Implicit conversion of concatenation result to immutable

[Per Nordlöw]

Can somebody explain the logic behind the compiler disallowing 
both line 3 and 4 in

```d
const(char)[] x;
string y;
string z1 = x ~ y; // errors
string z2 = y ~ x; // errors
```

erroring as

```
Error: cannot implicitly convert expression `x ~ 
cast(const(char)[])y` of type `char[]` to `string`
Error: cannot implicitly convert expression `cast(const(char)[])y 
~ x` of type `char[]` to `string
````

Has this something to do with the compiler being defensive about 
possible in-place appending or prepending to the arguments (in 
this case `x` and `y`) passed to the array concatenation 
expression?

For instance, could `x ~ y` return either
- a back-extended slice `x[0 .. x.length + y.length]` with `y` 
appended at the back or
- a front-extended slice `y[-x.length .. y.length]` with `x` 
prepended to the front

provided the GC has information about available free memory there?

This problem regularly crops up for me during assembling of 
strings passed as a string parameter for instance an exception 
constructor.