Observation about nulls in type patterns

Brian Goetz brian.goetz at oracle.com
Mon Jul 27 21:29:25 UTC 2020

> Too bad there isn’t such a thing as a “static cast”. (Oooh!  Ooh! 
>  “(static <type>) <expression>” !  :-). No, don’t do that; it’s a 
> terrible pun on the word “static”.

Alternately, what is too bad is that there _is_ such a thing as a 
"static cast", but it uses the same syntax, and has the same sort of 
contextual problem that Jens has complained about here!

Suppose we have:

     class Foo<T> {
         void m(Foo f) {
             Foo<String> fs = (Foo<String>) f;  // static cast!

The cast to `Foo<String>` is purely a static cast.  (Same with casts to 
things like `Foo<T>` or `T[]` or `T`.)  Essentially, if the cast type is 
not reifiable, but the two types are cast-convertible (perhaps with an 
unchecked warning), then its a static type, otherwise its a dynamic 
type.  This fact has cause plenty of trouble in Valhalla, since having 
such a cast magically become a dynamic one would surely cause problems.

But, yes, if casts could be explicitly denoted as static or dynamic, 
this would do the trick.

> But if this turns out to be a big problem, it does suggest that one 
> could have a form of switch in which the static type of the switch 
> expression is declared.  I don’t suggest pursuing it now, but it’s one 
> idea to keep in our back pocket (where we can firmly sit on it unless 
> and until needed).

... and we said the same thing the last time we were at this crossroads, 
with respect to totality in statement switches. Indeed, when the switch 
rehabiliation is complete, we can go back and ask what additional 
optional type checking we want to perform.

The mistake of declaring variables as `T t` rather than `t : T` (where 
you can elide the type ascription in some cases) haunts us ever day ... 
if we had gone this way, you could get what you want quite naturally with

     switch (x : T) { ... }

as a static type assertion.
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