Why are template alias parameters broken?

[Steven Schveighoffer]

On 8/27/21 2:47 PM, Menshikov wrote:
> It's work:
> 
> ```d
> template Foo(alias var)
> {
>      void inc() { var++; }
> }
> 
> void main()
> {
>      int v = 4;
>      alias foo = Foo!(v);
>      foo.inc();
>      assert(v == 5);
> }
> ```
> But it doesn't work:
> ```d
> template Foo(alias var)
> {
>      void inc() { var++; }
> }
> 
> void main()
> {
>      struct V{
>        int a;
>      }
>      auto v = V(4);
>      alias foo = Foo!(v.a);
>      foo.inc();//err
>      assert(v.a == 5);
> }
> ```
> Hence this applies to Alias!() and AliasSeq!()

Aliases to expressions don't work. They have to be symbols, type 
keywords (e.g. `int`), or compile-time values. `v.a` is an expression.

It's quite an annoying limitation, I agree.

To work around, you can do:

```d
template Foo(alias var)
{
    void inc() {var.a++}
}
```

and then pass `v` instead of `v.a`.

-Steve