1 files changed, 24 insertions, 1 deletions
diff --git a/src/passes/SignatureRefining.cpp b/src/passes/SignatureRefining.cpp
index 37ca091df..d094382c7 100644
--- a/src/passes/SignatureRefining.cpp
+++ b/src/passes/SignatureRefining.cpp
@@ -26,6 +26,7 @@
 // type, and all call_refs using it).
 //
 
+#include "ir/export-utils.h"
 #include "ir/find_all.h"
 #include "ir/lubs.h"
 #include "ir/module-utils.h"
@@ -69,6 +70,10 @@ struct SignatureRefining : public Pass {
 
       // A possibly improved LUB for the results.
       LUBFinder resultsLUB;
+
+      // Normally we can optimize, but some cases prevent a particular signature
+      // type from being changed at all, see below.
+      bool canModify = true;
     };
 
     ModuleUtils::ParallelFunctionAnalysis<Info> analysis(
@@ -106,6 +111,20 @@ struct SignatureRefining : public Pass {
       allInfo[func->type].resultsLUB.combine(info.resultsLUB);
     }
 
+    // We cannot alter the signature of an exported function, as the outside may
+    // notice us doing so. For example, if we turn a parameter from nullable
+    // into non-nullable then callers sending a null will break. Put another
+    // way, we need to see all callers to refine types, and for exports we
+    // cannot do so.
+    // TODO If a function type is passed we should also mark the types used
+    //      there, etc., recursively. For now this code just handles the top-
+    //      level type, which is enough to keep the fuzzer from erroring. More
+    //      generally, we need to decide about adding a "closed-world" flag of
+    //      some kind.
+    for (auto* exportedFunc : ExportUtils::getExportedFunctions(*module)) {
+      allInfo[exportedFunc->type].canModify = false;
+    }
+
     bool refinedResults = false;
 
     // Compute optimal LUBs.
@@ -116,6 +135,11 @@ struct SignatureRefining : public Pass {
         continue;
       }
 
+      auto& info = allInfo[type];
+      if (!info.canModify) {
+        continue;
+      }
+
       auto sig = type.getSignature();
 
       auto numParams = sig.params.size();
@@ -127,7 +151,6 @@ struct SignatureRefining : public Pass {
         }
       };
 
-      auto& info = allInfo[type];
       for (auto* call : info.calls) {
         updateLUBs(call->operands);
       }