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

[EricWF]

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");

[rsmith]

include/tuple

176–178

This line looks wrong to me. This disallows an rvalue reference tuple member from binding to an rvalue reference argument, no? Can you

return !is_reference<_Hp>::value || (is_reference<_Tp>::value && is_convertible<_RawTp*, _RawHp*>::value) || (reference wrapper special case);

instead? That should at least only reject valid code in cases where _RawTp is a class type that converts to a reference type.

177–178

It would be reasonable to consider is_base_of here too.

[rsmith]

This will still diagnose valid and reasonable programs, such as:

struct ConvertToRef { operator int&(); };
std::tuple<int&> t = {ConvertToRef()};

... on compilers that don't provide the trait. You could maybe try to work around that by checking to see if the type has a member .operator int&(). But perhaps it's better to remove the non-conforming check entirely, at least in the case where you can't reasonably get it right.

[EricWF]

In D41977#975403, @rsmith wrote:

This will still diagnose valid and reasonable programs, such as:

struct ConvertToRef { operator int&(); };
std::tuple<int&> t = {ConvertToRef()};

... on compilers that don't provide the trait. You could maybe try to work around that by checking to see if the type has a member .operator int&(). But perhaps it's better to remove the non-conforming check entirely, at least in the case where you can't reasonably get it right.

I agree with both your comments. I'll remove the non-conforming check.

[EricWF]

• Address @rsmith's comments by removing the fallback implementation of the diagnostics.

[rsmith]

Looks good, thanks!

Maybe the assertion message could be clearer about why this is a problem though? ("binds to a temporary whose lifetime has ended" maybe?)

[EricWF]

• Improve diagnostic as per @rsmiths suggestion.

[timshen]

__can_bind_reference() doesn't return anything when __reference_binds_to_temporary doesn't exist. This causes builds break with old compilers.

Should it just return true if __reference_binds_to_temporary doesn't exist?

[timshen]

Created D42510 for discussion. I'm not sure if it's a good idea, though.

include/tuple

 
 template <size_t _Ip, class _Hp, bool>
 class __tuple_leaf
 {
     _Hp __value_;
 
     template <class _Tp>
     static constexpr bool __can_bind_reference() {
-        using _RawTp = typename remove_reference<_Tp>::type;
-        using _RawHp = typename remove_reference<_Hp>::type;
-        using _CheckLValueArg = integral_constant<bool,
-            is_lvalue_reference<_Tp>::value
-        ||  is_same<_RawTp, reference_wrapper<_RawHp>>::value
-        ||  is_same<_RawTp, reference_wrapper<typename remove_const<_RawHp>::type>>::value
-        >;
-        return  !is_reference<_Hp>::value
-            || (is_lvalue_reference<_Hp>::value && _CheckLValueArg::value)
-            || (is_rvalue_reference<_Hp>::value && !is_lvalue_reference<_Tp>::value);
+#if __has_keyword(__reference_binds_to_temporary)
+      return !__reference_binds_to_temporary(_Hp, _Tp);
+#endif
     }
 
     __tuple_leaf& operator=(const __tuple_leaf&);
 public:
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR __tuple_leaf()
              _NOEXCEPT_(is_nothrow_default_constructible<_Hp>::value) : __value_()
        {static_assert(!is_reference<_Hp>::value,
               "Attempted to default construct a reference element in a tuple");}
template <class _Tp,
                       __lazy_not<is_same<typename decay<_Tp>::type, __tuple_leaf>>
                     , is_constructible<_Hp, _Tp>
                     >::value
                 >::type
             >
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
         explicit __tuple_leaf(_Tp&& __t) _NOEXCEPT_((is_nothrow_constructible<_Hp, _Tp>::value))
             : __value_(_VSTD::forward<_Tp>(__t))
-        {static_assert(__can_bind_reference<_Tp>(),
-       "Attempted to construct a reference element in a tuple with an rvalue");}
+        {static_assert(__can_bind_reference<_Tp&&>(),
+       "Attempted construction of reference element binds to a temporary whose lifetime has ended");}
 
     template <class _Tp, class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         explicit __tuple_leaf(integral_constant<int, 0>, const _Alloc&, _Tp&& __t)
             : __value_(_VSTD::forward<_Tp>(__t))
-        {static_assert(__can_bind_reference<_Tp>(),
-       "Attempted to construct a reference element in a tuple with an rvalue");}
+        {static_assert(__can_bind_reference<_Tp&&>(),
+       "Attempted construction of reference element binds to a temporary whose lifetime has ended");}
 
