[boost] [proto] subscript_wrapper and templated operator[]

Subject: [boost] [proto] subscript_wrapper and templated operator[]
From: David Deakins (ddeakins_at_[hidden])
Date: 2009-01-14 13:40:32

While using proto with some classes that have a templated operator[] on 
VC8 and VC9, I have run into a '2 overloads have similar conversions' 
ambiguity error (C2666).  The problem seems to be with subscript_wrapper 
in detail/decltype.hpp, where neither the templated operator[] from the 
original class nor the fallback 'any operator[](any)' function in 
subscript_wrapper have priority.  Any thoughts on how to tweak 
subscript_wrapper so that the 'any operator[](any)' function looks like 
a worse match than the templated operator[] and thus prevents the 
ambiguity?  I attached a small test case that reproduces the problem. 
Also, for reference, I am using the proto code from the release branch 
as of a couple days ago (~12 Jan 2009).

Thanks,
-Dave


#include <boost/proto/proto.hpp>
using namespace boost;


struct Abc
{
    template <typename T>
    void operator[](T const & t)
    {
    }
};


template< typename Expr >
void evaluate( Expr const & expr )
{
    proto::default_context ctx;
    proto::eval(expr, ctx);
}

int main()
{
    Abc a;
    evaluate( proto::lit(a) [ 0 ] );
    return 0;
}