proposed @noreturn attribute

[Timon Gehr via Digitalmars-d]

On 10.07.2017 04:44, Walter Bright wrote:
> Let's call the bottom type 'B' for the moment, for convenience.
> 
> If we think of it like a floating point NaN,

It's not. It's an empty type.

> then any type construction 
> of B yields a B:
> 
>      const(B) -> B

I don't see why to add this special case.

>      B* -> B
>      B[] -> B
>      X[B] -> B
>      B[X] -> B
> ...

This would be wrong. All of the types on the left have valid values, 
they are not isomorphic to B.

> Since B cannot have a value, any expression that forms a B can be 
> replaced with an assert(0).
> 
>      B foo(); // declaration
>      foo() -> foo(); assert(0);
> 

Yes.


>      cast(B)exp -> exp; assert(0);
> 
>      B b; -> assert(0);
> 

Those wouldn't compile. (Nothing can be cast to bottom, there is no 
default initializer that the variable declaration can use.)

> Given a tuple of types:
> 
>      alias T = tuple(B,X);
> 
> is T equivalent to which of:
> 
>      B           (1)
>      tuple(X)    (2)
>      tuple(Y,X)  (3)
>      tuple(B,X)  (4)
> 
> ? I'm leaning toward (4) as making the most sense.
> ...

It's the only one that makes any sense.

>      struct S {
>         T t;
>      }
> 
> should then yield an error, while:
> 
>      struct S {
>         T[1..1] t; // t is of type X
>      }
> 
> would not.

I guess you meant T[1..2].