ReturnType of lambda templates

[Roland Hadinger via Digitalmars-d-learn]

Hi!

Suppose I wanna do this (which already works, which is why D is 
pretty cool):

     import std.traits;
     import std.typecons : Nullable;

     // Retrofit Nullable to allow for monadic chaining
     //
     auto apply( alias fun, T )( Nullable!T nullable )
         if( isSomeFunction!fun )
     {
         enum returnsVoid     = is( ReturnType!fun == void );
         enum returnsNullable = is( ReturnType!fun : Nullable!X, X 
);

         static if( returnsVoid ) {

             if( !nullable.isNull )
                 fun( nullable.get );

         } else static if( returnsNullable ) {

             alias NR = ReturnType!fun;

             if( nullable.isNull )
                 return NR();
             else
                 return fun( nullable.get );

         } else {

             alias NR = Nullable!( ReturnType!fun );

             if( nullable.isNull )
                 return NR();
             else
                 return NR( fun( nullable.get ) );
         }
     }

     void main()
     {
         Nullable!int a = 1;
         Nullable!int b = a.apply!( (int x) => 2 * x );
                        // .apply!( (double x) => foo( x, u ) )
                        // .apply!( (double x) => bar( x, v ) )
                        // .apply!( (string x) => baz( x, w ) );
     }

...but without explicitly specifying the type of the lambda 
parameter:

     void main()
     {
         Nullable!int a = 1;
         Nullable!int b = a.apply!( x => 2 * x );
                        // .apply!( x => foo( x, u ) )
                        // .apply!( x => bar( x, v ) )
                        // .apply!( x => baz( x, w ) );
     }

Problem is, the shorter form involves lambda templates, and both 
'isSomeFunction' and 'ReturnType' currently don't seem to work
with those. No problems with the other version.

Can I fix my 'apply' function to still allow for the less verbose 
form?

If that's impossible, is it a theoretical possibility to improve 
D's type deduction to handle this?

Or should I just not attempt to write code like this for the time 
being (after all, functional code like this incurs some overhead, 
but in this particular case, I'd prefer legibility over 
performance)?

Thanks!