DIP 1017--Add Bottom Type--Final Review

[Neia Neutuladh]

On Tue, 22 Jan 2019 12:57:10 +0000, Olivier FAURE wrote:
> On Monday, 21 January 2019 at 17:52:52 UTC, Neia Neutuladh wrote:
>> The more extreme variant, where everything is a tuple that implicitly
>> casts to any prefix of itself, is not very useful.
> 
> Why?

Because it makes field order significant. It means that these two types 
are different:

    struct A { int x; string y; }
    struct B { string y; int x; }

The first is Tuple!(int, string); the second is Tuple!(string, int). They 
look like they should convert to each other, but they don't.

And these two types *do* convert to each other, but not in the way you'd 
want:

    struct A { bool destroyEarth; bool preserveHumanity; }
    struct B { bool preserveHumanity; bool destroyEarth; }

Passing the wrong type flips the fields around, because the order of the 
fields is what matters, because that's part of the structure. And that's 
why nobody actually suggests this.