error: explicit specialization of non-template class 'hash'

[njlr]

I get this error when building the tests on macOS:

In file included from test/optional-lite.t.cpp:7:
In file included from test/optional-lite.t.h:12:
include/nonstd/optional.hpp:1070:7: error: explicit specialization of non-template class 'hash'
class hash< nonstd::optional >
^ ~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

My Clang version:

$ clang --version
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

My compiler flags:

-std=c++11

[martinmoene]

Note to self: investigate CGAL/Nested specialization syntax?.

[martinmoene]

Above CGAL link refers to Why can't I specialize std::tuple_element which states the location of the original declaration of hash is not std and the declaration should not be repeated differently:

namespace std {

//    template < class T>
//    struct hash;

template< class T >
class hash< nonstd::optional<T> >
{
public:
    std::size_t operator()( nonstd::optional<T> const & v ) const optional_noexcept
    {
        return bool( v ) ? hash<T>()( *v ) : 0;
    }
};
}

However this situation is not (directly) the case here.

[martinmoene]

gsl lite's issue 63 provides more information and circumvents the Clang Xcode problem by using gsl::to_integer<std::size_t>( v ) instead of hash<T>()( v ).

[martinmoene]

I gather #include <functional> is missing.

[martinmoene]

@njlr Question is: do we now get gsl lite's issue 63 ?

[njlr]

The tests work now.

01-to_int works if I change line 19 to const:

    const char * text = argc > 1 ? argv[1] : "42";

[njlr]

See: #6

[martinmoene]

Thanks!