[boost] detecting a constructor with a specific signature

Subject: [boost] detecting a constructor with a specific signature
From: Kenny Riddile (kfriddile_at_[hidden])
Date: 2010-01-26 17:36:25

I'm trying to create a type trait to detect the presence of a 
constructor with a very specific signature.  Types passed to this trait 
will *always* have a constructor of the form:

T( const std::string& );

but some of them will have this constructor as well:

T( const std::string&, Foo& ); // Second parameter will always be a Foo&

I would like to create a type trait that returns true when the second 
constructor is present.  I can't simply do:

!is_convertible<std::string, T>::value

since that will return false when the second constructor is present 
since all Ts are convertible from std::string.  Is it possible to create 
such a trait?