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

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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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323380 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eric Fiselier
2018-01-24 22:14:01 +00:00
parent e54a22f809
commit 8286acce44
4 changed files with 159 additions and 61 deletions
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21
include/tuple
View File

@@ -173,16 +173,9 @@ class __tuple_leaf
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&);
@@ -224,15 +217,15 @@ public:
_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