Feedback Thread: DIP 1034--Add a Bottom Type (reboot)--Final Review

Dennis dkorpel at
Tue Sep 22 15:08:04 UTC 2020

On Tuesday, 22 September 2020 at 14:16:19 UTC, Steven 
Schveighoffer wrote:
> ```
> int x;
> int bar();
> auto foo() {
>     cast(noreturn) (x = bar()); // same as {x = bar(); 
> assert(0);}
> }
> ```
> This is odd, and I'm not understanding the reason for the cast 
> if you aren't assigning.

This code has no reason, it's just an example of the rewrite that 
happens with `cast(noreturn)`. It could actually be useful in 
generic code, or when you are using a function that is noreturn 
but does not have the correct return type.
// someone else's code
void panic(string msg);

// your code:
auto f = () => cast(noreturn) panic("error");

> Also, earlier you say that you can declare a noreturn variable, 
> but as long as you don't use it, you don't generate the assert 
> 0. Is this example a contradiction of that?

I don't think so, the example does not declare a local variable. 
I could rewrite it to this:

import std;
int x;
int bar();
auto foo() {
     noreturn y; // no assert here yet
     writeln("this will get printed");
     y = cast(noreturn) (x = bar());
     // here it results in assert(0)

The reason for this is argued by Johannes Loher here:$14fk$

> ```
> /* 6 */ !(is(T == function)) || is(noreturn* : T)
> /* 7 */ !(is(T == delegate)) || is(noreturn* : T)
> ```
> What are these trying to say? I think these rules need some 
> clarification or rewriting

In formal logic, "(not p) or q" is equivalent to "if p then q", 
so it means "if T is a function/delegate, noreturn* implicitly 
converts to it". The DIP follows the rules up with a short 

> 5, 6 and 7 mean that null (which is of type noreturn*) may 
> still be assigned to a function pointer or array.

The point is that these things should still be allowed when 
`typeof(null)` becomes equivalent to `noreturn*`:
int[] a = null;
void function() f = null;
void delegate() d = null;
Object o = null;
(I just realized I have no rule for `T == class`. I sometimes 
forget classes exist in D :p)

I'll try to make it clearer.

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