[ 2017-03-12, post-Kona ]

Resolved by p0604r0.

[ 2017-02-24, Daniel comments ]

I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.

[ 2017-02-22, Daniel comments and provides concrete wording ]

The approach chosen to resolve this issue is a merger with LWG 2928, that is the callable traits are also renamed to invocable.

Previous resolution [SUPERSEDED]:

This wording is relative to N4640.

1. Modify [meta.type.synop], header <type_traits> synopsis, as indicated:

   […]
   // 20.15.6, type relations
   […]
   
   template <class, class R = void> struct is_callable; // not defined
   template <class Fn, class... ArgTypes, class R>
   struct is_callable<Fn(ArgTypes...), R>;
   template <class Fn, class... ArgTypes> struct is_invocable;
   template <class R, class Fn, class... ArgTypes> struct is_invocable_r;
   
   template <class, class R = void> struct is_nothrow_callable; // not defined
   template <class Fn, class... ArgTypes, class R>
   struct is_nothrow_callable<Fn(ArgTypes...), R>;
   template <class Fn, class... ArgTypes> struct is_nothrow_invocable;
   template <class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r;
   
   […]
   
   // 20.15.6, type relations
   […]
   template <class T, class R = void> constexpr bool is_callable_v
   = is_callable<T, R>::value;
   template <class T, class R = void> constexpr bool is_nothrow_callable_v
   = is_nothrow_callable<T, R>::value;
   template <class Fn, class... ArgTypes> constexpr bool is_invocable_v
   = is_invocable<Fn, ArgTypes...>::value;
   template <class R, class Fn, class... ArgTypes> constexpr bool is_invocable_r_v
   = is_invocable_r<R, Fn, ArgTypes...>::value;
   template <class Fn, class... ArgTypes> constexpr bool is_nothrow_invocable_v
   = is_nothrow_invocable<Fn, ArgTypes...>::value;
   template <class R, class Fn, class... ArgTypes> constexpr bool is_nothrow_invocable_r_v
   = is_nothrow_invocable_r<R, Fn, ArgTypes...>::value;
   […]
   

2. Modify [meta.rel], Table 44 — "Type relationship predicates", as indicated:

   Table 44 — Type relationship predicates

   […]
   template <class Fn, class... ArgTypes, class R> struct is_invocablecallable< Fn(ArgTypes...), R>;	The expression INVOKE(declval<Fn>(), declval<ArgTypes>()..., R) is well formed when treated as an unevaluated operand	Fn, R, and all types in the parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
   template <class R, class Fn, class... ArgTypes> struct is_invocable_r;	The expression INVOKE(declval<Fn>(), declval<ArgTypes>()..., R) is well formed when treated as an unevaluated operand	Fn, R, and all types in the parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
   template <class Fn, class... ArgTypes, class R> struct is_nothrow_invocablecallable< Fn(ArgTypes...), R>;	is_invocablecallable_v< Fn, ArgTypes...Fn(ArgTypes...), R> is true and the expression INVOKE(declval<Fn>(), declval<ArgTypes>()..., R) is known not to throw any exceptions	Fn, R, and all types in the parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
   template <class R, class Fn, class... ArgTypes, class R> struct is_nothrow_invocable_r;	is_invocable_r_v< R, Fn, ArgTypes...> is true and the expression INVOKE(declval<Fn>(), declval<ArgTypes>()..., R) is known not to throw any exceptions	Fn, R, and all types in the parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.

[ 2017-02, pre-Kona ]

See also LWG 2927.

It is becoming more and more apparent that using a function type as the template argument to result_of causes annoying problems. That was done because C++03 didn't have variadic templates, so it allowed an arbitrary number of types to be smuggled into the template via a single parameter, but it's a hack and unnecessary in C++ today. result_of<F(Args...)> has absolutely nothing to do with a function type that returns F, and the syntactic trickery using a function type has unfortunate consequences such as top-level cv-qualifiers and arrays decaying (because those are the rules for function types).

It might be too late to change result_of, but we should not repeat the same mistake for std::is_callable.

Proposed change: Possibly get rid of the is_callable<Fn(ArgTypes?...), R> specialization. Change the primary template is_callable<class, class R = void> to is_callable<class Fn, class.. ArgTypes?> and define a separate template such as is_callable_r<class R, class Fn, class... ArgTypes?> for the version that checks the return type. The resulting inconsistency might need to be resolved/improved upon.