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, 25 insertions, 15 deletions

diff --git a/source/slang/slang-check-constraint.cpp b/source/slang/slang-check-constraint.cpp
index e5551a875..3e3ed5297 100644
--- a/source/slang/slang-check-constraint.cpp
+++ b/source/slang/slang-check-constraint.cpp
@@ -83,16 +83,22 @@ namespace Slang
         Type*                   interfaceType)
     {
         // The most basic test here should be: does the type declare conformance to the trait.
-        if (isSubtype(type, interfaceType, constraints->additionalSubtypeWitnesses ? IsSubTypeOptions::NoCaching : IsSubTypeOptions::None))
-            return type;
-
-        // If additional subtype witnesses are provided for `type` in `constraints`,
-        // try to use them to see if the interface is satisfied.
+       
         if (constraints->subTypeForAdditionalWitnesses == type)
         {
+            // If additional subtype witnesses are provided for `type` in `constraints`,
+            // try to use them to see if the interface is satisfied.
             if (constraints->additionalSubtypeWitnesses->containsKey(interfaceType))
                 return type;
         }
+        else
+        {
+            if (isSubtype(
+                type,
+                interfaceType,
+                constraints->additionalSubtypeWitnesses ? IsSubTypeOptions::NoCaching : IsSubTypeOptions::None))
+                return type;
+        }
 
         // Just because `type` doesn't conform to the given `interfaceDeclRef`, that
         // doesn't necessarily indicate a failure. It is possible that we have a call
@@ -653,18 +659,22 @@ namespace Slang
             }
 
             // Search for a witness that shows the constraint is satisfied.
-            auto subTypeWitness = isSubtype(
-                sub,
-                sup,
-                system->additionalSubtypeWitnesses ? IsSubTypeOptions::NoCaching : IsSubTypeOptions::None);
-            if (!subTypeWitness)
+            SubtypeWitness* subTypeWitness = nullptr;
+            if (sub == system->subTypeForAdditionalWitnesses)
             {
-                if (sub == system->subTypeForAdditionalWitnesses)
-                {
-                    // If no witness was found, try to find the witness from additional witness.
-                    system->additionalSubtypeWitnesses->tryGetValue(sup, subTypeWitness);
-                }
+                // If we are trying to find the subtype info for a type whose inheritance info is
+                // being calculated, use what we have already known about the type.
+                system->additionalSubtypeWitnesses->tryGetValue(sup, subTypeWitness);
             }
+            else
+            {
+                // The general case is to initiate a subtype query.
+                subTypeWitness = isSubtype(
+                    sub,
+                    sup,
+                    system->additionalSubtypeWitnesses ? IsSubTypeOptions::NoCaching : IsSubTypeOptions::None);
+            }
+
             if(subTypeWitness)
             {
                 // We found a witness, so it will become an (implicit) argument.