[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, 18 insertions, 0 deletions

diff --git a/test/exception-handling.wast b/test/exception-handling.wast
index d37cc322b..1a484f9ac 100644
--- a/test/exception-handling.wast
+++ b/test/exception-handling.wast
@@ -8,6 +8,9 @@
   (func $foo)
   (func $bar)
 
+  ;; ---------------------------------------------------------------------------
+  ;; Old Phase 3 exception handling
+
   (func $eh-test (local $x (i32 i64))
     ;; Simple try-catch
     (try
@@ -365,4 +368,19 @@
     )
     (nop)
   )
+
+  ;; ---------------------------------------------------------------------------
+  ;; New exception handling
+
+  (func $exnref-test (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)
+    )
+  )
 )