static alias this and if/do/while/for

[Q. Schroll]

On Thursday, 22 October 2020 at 21:55:59 UTC, Jack Applegame 
wrote:
> There is a funny feature (or bug) in the D language:
>
> static alias this and static operator overloading.
>
> For example
>
>> interface Foo {
>>     static {
>>         int value;
>>         void opAssign(int v) { value = v; }
>>         int get() { return value; }
>>         alias get this;
>>     }
>> }
>
> Now we can use type Foo as if it were an lvalue/rvalue:
>
>> Foo = 5;
>> int a = Foo;
>> int b = Foo + a;
>
> I heavily use this feature in my MCU library.

It doesn't surprise me that much. What actually did was that 
static opIndex() works. Consider this:

     struct X
     {
         static int opIndex() { return 1; }
     }

     alias SliceOf(T) = T[];
     enum ValueOf(T) = T[];

     static assert( ! is(int[SliceOf!X] == int[ValueOf!X]));

But static opIndex and friends can be useful: 
https://run.dlang.io/is/s15zS0 I'd actually find it awesome if we 
had opCallAssign and opCallOpAssign; the reason I used opIndex 
and not opCall is that way, one gets access to the right-hand 
side and to-assign parameters by reference, so no internal 
pointer stuff's needed.

> But it doesn't work inside conditionals:
>
>> if(Foo) {}    // Error: type Foo is not an expression
>> while(Foo) {} // Error: type Foo is not an expression
>
> Even if we define `static opCast!bool()`. It doesn't help.

Only using Foo.opCast!bool works; cast(bool) won't compile.

> Should this be fixed?

The bool stuff should be fixed. The other direction would be a 
breaking change.