1 files changed, 3 insertions, 9 deletions
diff --git a/source/slang/slang-check-conformance.cpp b/source/slang/slang-check-conformance.cpp
index b95df520d..e77ea4981 100644
--- a/source/slang/slang-check-conformance.cpp
+++ b/source/slang/slang-check-conformance.cpp
@@ -301,19 +301,13 @@ namespace Slang
             // is a subtype of I.
             // We need to check and make sure the interface type of the `ExtractExistentialType`
             // is equal to `superType`.
-            auto interfaceDeclRef = extractExistentialType->interfaceDeclRef;
-            auto thisTypeSubst = findThisTypeSubstitution(interfaceDeclRef.substitutions.substitutions, interfaceDeclRef.getDecl());
-            SLANG_ASSERT(thisTypeSubst && thisTypeSubst == interfaceDeclRef.substitutions.substitutions);
-            // The interfaceDeclRef in `extractExistentialType` contains a `ThisTypeSubstitution`
-            // to allow member lookup to return correct substituted types. Here we just need
-            // to know if that interface is the same as the superType, so we need to exclude
-            // the `ThisTypeSubstitution` from comparison.
-            interfaceDeclRef.substitutions.substitutions = thisTypeSubst->outer;
+            //
+            auto interfaceDeclRef = extractExistentialType->originalInterfaceDeclRef;
             if (interfaceDeclRef.equals(superTypeDeclRef))
             {
                 if (outWitness)
                 {
-                    *outWitness = thisTypeSubst->witness;
+                    *outWitness = extractExistentialType->getSubtypeWitness();
                 }
                 return true;
             }