[llvm-bugs] [Bug 30763] New: SFINAE issue with constexpr operator

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30763

            Bug ID: 30763
           Summary: SFINAE issue with constexpr operator
           Product: clang
           Version: 3.9
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: officesamurai at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 17476
  --> https://llvm.org/bugs/attachment.cgi?id=17476&action=edit
test code (also in the text)

The following code fails to compile:

#include <type_traits>
#include <utility>

template <typename TT1, typename TT2>
struct Helper
{
    template <typename T1, typename T2>
    static auto test(int) -> decltype(std::declval<T1>() == std::declval<T2>(),
std::true_type());

    template <typename T1, typename T2>
    static auto test(...) -> std::false_type;

    using Result = decltype(test<TT1, TT2>(0));
};

template <typename TT1, typename TT2>
constexpr bool isEqualityComparable()
{
    using Result = typename Helper<TT1, TT2>::Result;
    return Result::value;
}

struct S
{
};

template <typename T>
struct Wrapper
{
    T t;
};

// Removing constexpr here will make the code compilable
template <typename T1, typename T2>
constexpr bool operator==(const Wrapper<T1>& w1, const Wrapper<T2>& w2)
{
    return w1.t == w2.t;
}

using W = Wrapper<S>;

int main()
{
    isEqualityComparable<W, W>();
}



The result is:

$ clang++-3.9 error_test.cpp -std=c++11
error_test.cpp:37:17: error: invalid operands to binary expression ('const S'
and 'const S')
    return w1.t == w2.t;
           ~~~~ ^  ~~~~
error_test.cpp:8:58: note: in instantiation of function template specialization
'operator==<S, S>' requested here
    static auto test(int) -> decltype(std::declval<T1>() == std::declval<T2>(),
std::true_type());
                                                         ^
error_test.cpp:13:29: note: while substituting explicitly-specified template
arguments into function template 'test'
    using Result = decltype(test<TT1, TT2>(0));
                            ^
error_test.cpp:19:29: note: in instantiation of template class
'Helper<Wrapper<S>, Wrapper<S> >' requested here
    using Result = typename Helper<TT1, TT2>::Result;
                            ^
error_test.cpp:44:5: note: in instantiation of function template specialization
'isEqualityComparable<Wrapper<S>, Wrapper<S> >' requested here
    isEqualityComparable<W, W>();
    ^
error_test.cpp:35:16: note: candidate template ignored: could not match
'Wrapper<type-parameter-0-0>' against 'const S'
constexpr bool operator==(const Wrapper<T1>& w1, const Wrapper<T2>& w2)


Also:
1) -std=c++14 gives the same result,
2) removing constexpr from operator== fixes the problem,
3) GCC 4.9.3 produces a similar error while GCC 5.2.0 compiles it without
errors.

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