Type use annotations on instanceof patterns

[Alex Buckley]

On 11/22/2023 9:55 AM, Werner Dietl wrote:
>          if (x instanceof @TA Float) {
>          if (x instanceof @TA Boolean b) {
> 
>      @Target(ElementType.TYPE_USE)
>      @Retention(RetentionPolicy.RUNTIME)
>      @interface TA {}
> 
> 1) When looking at the bytecode with `javap -v 
> InstanceOfPatternVariable.class` I only see one use of @TA:
> 
>        RuntimeVisibleTypeAnnotations:
>          0: #19(): INSTANCEOF, offset=1
>            InstanceOfPatternVariable$TA
> 
> The JSL 15.20.2 [1] says "An instanceof expression may perform either 
> type comparison or pattern matching."
> The JVMS 4.7.20-B [2] talks about how type annotations on "instanceof 
> expressions" are stored.
> It would therefore seem to me that both occurrences of TA should be 
> stored in the class file.

(You mean both occurrences of the annotation `@TA`. The annotation 
interface `TA` is a distinct thing.)

Yes, both occurrences of `@TA` should be stored. Let's "prove it" for 
the second occurrence.

The type pattern `Boolean b` is defined (per 14.30.1) as a local 
variable declaration. The annotation `@TA` which syntactically appears 
as a modifier on that declaration is interpreted according to 9.7.4. How?

The annotation interface `TA` is defined with TYPE_USE, which means (per 
9.6.4.1) that `TA` is applicable in type contexts. One of the type 
contexts (per 4.11) is: "The type in a local variable declaration in 
either a statement ... or a pattern". So, the following rule in 9.7.4 
applies:

```
- If the annotation's interface is applicable in type contexts, and not 
in the declaration context corresponding to the declaration, then the 
annotation is deemed to apply only to the type which is closest to the 
annotation.
```

This means that the annotation `@TA` is a type annotation (applying to 
the use of the `Boolean` type within a local variable declaration) and 
not a declaration annotation on the overall local variable declaration 
`Boolean b`.

Great -- type annotations integrate with type patterns in the JLS as you 
would expect. Consequently, `@TA` should appear in the RVTA attribute 
via a localvar_target item. Perhaps javac is emitting the item but javap 
is unable to render it.

Alex