Implicit conversion of concatenation result to immutable

[Per Nordlöw]

On Thursday, 1 April 2021 at 21:21:02 UTC, Per Nordlöw wrote:
> Can somebody explain the logic behind the compiler disallowing 
> both line 3 and 4 in

I'm very intrigued by the fact the this issue hasn't been 
discussed more. It smells to me like this behaviour is somewhat 
by design...because I don't think it is at all difficult to fix.

During implicit conversion checking, just peek into the 
expression and see if it's CatExp and if so treat it as a unique 
reference and allow conversion from mutable non-mutable.

However, note that to solve this in the general case we have to 
involve data flow and escap e analysis. And have a tag associated 
with a variable that indicates whether it is aliased or not (on, 
yes, maybe).

Consider, for instance,

```d
auto c = a ~ b;
e = d; // `d` aliased to `e`
f = foo(e); // `e` maybe aliased to `f`
immutable g = e; // allowed?
```