ReturnType of lambda templates

[Roland Hadinger via Digitalmars-d-learn]

On Tuesday, 18 August 2015 at 15:11:34 UTC, John Colvin wrote:
>
> for simple lambdas like that (i.e. function templates with one 
> template arguments that corresponds to the type of the first 
> and only argument), just add this template overload:
>
> auto apply( alias fun, T )( Nullable!T nullable )
>     if( !isSomeFunction!fun )
> {
>     return .apply!(fun!T, T)(nullable);
> }
>
> and it should work (give or a take a few typos on my part).

That was quick - and it works :)

Now, where is my thinking cap again?