[Custom Descriptors] Fix ref.cast_desc of i31ref (#7719)

We previously hit an assertion failure when accessing the GCData of the
ref literal because i31 literals do not have GCData.

Author: tlively

src/wasm-interpreter.h

@@ -1665,6 +1665,10 @@ class ExpressionRunner : public OverriddenVisitor<SubType, Flow> {
       trap("null descriptor");
     }
     Literal val = ref.getSingleValue();
+    if (!val.isData() && !val.isNull()) {
+      // For example, i31ref.
+      return typename Cast::Failure{val};
+    }
     auto data = val.getGCData();
     if (!data) {
       // Check whether null is allowed.

test/spec/ref.cast_desc.wast

@@ -164,6 +164,15 @@
     )
     (i32.const 0)
   )
+
+  (func (export "cast-i31ref")
+    (drop
+      (ref.cast_desc (ref $super)
+        (ref.i31 (i32.const 0))
+        (struct.new $super.desc)
+      )
+    )
+  )
 )
 
 (assert_return (invoke "cast-success"))
@@ -178,6 +187,7 @@
 (assert_trap (invoke "cast-nn-fail-null-desc") "null descriptor")
 (assert_return (invoke "cast-branch-ref") (i32.const 1))
 (assert_return (invoke "cast-branch-desc") (i32.const 1))
+(assert_trap (invoke "cast-i31ref") "cast error")
 
 (assert_malformed
   ;; Cast type must be a reference.