How to get the UDAs for a function parameter correctly?

[Adam D. Ruppe]

On Monday, 11 May 2020 at 02:25:39 UTC, Heromyth wrote:
> I want to get the UDAs for for a function parameter. Here the 
> test code and I got some errors:

I think my blog aside is one of the few if only write-ups on how 
to do this:

http://dpldocs.info/this-week-in-d/Blog.Posted_2019_02_11.html#how-to-get-uda-on-a-function-param


the library will not help much, you need to slice the parameters 
yourself. (It might work combine lib stuff with slice though, 
e.g. getUDA!(Parameters!F[1 .. 2]) maybe, i have never tried).

but this does work:

         static if(is(typeof(method) P == __parameters))
         foreach(idx, _; P) {{
		// alias for convenience later
                 alias param = P[idx .. idx + 1]; // this slice is 
the trick
		// can get the identifier now
                 string ident = __traits(identifier, param);
		// and the attributes
                 foreach(attr; __traits(getAttributes, param))
			{}
	}}