http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56283
Bug #: 56283
Summary: std::result_of does not gracefully SFINAE (not define
::type)
Classification: Unclassified
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: [email protected]
ReportedBy: [email protected]
>From the spec of std::result_of,
If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well formed
when treated as an unevaluated operand (Clause 5), the member typedef type
shall name the type decltype(INVOKE(declval<Fn>(), declval<ArgTypes>()...));
otherwise, there shall be no member type.
If the given INVOKE expression is invalid, ::type should not exist. But an
attempt to use it this way to detect a valid call fails.
Below is a short demo program. A non-overloaded function name and a functor
type with an overloaded call operator are passed to result_of through a type
traits style SFINAE wrapper. None of the cases work. In 2 of the 3 cases the
failure fires the static assertion, but not for the multiple overload issue.
#include <type_traits>
template< typename fn, typename = void >
struct is_invokable : std::false_type {};
template< typename fn >
struct is_invokable< fn, typename std::conditional< true, void, typename
std::result_of< fn >::type >::type >
: std::true_type {};
void f( int );
static_assert( ! is_invokable< decltype( f ) &( char const * ) >(), "can
convert c-string to int" );
struct s {
void operator() ( int ) {}
void operator() ( float ) {}
};
static_assert( ! is_invokable< s( double ) >(), "resolved ambiguous conversion"
);
static_assert( ! is_invokable< s( char * ) >(), "converted pointer to number"
);
