Remove type annotations on ref.as_non_null and br_on_null

[rossberg]

Following WebAssembly/reference-types#99 and addressing issue #27, this removes the type annotations from ref.as_non_null and br_on_null (they already were absent on call_ref and func.bind).

One small caveat is that this requires extending the bottom type to a bottom heap type, to be able to type an unknown reference type in an instruction output.

[rossberg]

@jakobkummerow, @lars-t-hansen, let me know if you think this looks reasonable.

(See the pseudo code algorithm in the appendix to get an impression of how it affects a validator.)

[jakobkummerow]

Looks reasonable.

[RossTate]

Wait, what bottom type?

[lars-t-hansen]

Looks plausible.

[taralx]

Wait, what bottom type?

Polymorphic type used when stack underflows in unreachable code.

[taralx]

Is Ref(Bot, false) just simpler to implement than Bot, or is there some other consideration?

[taralx]

(did anyone notice that matches is not symmetric w.r.t. Bot?)

[RossTate]

I can't find anything about a bottom type in the overview. Where should I look for more details about it?

[taralx]

If I understand the history correctly:

Added as Unknown in WebAssembly/reference-types#3.
Renamed to Bot in WebAssembly/reference-types#43.
Removed from reference-types along with subtyping in WebAssembly/reference-types#87.

[RossTate]

Oh, nullref? But the current overview says ref.null ht: [] -> [(ref null ht)]. I'm confused.

[taralx]

No, not nullref. That's not polymorphic. There is no way to instantiate the bottom type in real code -- it is a placeholder that exists only during validation.

[RossTate]

Oh, I see. It’s not part of the spec; it’s just a device used in the reference implementation. Is that right?

[taralx]

I believe so, but I'll let @rossberg confirm.

[rossberg]

It will appear in the spec, but only as an internal type. You cannot use it in a program. Similar as with the bottom value type that was previously in the reftypes proposal.

[rossberg]

@taralx:

Is Ref(Bot, false) just simpler to implement than Bot, or is there some other consideration?

You'll need both. The latter matches any value type whatsoever, the former only reference types. When you have determined that an operand is a ref type you must not lose that information. Rules out

(unreachable)
(ref.as_non_null)
(i32.add)

(cf. unreached-invalid.wast)

(did anyone notice that matches is not symmetric w.r.t. Bot?)

It shouldn't be. It's a bottom type, not a top type.

[RossTate]

Got it. Thanks for the explanation!

[taralx]

LGTM, thanks!