Restore the `extern` heap type (#4898)

The GC proposal has split `any` and `extern` back into two separate types, so
reintroduce `HeapType::ext` to represent `extern`. Before it was originally
removed in #4633, externref was a subtype of anyref, but now it is not. Now that
we have separate heaptype type hierarchies, make `HeapType::getLeastUpperBound`
fallible as well.

2 files changed, 15 insertions, 3 deletions

diff --git a/src/ir/possible-constant.h b/src/ir/possible-constant.h
index b3272461d..21f1cfa65 100644
--- a/src/ir/possible-constant.h
+++ b/src/ir/possible-constant.h
@@ -114,8 +114,14 @@ public:
       auto type = getConstantLiteral().type.getHeapType();
       auto otherType = other.getConstantLiteral().type.getHeapType();
       auto lub = HeapType::getLeastUpperBound(type, otherType);
-      if (lub != type) {
-        value = Literal::makeNull(lub);
+      if (!lub) {
+        // TODO: Remove this workaround once we have bottom types to assign to
+        // null literals.
+        value = Many();
+        return true;
+      }
+      if (*lub != type) {
+        value = Literal::makeNull(*lub);
         return true;
       }
       return false;
diff --git a/src/ir/possible-contents.cpp b/src/ir/possible-contents.cpp
index 7079a9446..d9c608bda 100644
--- a/src/ir/possible-contents.cpp
+++ b/src/ir/possible-contents.cpp
@@ -56,7 +56,13 @@ void PossibleContents::combine(const PossibleContents& other) {
       assert(other.isNull());
       auto lub = HeapType::getLeastUpperBound(type.getHeapType(),
                                               otherType.getHeapType());
-      value = Literal::makeNull(lub);
+      if (!lub) {
+        // TODO: Remove this workaround once we have bottom types to assign to
+        // null literals.
+        value = Many();
+        return;
+      }
+      value = Literal::makeNull(*lub);
     }
     return;
   }