https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83035
Bug ID: 83035
Summary: Using decltype fails with an operator using non-ADL
lookup
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: tonvandenheuvel at gmail dot com
Target Milestone: ---
Created attachment 42639
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42639&action=edit
preprocessed source file
The following code generates an incorrect compiler error using GCC 7.2.0:
#include <utility>
namespace N
{
struct A {};
struct B {};
}
namespace operators
{
bool operator|(N::A, N::B) { return true; };
}
namespace details
{
using namespace operators;
template<typename T, typename U>
using UnionResultType = decltype(std::declval<T>() | std::declval<U>());
}
int main()
{
using UnionAB = details::UnionResultType<N::A, N::B>;
return 0;
}
(see: https://godbolt.org/g/2rfkh5)
'g++ -std=c++17 -save-temps main.cpp' reports:
main.cpp: In substitution of ‘template<class T, class U> using UnionResultType
= decltype ((declval<T>() | declval<U>())) [with T = N::A; U = N::B]’:
main.cpp:24:54: required from here
main.cpp:19:54: error: no match for ‘operator|’ (operand types are ‘N::A’ and
‘N::B’)
using UnionResultType = decltype(std::declval<T>() | std::declval<U>());
The operator is in scope, so this should not be an issue. Writing instead:
template<typename T, typename U>
using UnionResultType = decltype(operator|(std::declval<T>(),
std::declval<U>()));
does compile. See also:
https://stackoverflow.com/questions/47356994/using-decltype-with-an-operator-using-non-adl-lookup.
