Effects.h: Fix RefAs (#4312)

We marked that as only trapping if the input as nullable. But ref.as_func
will trap if it isn't a func, for example.

We could in theory try to check if a trap is possible, like checking if the
input is already non-nullable or already a function, etc., but we have
optimization passes to get rid of RefAs when they are not needed
anyhow, so there is no point to duplicate that here.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/ir/effects.h b/src/ir/effects.h
index f24f0ac3f..ced8e63cb 100644
--- a/src/ir/effects.h
+++ b/src/ir/effects.h
@@ -725,9 +725,15 @@ private:
     }
     void visitRefAs(RefAs* curr) {
       // traps when the arg is not valid
-      if (curr->value->type.isNullable()) {
-        parent.implicitTrap = true;
-      }
+      parent.implicitTrap = true;
+      // Note: We could be more precise here and report the lack of a possible
+      // trap if the input is non-nullable (and also of the right kind for
+      // RefAsFunc etc.). However, we have optimization passes that will
+      // remove a RefAs in such a case (in OptimizeInstructions, and also
+      // Vacuum in trapsNeverHappen mode), so duplicating that code here would
+      // only help until the next time those optimizations run. As a tradeoff,
+      // we keep the code here simpler, but it does mean another optimization
+      // cycle may be needed in some cases.
     }
   };