mode 100644
index 00000000000..4680635441d
--- /dev/null
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
@@ -0,0 +1,136 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <tuple>
+
+// See llvm.org/PR20855
+
+#include <tuple>
+#include <string>
+#include "test_macros.h"
+
+#if TEST_HAS_BUILTIN_IDENTIFIER(__reference_binds_to_temporary)
+# define ASSERT_REFERENCE_BINDS_TEMPORARY(...) static_assert(__reference_binds_to_temporary(__VA_ARGS__), "")
+# define ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(...) static_assert(!__reference_binds_to_temporary(__VA_ARGS__), "")
+#else
+# define ASSERT_REFERENCE_BINDS_TEMPORARY(...) static_assert(true, "")
+# define ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(...) static_assert(true, "")
+#endif
+
+template <class Tp>
+struct ConvertsTo {
+  using RawTp = typename std::remove_cv< typename std::remove_reference<Tp>::type>::type;
+
+  operator Tp() const {
+    return static_cast<Tp>(value);
+  }
+
+  mutable RawTp value;
+};
+
+struct Base {};
+struct Derived : Base {};
+
+
+static_assert(std::is_same<decltype("abc"), decltype(("abc"))>::value, "");
+ASSERT_REFERENCE_BINDS_TEMPORARY(std::string const&, decltype("abc"));
+ASSERT_REFERENCE_BINDS_TEMPORARY(std::string const&, decltype(("abc")));
+ASSERT_REFERENCE_BINDS_TEMPORARY(std::string const&, const char*&&);
+
+ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(int&, const ConvertsTo<int&>&);
+ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(const int&, ConvertsTo<int&>&);
+ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(Base&, Derived&);
+
+
+static_assert(std::is_constructible<int&, std::reference_wrapper<int>>::value, "");
+static_assert(std::is_constructible<int const&, std::reference_wrapper<int>>::value, "");
+
+template <class T> struct CannotDeduce {
+ using type = T;
+};
+
+template <class ...Args>
+void F(typename CannotDeduce<std::tuple<Args...>>::type const&) {}
+
+void compile_tests() {
+  {
+    F<int, int const&>(std::make_tuple(42, 42));
+  }
+  {
+    F<int, int const&>(std::make_tuple<const int&, const int&>(42, 42));
+    std::tuple<int, int const&> t(std::make_tuple<const int&, const int&>(42, 42));
+  }
+  {
+    auto fn = &F<int, std::string const&>;
+    fn(std::tuple<int, std::string const&>(42, std::string("a")));
+    fn(std::make_tuple(42, std::string("a")));
+  }
+  {
+    Derived d;
+    std::tuple<Base&, Base const&> t(d, d);
+  }
+  {
+    ConvertsTo<int&> ct;
+    std::tuple<int, int&> t(42, ct);
+  }
+}
+
+void allocator_tests() {
+    std::allocator<void> alloc;
+    int x = 42;
+    {
+        std::tuple<int&> t(std::ref(x));
+        assert(&std::get<0>(t) == &x);
+        std::tuple<int&> t1(std::allocator_arg, alloc, std::ref(x));
+        assert(&std::get<0>(t1) == &x);
+    }
+    {
+        auto r = std::ref(x);
+        auto const& cr = r;
+        std::tuple<int&> t(r);
+        assert(&std::get<0>(t) == &x);
+        std::tuple<int&> t1(cr);
+        assert(&std::get<0>(t1) == &x);
+        std::tuple<int&> t2(std::allocator_arg, alloc, r);
+        assert(&std::get<0>(t2) == &x);
+        std::tuple<int&> t3(std::allocator_arg, alloc, cr);
+        assert(&std::get<0>(t3) == &x);
+    }
+    {
+        std::tuple<int const&> t(std::ref(x));
+        assert(&std::get<0>(t) == &x);
+        std::tuple<int const&> t2(std::cref(x));
+        assert(&std::get<0>(t2) == &x);
+        std::tuple<int const&> t3(std::allocator_arg, alloc, std::ref(x));
+        assert(&std::get<0>(t3) == &x);
+        std::tuple<int const&> t4(std::allocator_arg, alloc, std::cref(x));
+        assert(&std::get<0>(t4) == &x);
+    }
+    {
+        auto r = std::ref(x);
+        auto cr = std::cref(x);
+        std::tuple<int const&> t(r);
+        assert(&std::get<0>(t) == &x);
+        std::tuple<int const&> t2(cr);
+        assert(&std::get<0>(t2) == &x);
+        std::tuple<int const&> t3(std::allocator_arg, alloc, r);
+        assert(&std::get<0>(t3) == &x);
+        std::tuple<int const&> t4(std::allocator_arg, alloc, cr);
+        assert(&std::get<0>(t4) == &x);
+    }
+}
+
+
+int main() {
+  compile_tests();
+  allocator_tests();
+}