Fix assertion failure in RemoveUnusedBrs (#5895)

The improvements to RemoveUnusedBrs in #5887 also introduced a regression where
the pass did not correctly handle unreachable fallthrough values and crashed
with an assertion failure. Fix the problem by returning early when a fallthrough
value is unreachable and add a regression test.

Fixes #5892.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/passes/RemoveUnusedBrs.cpp b/src/passes/RemoveUnusedBrs.cpp
index 4fda7afb2..a5950f2fa 100644
--- a/src/passes/RemoveUnusedBrs.cpp
+++ b/src/passes/RemoveUnusedBrs.cpp
@@ -719,6 +719,10 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
 
         Type refType =
           Properties::getFallthroughType(curr->ref, passOptions, *getModule());
+        if (refType == Type::unreachable) {
+          // Leave this to DCE.
+          return;
+        }
         assert(refType.isRef());
 
         // When we optimize based on all the fallthrough type information