Passing private functions to template algorithms

[Timon Gehr via Digitalmars-d]

On 07.06.2016 20:45, FreeSlave wrote:
> I want to bring up this unpleasant topic for discussion, because I'm
> really tired of hacks I need to do in my code.
>
> Look at this snippet:
>
> private bool isFileNothrow(string path)
> {
>      import std.file : isFile;
>      import std.exception : collectException;
>      bool ok;
>      collectException(path.isFile, ok);
>      return ok;
> }
>
> void main(string[] args)
> {
>      import std.algorithm : filter;
>      auto r = args.filter!(isFileNothrow);
> }
>
> Looks good, but it won't compile because isFileNothrow is invisible to
> std.algorithm.
>
> Technically it's right: function is private, so no access from other
> modules. But it makes me to do hacks like
>
> auto r = args.filter!(p => p.isFileNothrow);
>
> I don't see a way how can I reuse my functions while keeping it private
> without ugly hacks.
>
> How about allowing templates from other modules to have access to
> private functions? I think this would be pretty legal behavior, since
> template is instantiated in this module anyway.

I think it is obvious that this should work. Visibility checks need to 
happen during identifier lookup. This lookup is happening in the module 
where isFileNothrow is visible.