should std::common_type use std::decay?

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should std::common_type use std::decay?

Yes, see Library Working Group Defect #2141.

Short version (long version, see link above):

  • declval<A>() returns a A&&

  • common_type is specified via declval, n3337:

    template <class T, class U>
struct common_type<T, U> {
    typedef decltype(true ? declval<T>() : declval<U>()) type;
};

  • common_type<int, int>::type therefore yields int&&, which is unexpected

  • proposed resolution is to add decay

    template <class T, class U>
struct common_type<T, U> {
    typedef decay_t < decltype(true ? declval<T>() : declval<U>()) > type;
};

  • common_type<int, int>::type now yields int

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