[Wasm GC] Handle uses of default values in LocalSubtyping (#4024)

It is ok to use the default value of a reference even if we refine the type,
as it would be a more specifically-typed null, and all nulls compare the
same. However, if the default is used then we *cannot* alter the type to
be non-nullable, as then we'd use a null where that is not allowed.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/passes/Precompute.cpp b/src/passes/Precompute.cpp
index 44227a50f..99741ebe3 100644
--- a/src/passes/Precompute.cpp
+++ b/src/passes/Precompute.cpp
@@ -351,8 +351,10 @@ private:
           if (set == nullptr) {
             if (getFunction()->isVar(get->index)) {
               auto localType = getFunction()->getLocalType(get->index);
-              assert(!localType.isNonNullable() &&
-                     "Non-nullable locals must not use the default value");
+              if (localType.isNonNullable()) {
+                Fatal() << "Non-nullable local accessing the default value in "
+                        << getFunction()->name << " (" << get->index << ')';
+              }
               curr = Literal::makeZeros(localType);
             } else {
               // it's a param, so it's hopeless