Weird template instantiation problem

[Arafel via Digitalmars-d-learn]

On Monday, 12 June 2017 at 19:23:10 UTC, ketmar wrote:
> p.s.: while i understand the technical reason for second error 
> message, it is still random and confusing.

I think the reason for the typeof problem is that it works with 
expressions, not with types (so, typeof (int) is also not valid), 
and the alias resolves ultimately to a type.

I actually found a workaround for the original issue:

```
enum defaultChooser(T) = function size_t(T[] queue) {
	return 0;
};

struct S(T, alias chooser = defaultChooser!int) if 
(is(typeof(chooser) : size_t function(T[]))) {
}

void main() {
	S!(int, defaultChooser!int) s;
}
```

This works, but strangely if I try "==" instead of ":" in the 
template condition, then it fails again. Honestly I don't know 
why it makes a difference, I guess attribute inference might be 
at fault... but in the version with the "static assert" I was 
explicitly checking them, and they apparently matched...

Also, this is just a(n ugly) workaround, and there might be side 
effects of using an alias parameter that I'm not aware of... and 
more importantly, I still think the original version should work! 
;-)