transporting qualifier from parameter to the return value

[Kagamin]

Walter Bright Wrote:

> Unmentioned in the proposal is is inout a type constructor or a storage 
> class? For example,
> 
>      U[inout(T)]* foo(inout(X)*** p) { ... }
> 
> This is much more complex to implement than only allowing inout at the 
> top level, i.e. as a storage class.

It's a type constructor just like const and immutable. As to your example, imagine inout expands as follows

U[T]* foo(X*** p) { ... }
U[const(T)]* foo(const(X)*** p) { ... }
U[immutable(T)]* foo(immutable(X)*** p) { ... }

If this works, then it's ok.

Giving it a quick thought, it should work.

> I also prefer the idea of inout on the return type being assumed, rather 
> than explicit:
> 
>      T foo(inout U p) { ... }

Given inout is a type constructor, it's critical for type safety to properly decorate return type: T is mutable and can't play role of inout-qualified type.