Template alias parameters to local variables

[via Digitalmars-d-learn]

Consider the following code:

       alias Sink = scope void delegate(const(char)[]);

       private template generateAliases(int __i, __vars...) {
           import std.conv : to;

           static if(__i < __vars.length)
               enum generateAliases = "alias " ~
__vars[__i].stringof ~ " =
                    __vars[" ~ __i.to!string ~ "];\n" ~
                    generateAliases!(__i+1, __vars);
           else
               enum generateAliases = "";
       }

       template render(string __tpl, __vars...) {
           void render(Sink sink) {
               static void render2(string __tpl2, __vars2...)(Sink
sink2 = sink) {
                   .render!(__tpl2, __vars2)(sink2);
               }
               void render3(string __tpl2, __vars2...)(Sink sink2 =
sink) {
                   .render!(__tpl2, __vars2)(sink2);
               }

               alias __sink = sink;

               mixin(generateAliases!(0, __vars));
               //mixin(generateRenderer(__tpl));

               alias x = __vars[0];
               alias y = __vars[1];
			
               render2!("...", x, y);    // ERROR
               render3!("...", x, y);    // ERROR
           }
       }

       void main() {
           import std.stdio;
           int a, b;
           render!("...", a, b)(s => write(s));
       }

Inside the `render` function, which takes a string and several
variables as template parameters, as well as a delegate as
runtime parameter, I want another function to be available, which
should again accept a string and several aliases (including some
that `render` previously received as aliases). At the same time,
the function should implicitly pick up the sink that has been
passed to `render`. I'm trying to slightly different
implementations, `render2` and `render3`. They both produce
compile errors:

render2:
test.d(16): Error: static function test.main.render!("...", a,
b).render2 cannot access frame of function D main
test.d(29): Error: static function test.main.render!("...", a,
b).render2 cannot access frame of function D main

render3:
test.d(30): Error: template instance render3!("...", a, b) cannot
use local 'a' as parameter to non-global template render3(string
__tpl2, __vars2...)(Sink sink2 = sink)

I would prefer `render2`, because it is static and thus doesn't
need to allocate a context for the sink. Indeed, it used to
work before this issue was fixed:
https://issues.dlang.org/show_bug.cgi?id=11946
https://github.com/D-Programming-Language/dmd/pull/3884

Now, has this fix gone too far? On first glance, the `render2`
error message seems to make sense, but why doesn't the same error
occur for `render`?

However, why doesn't at least `render3` work? It's strange local
templates cannot accept local variables as aliases...

Alternatively, does anyone know of another way to achieve what I
want?