1 files changed, 21 insertions, 3 deletions
diff --git a/src/passes/Precompute.cpp b/src/passes/Precompute.cpp
index c07286ec9..3e2e7a64f 100644
--- a/src/passes/Precompute.cpp
+++ b/src/passes/Precompute.cpp
@@ -311,9 +311,27 @@ private:
         // somehow know the entire expression precomputes to a 42, then we can
         // propagate that 42 along to the users, regardless of whatever the call
         // did globally.)
-        auto values = setValues[set] =
-          precomputeValue(Properties::getFallthrough(
-            set->value, getPassOptions(), getModule()->features));
+        auto values = precomputeValue(Properties::getFallthrough(
+          set->value, getPassOptions(), getModule()->features));
+        // Fix up the value. The computation we just did was to look at the
+        // fallthrough, then precompute that; that looks through expressions
+        // that pass through the value. Normally that does not matter here,
+        // for example, (block .. (value)) returns the value unmodified.
+        // However, some things change the type, for example RefAsNonNull has
+        // a non-null type, while its input may be nullable. That does not
+        // matter either, as if we managed to precompute it then the value had
+        // the more specific (in this example, non-nullable) type. But there
+        // is a situation where this can cause an issue: RefCast. An attempt to
+        // perform a "bad" cast, say of a function to a struct, is a case where
+        // the fallthrough value's type is very different than the actually
+        // returned value's type. To handle that, if we precomputed a value and
+        // if it has the wrong type then precompute it again without looking
+        // through to the fallthrough.
+        if (values.isConcrete() &&
+            !Type::isSubType(values.getType(), set->value->type)) {
+          values = precomputeValue(set->value);
+        }
+        setValues[set] = values;
         if (values.isConcrete()) {
           for (auto* get : localGraph.setInfluences[set]) {
             work.insert(get);