[EH] Add exnref type back (#6149)

At the Oct hybrid CG meeting, we decided to add back `exnref`, which was
removed in 2020:
https://github.com/WebAssembly/meetings/blob/main/main/2023/CG-10.md

The new version of the proposal reflected in the explainer:
https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md

While adding support for `exnref` in the current codebase which has all
GC subtype hierarchies, I noticed we might need `noexn` heap type for
the bottom type of `exn`. We don't have it now so I just set it to 0xff
for the moment.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/exception-handling.wast.fromBinary b/test/exception-handling.wast.fromBinary
index e9cfce409..93678bc6f 100644
--- a/test/exception-handling.wast.fromBinary
+++ b/test/exception-handling.wast.fromBinary
@@ -4,6 +4,7 @@
  (type $2 (func (param i64)))
  (type $3 (func (param i32 i64)))
  (type $4 (func (param eqref)))
+ (type $5 (func (result exnref)))
  (tag $e-i32 (param i32))
  (tag $e-i64 (param i64))
  (tag $e-i32-i64 (param i32 i64))
@@ -428,5 +429,18 @@
   )
   (nop)
  )
+ (func $exnref-test (type $5) (result exnref)
+  (local $exn exnref)
+  (local $null-exn nullexnref)
+  (if (result exnref)
+   (i32.const 1)
+   (if (result nullexnref)
+    (i32.const 1)
+    (local.get $null-exn)
+    (ref.null noexn)
+   )
+   (local.get $exn)
+  )
+ )
 )