     template <class _Tp, class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         explicit __tuple_leaf(integral_constant<int, 1>, const _Alloc& __a, _Tp&& __t)
             : __value_(allocator_arg_t(), __a, _VSTD::forward<_Tp>(__t))
         {static_assert(!is_reference<_Hp>::value,
             "Attempted to uses-allocator construct a reference element in a tuple");}
 

test/libcxx/utilities/tuple/tuple.tuple/diagnose_reference_binding.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-//                     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>
-
-// Test the diagnostics libc++ generates for invalid reference binding.
-// Libc++ attempts to diagnose the following cases:
-//  * Constructing an lvalue reference from an rvalue.
-//  * Constructing an rvalue reference from an lvalue.
-
-#include <tuple>
-#include <string>
-
-int main() {
-    std::allocator<void> alloc;
-
-    // expected-error-re@tuple:* 4 {{static_assert failed{{.*}} "Attempted to construct a reference element in a tuple with an rvalue"}}
-
-    // bind lvalue to rvalue
-    std::tuple<int const&> t(42); // expected-note {{requested here}}
-    std::tuple<int const&> t1(std::allocator_arg, alloc, 42); // expected-note {{requested here}}
-    // bind rvalue to constructed non-rvalue
-    std::tuple<std::string &&> t2("hello"); // expected-note {{requested here}}
-    std::tuple<std::string &&> t3(std::allocator_arg, alloc, "hello"); // expected-note {{requested here}}
-
-    // FIXME: The below warnings may get emitted as an error, a warning, or not emitted at all
-    // depending on the flags used to compile this test.
-  {
-    // expected-warning@tuple:* 0+ {{binding reference member '__value_' to a temporary value}}
-    // expected-error@tuple:* 0+ {{binding reference member '__value_' to a temporary value}}
-  }
-}

test/libcxx/utilities/tuple/tuple.tuple/diagnose_reference_binding.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-//                     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>
-
-// Test the diagnostics libc++ generates for invalid reference binding.
-// Libc++ attempts to diagnose the following cases:
-//  * Constructing an lvalue reference from an rvalue.
-//  * Constructing an rvalue reference from an lvalue.
-
-#include <tuple>
-#include <string>
-#include <functional>
-#include <cassert>
-
-static_assert(std::is_constructible<int&, std::reference_wrapper<int>>::value, "");
-static_assert(std::is_constructible<int const&, std::reference_wrapper<int>>::value, "");
-
-
-int main() {
-    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);
-    }
-}

test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.fail.cpp

+// -*- 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
+
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wdangling-field"
+#endif
+
+#include <tuple>
+#include <string>
+#include "test_macros.h"
+
+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 {};
+
+template <class T> struct CannotDeduce {
+ using type = T;
+};
+
+template <class ...Args>
+void F(typename CannotDeduce<std::tuple<Args...>>::type const&) {}
+
+
+int main() {
+#if TEST_HAS_BUILTIN_IDENTIFIER(__reference_binds_to_temporary)
+  // expected-error@tuple:* 8 {{"Attempted construction of reference element binds to a temporary whose lifetime has ended"}}
+  {
+    F<int, const std::string&>(std::make_tuple(1, "abc")); // expected-note 1 {{requested here}}
+  }
+  {
+    std::tuple<int, const std::string&> t(1, "a"); // expected-note 1 {{requested here}}
+  }
+  {
+    F<int, const std::string&>(std::tuple<int, const std::string&>(1, "abc")); // expected-note 1 {{requested here}}
+  }
+  {
+    ConvertsTo<int&> ct;
+    std::tuple<const long&, int> t(ct, 42); // expected-note {{requested here}}
+  }
+  {
+    ConvertsTo<int> ct;
+    std::tuple<int const&, void*> t(ct, nullptr); // expected-note {{requested here}}
+  }
+  {
+    ConvertsTo<Derived> ct;
+    std::tuple<Base const&, int> t(ct, 42); // expected-note {{requested here}}
+  }
+  {
+    std::allocator<void> alloc;
+    std::tuple<std::string &&> t2("hello"); // expected-note {{requested here}}
+    std::tuple<std::string &&> t3(std::allocator_arg, alloc, "hello"); // expected-note {{requested here}}
+  }
+#else
+#error force failure
+// expected-error@-1 {{force failure}}
+#endif
+}

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp

+// -*- 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();
+}