Partially applied template alias messes up template inferance

[Paul Backus]

On Wednesday, 14 October 2020 at 02:37:25 UTC, Jonathan Levi 
wrote:
> This gist gives a full example: 
> https://gist.github.com/run-dlang/a28372ffaa05c3f2a4d852ca4b0a7bf5  (See the compiler error.)
>
> In essence:
> ```
> struct Vec(T, size_t size) {}
> alias Vec2(T) = Vec(T, size);
> void fun(T)(Vec2!T vec) {}
>
> void main() {
>     // Error: template fun cannot deduce function from argument 
> types !()(Vec!(float, 2LU))
>     fun(Vec2!float());
> }
> ```
>
> The template function can be written without using the alias 
> and everything works great:
> ```
> void fun(T)(Vec!(T,2) vec) {}
> ```
>
> Is this fixable in the language, or is it just not feasible?

This is a known limitation of template argument deduction:

https://issues.dlang.org/show_bug.cgi?id=1807

Attempts have been made to fix it in the past, but so far none 
have succeeded. It's probably not feasible given the compiler's 
current implementation of templates.