Skip creating transitive witnesses for interface conformances that are inherited through structs (#7597)

* Eliminate null-pointer reference in multilevel struct interface inheritance

* Add test

* Replace witness nullptr guard with conformance check to skip the witness

* Update test

* Skip creating transitive witness entirely for struct-struct-interface

* format code (#14)

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

* Removes casts, check for identity witness

---------

Co-authored-by: slangbot <ellieh+slangbot@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

2 files changed, 60 insertions, 5 deletions

diff --git a/source/slang/slang-check-inheritance.cpp b/source/slang/slang-check-inheritance.cpp
index 164828bac..49ca3d3ac 100644
--- a/source/slang/slang-check-inheritance.cpp
+++ b/source/slang/slang-check-inheritance.cpp
@@ -858,13 +858,37 @@ void SharedSemanticsContext::_mergeFacetLists(
             //
             SubtypeWitness* baseIsSubtypeOfFacet = foundFacet->subtypeWitness;
 
-            auto selfIsSubtypeOfFacet = _getASTBuilder()->getTransitiveSubtypeWitness(
-                selfIsSubtypeOfBase,
-                baseIsSubtypeOfFacet);
+            // Check if this would create an unwanted struct->struct->interface conformance
+            // We want to prevent transitive witnesses of the form: struct Child -> struct Parent ->
+            // interface IFoo
+            bool shouldSkipTransitiveWitness = false;
 
-            indirectFacet->setSubtypeWitness(_getASTBuilder(), selfIsSubtypeOfFacet);
+            auto selfType = selfIsSubtypeOfBase->getSub();
+            auto baseType = selfIsSubtypeOfBase->getSup();
+            auto facetType = baseIsSubtypeOfFacet->getSup();
 
-            ioMergedFacets.add(indirectFacet);
+            if (selfType && baseType && facetType)
+            {
+                auto selfDeclRef = isDeclRefTypeOf<StructDecl>(selfType);
+                auto baseDeclRef = isDeclRefTypeOf<StructDecl>(baseType);
+                auto facetDeclRef = isDeclRefTypeOf<InterfaceDecl>(facetType);
+
+                // Only skip if we have struct->struct->interface
+                // and the struct->struct witness is not the identity witness
+                shouldSkipTransitiveWitness = selfDeclRef && baseDeclRef && facetDeclRef &&
+                                              !isTypeEqualityWitness(selfIsSubtypeOfBase);
+            }
+
+            if (!shouldSkipTransitiveWitness)
+            {
+                auto selfIsSubtypeOfFacet = _getASTBuilder()->getTransitiveSubtypeWitness(
+                    selfIsSubtypeOfBase,
+                    baseIsSubtypeOfFacet);
+
+                indirectFacet->setSubtypeWitness(_getASTBuilder(), selfIsSubtypeOfFacet);
+
+                ioMergedFacets.add(indirectFacet);
+            }
         }
 
         // We picked one `foundFacet` above to be added to the merged
diff --git a/tests/experiments/generic/multilevel-inheritance.slang b/tests/experiments/generic/multilevel-inheritance.slang
new file mode 100644
index 000000000..f3db67d70
--- /dev/null
+++ b/tests/experiments/generic/multilevel-inheritance.slang
@@ -0,0 +1,31 @@
+//TEST:SIMPLE(filecheck=CHECK): -target spirv -gnone -std 2026
+//TEST:SIMPLE(filecheck=CHECK): -target spirv -gminimal -std 2026
+//TEST:SIMPLE(filecheck=CHECK_LEGACY): -target spirv -gnone -std 2025
+//TEST:SIMPLE(filecheck=CHECK_LEGACY): -target spirv -gminimal -std 2025
+//TEST:SIMPLE(filecheck=CHECK_LEGACY): -target spirv -gnone -std legacy
+//TEST:SIMPLE(filecheck=CHECK_LEGACY): -target spirv -gminimal -std legacy
+
+// CHECK: error 30811
+// CHECK: struct Child : Parent {};
+// CHECK: error 38029
+// CHECK: getObject<Child>();
+
+// CHECK_LEGACY: warning 30816
+// CHECK_LEGACY: struct Child : Parent {};
+// CHECK_LEGACY: error 38029
+// CHECK_LEGACY: getObject<Child>();
+
+interface IFoo {};
+struct Parent : IFoo {};
+struct Child : Parent {};
+
+T getObject<T>() where T : IFoo
+{
+    return T();
+}
+
+[shader("compute")]
+void main()
+{
+    getObject<Child>();
+}