[NominalFuzzing] Fix SignaturePruning on types with a super (#4657)

Do not prune parameters if there is a supertype that is a signature.

Without this we crash on an assertion in TypeBuilder when we try to
recreate the types (as we try to make a a subtype with fewer fields
than the super).

1 files changed, 9 insertions, 0 deletions

diff --git a/src/passes/SignaturePruning.cpp b/src/passes/SignaturePruning.cpp
index 033333758..4f142eb97 100644
--- a/src/passes/SignaturePruning.cpp
+++ b/src/passes/SignaturePruning.cpp
@@ -143,6 +143,15 @@ struct SignaturePruning : public Pass {
         continue;
       }
 
+      // A type with a signature supertype cannot be optimized: we'd need to
+      // remove the field from the super as well, which atm we don't attempt to
+      // do. TODO
+      if (auto super = type.getSuperType()) {
+        if (super->isSignature()) {
+          continue;
+        }
+      }
+
       // Apply constant indexes: find the parameters that are always sent a
       // constant value, and apply that value in the function. That then makes
       // the parameter unused (since the applied value makes us ignore the value