[Wasm GC] Fix precomputing of incompatible fallthrough values (#3875)

Precompute not only computes values, but looks at the fallthrough,

(local.set 0
  (block
    ..stuff we can ignore..
    ;; the fallthrough we care about - if a value is set to local 0, it is this
    (i32.const 10)
  )
)

Normally that is fine, but the fuzzer found a case where it is not: RefCast may
return a different type than the fallthrough, even an incompatible type if we
try to do something bad like cast a function to a struct. As we may then
propagate the value to a place that expects the proper type, this can cause an
error.

To fix this, check if the precomputed value is a proper subtype. If it is not,
then do not look through into the fallthrough, but compute the entire thing.
(In the case of a bad RefCast of a func to a struct, it would then indicate a
trap happens, and we would not precompute the value.)

0 files changed, 0 insertions, 0 deletions