alias this constructor

[Adam D. Ruppe]

On Sunday, 25 March 2018 at 00:47:20 UTC, Dave Jones wrote:
> struct Foo
> {
>     int x;
>     alias x this;
> }
>
> Foo f = 12; // error

You just need to define a constructor that takes an int. alias 
this is all about taking an existing object and substituting a 
member for it - emphasis on *existing* object, so it needs to 
already be constructed.

But you can construct from other types

struct Foo {
    int x;
    this(int x) { this.x = x; }
}

Foo f = 12; // allowed, calls that constructor

> Foo foo;
> f = 12;     // this is OK

Important to note that foo already exists here and is NOT 
reconstructed - the compiler just rewrites that into `f.x = 12;`, 
doing a member assignment.