math_fwd.hpp contains declarations of undefined quaternion template specializations

[Vladimir S. <vladimir.shlobin@…>]

This code no needed in math_fwd.hpp since no template specializations in quaternion.hpp. This code produces compile errors in some cases.

template < >
    class quaternion< float >;
template < >
    class quaternion< double >;
template < >
    class quaternion< long double >;

[John Maddock]

Fixed thanks.