Fix extension override behavior, and disallow extension on interface types. (#4977)

* Add a test to ensure extension does not override existing conformance.

* Fix doc.

* Update documentation.

* Fix doc.

* Add diagnostic test.

1 files changed, 9 insertions, 3 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 5654ac7a6..e5e8e8acc 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -8246,7 +8246,12 @@ namespace Slang
         if (auto targetDeclRefType = as<DeclRefType>(decl->targetType))
         {
             // Attach our extension to that type as a candidate...
-            if (auto aggTypeDeclRef = targetDeclRefType->getDeclRef().as<AggTypeDecl>())
+            if (targetDeclRefType->getDeclRef().as<InterfaceDecl>())
+            {
+                getSink()->diagnose(decl->targetType.exp, Diagnostics::invalidExtensionOnInterface, decl->targetType);
+                return;
+            }
+            else if (auto aggTypeDeclRef = targetDeclRefType->getDeclRef().as<AggTypeDecl>())
             {
                 auto aggTypeDecl = aggTypeDeclRef.getDecl();
 
@@ -8303,6 +8308,7 @@ namespace Slang
         // to extend.
         //
         decl->targetType = CheckProperType(decl->targetType);
+
         _validateExtensionDeclTargetType(decl);
 
         _validateExtensionDeclMembers(decl);
@@ -9188,13 +9194,13 @@ namespace Slang
                 // look up extensions based on what would be visible to that
                 // module.
                 //
-                // We need to consider the extensions declared in the module itself,
+                // Extensions declared in the module itself should have already
+                // been registered when we check them, but we still need to bring
                 // along with everything the module imported.
                 //
                 // Note: there is an implicit assumption here that the `importedModules`
                 // member on the `SharedSemanticsContext` is accurate in this case.
                 //
-                _addCandidateExtensionsFromModule(m_module->getModuleDecl());
                 for( auto moduleDecl : this->importedModulesList )
                 {
                     _addCandidateExtensionsFromModule(moduleDecl);