Variadic templates: braced pack expansion in std::array (gcc vs clang)

The following code will compile with GCC, but clang++ and icpc complain about a missing pair of braces at the end:

template < typename T, int m >
struct PowBogus {
    static T pow(const T t) noexcept { return t; }
};

template < typename T, int... mm >
struct CompoundPow {
    typedef std::array< T, sizeof...(mm) > return_type;
    static return_type pow(T t) noexcept {
          return {PowBogus< T, mm >::pow(t)...};  // error in clang, icc
    }
    static return_type pow_extra_brace(T t) noexcept {
          return {{PowBogus< T, mm >::pow(t)...}};  // fine in clang, icc, gcc
    }
};

I wonder why the extra pair of braces is necessary. My guess is that icpc and clang always interpret the pack expansion in braces as an initializer list, whereas gcc calls a std::array constructor. What does the standard have to say about this?