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libstdc++: Fix common_reference for non-reference results [PR100894]

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The result of COMMON-REF(A&, B&&) where they have no common reference
type should not be a reference. The implementation of COMMON-REF fails
to check that the result is a reference, so is well-formed when it
shouldn't be. This means that common_reference uses that result when it
shouldn't.

The fix is to reject the result of COMMON-REF(A, B) if it's not a
reference, so that common_reference falls through to the next case,
which uses COND-RES, which yields a non-reference result.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/100894
	* include/std/type_traits (__common_ref_impl<X&, Y&>): Only
	use the type if it's a reference.
	* testsuite/20_util/common_reference/100894.cc: New test.
This commit is contained in:
Jonathan Wakely 2021-06-14 20:31:00 +01:00
parent 4986946f3b
commit c37b5ddcc8
2 changed files with 18 additions and 3 deletions
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12
libstdc++-v3/include/std/type_traits
View file

@ -3340,11 +3340,17 @@ template <typename _From, typename _To>
template<typename _Ap, typename _Bp>
using __common_ref = typename __common_ref_impl<_Ap, _Bp>::type;
// COND-RES(COPYCV(X, Y) &, COPYCV(Y, X) &)
template<typename _Xp, typename _Yp>
using __condres_cvref
= __cond_res<__copy_cv<_Xp, _Yp>&, __copy_cv<_Yp, _Xp>&>;
// If A and B are both lvalue reference types, ...
template<typename _Xp, typename _Yp>
struct __common_ref_impl<_Xp&, _Yp&,
__void_t<__cond_res<__copy_cv<_Xp, _Yp>&, __copy_cv<_Yp, _Xp>&>>>
{ using type = __cond_res<__copy_cv<_Xp, _Yp>&, __copy_cv<_Yp, _Xp>&>; };
struct __common_ref_impl<_Xp&, _Yp&, __void_t<__condres_cvref<_Xp, _Yp>>>
: enable_if<is_reference_v<__condres_cvref<_Xp, _Yp>>,
__condres_cvref<_Xp, _Yp>>
{ };
// let C be remove_reference_t<COMMON-REF(X&, Y&)>&&
template<typename _Xp, typename _Yp>

9
libstdc++-v3/testsuite/20_util/common_reference/100894.cc Normal file
View file

@ -0,0 +1,9 @@
// { dg-options "-std=gnu++20" }
// { dg-do compile { target c++20 } }
// PR libstdc++/100894 - common_reference implementation seems to be wrong
#include <type_traits>
struct A {};
struct B { B(A); };
static_assert( std::is_same_v<std::common_reference_t<A&, B&&>, B> );