I made some progress. If I specialize struct VectorSubscriptCtx::eval with Vector10, like


struct VectorSubscriptCtx
{
VectorSubscriptCtx(std::size_t i) : i_(i) {}

template<typename Expr, typename EnableIf = void>
struct eval
: proto::default_eval<Expr, VectorSubscriptCtx const>
{};

template<typename Expr>
struct eval<
Expr
, typename boost::enable_if<
proto::matches<Expr, proto::terminal< Vector10 > >
>::type
>
{
//..
}
};

then it works (is was specialized with Vector). It also works when using the Boost _ literal (match anything), like

template<typename Expr>
struct eval<
Expr
, typename boost::enable_if<
proto::matches<Expr, proto::terminal< _ > >
>::type


However, I feel this is not good style. Can this be expressed with the is_base_of trait instead?





On 04/14/2016 10:10 AM, Mathias Gaunard wrote:
On 14 April 2016 at 14:43, Frank Winter <fwin...@jlab.org
<mailto:fwin...@jlab.org>> wrote:

    Hi all!

    Suppose you'd want to implement a simple EDSL (Embedded Domain
    Specific Language) with Boost.proto with the following requirements:

         Custom class 'Vector' as terminal
         Classes derived from 'Vector' are working terminals too, e.g.
    Vector10

    [...]

    template<typename T>
    struct IsVector
       : mpl::false_
    {};


    template<>
    struct IsVector< Vector >
       : mpl::true_
    {};


Surely this should be true for all types derived from Vector.

template<typename T, typename Enable = void>
struct IsVector
   : mpl::false_
{};

template<typename T>
struct IsVector<T, typanem enable_if< is_base_of<Vector, T> >::type>
   : mpl::true_
{};


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