Use non-nullable ref.cast for non-nullable input (#5335)

We switched from emitting the legacy `ref.cast_static` instruction to emitting
`ref.cast null` in #5331, but that wasn't quite correct. The legacy instruction
had polymorphic typing so that its output type was nullable if and only if its
input type was nullable. In contrast, `ref.cast null` always has a a nullable
output type.

Fix our output by instead emitting non-nullable `ref.cast` if the output should
be non-nullable. Parse `ref.cast` in binary and text forms as well. Since the IR
can only represent the legacy polymorphic semantics, disallow unsupported casts
from nullable to non-nullable references or vice versa for now.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index d700f85f3..68a2d16dc 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -2116,7 +2116,12 @@ struct PrintExpressionContents
     if (curr->safety == RefCast::Unsafe) {
       printMedium(o, "ref.cast_nop ");
     } else {
-      printMedium(o, "ref.cast null ");
+      // Emulate legacy polymorphic behavior for now.
+      if (curr->ref->type.isNullable()) {
+        printMedium(o, "ref.cast null ");
+      } else {
+        printMedium(o, "ref.cast ");
+      }
     }
     printHeapType(o, curr->intendedType, wasm);
   }