Fix an issue in extension override. (#7402)

* Fix an issue in extension override.

* Fix typo in comment.

1 files changed, 28 insertions, 4 deletions

diff --git a/source/slang/slang-check-overload.cpp b/source/slang/slang-check-overload.cpp
index 8fbdceabb..8d76d2554 100644
--- a/source/slang/slang-check-overload.cpp
+++ b/source/slang/slang-check-overload.cpp
@@ -1321,6 +1321,34 @@ int SemanticsVisitor::CompareLookupResultItems(
     LookupResultItem const& left,
     LookupResultItem const& right)
 {
+    auto leftDeclRefParent = getParentDeclRef(left.declRef);
+    auto rightDeclRefParent = getParentDeclRef(right.declRef);
+
+    bool leftIsExtension = false;
+    bool rightIsExtension = false;
+    bool leftIsFreeFormExtension = false;
+    bool rightIsFreeFormExtension = false;
+
+    // Prefer declarations that are not in free-form generic extensions, i.e.
+    // `extension<T:IFoo> T { /* declaration here should have lower precedence. */ }
+    if (auto leftExt = as<ExtensionDecl>(leftDeclRefParent.getDecl()))
+    {
+        leftIsExtension = true;
+        if (isDeclRefTypeOf<GenericTypeParamDeclBase>(leftExt->targetType))
+            leftIsFreeFormExtension = true;
+    }
+    if (auto rightExt = as<ExtensionDecl>(rightDeclRefParent.getDecl()))
+    {
+        rightIsExtension = true;
+        if (isDeclRefTypeOf<GenericTypeParamDeclBase>(rightExt->targetType))
+            rightIsFreeFormExtension = true;
+    }
+
+    // If one of the candidates is a free-form extension, it is always worse than
+    // a non-free-form extension.
+    if (leftIsFreeFormExtension != rightIsFreeFormExtension)
+        return int(leftIsFreeFormExtension) - int(rightIsFreeFormExtension);
+
     // It is possible for lookup to return both an interface requirement
     // and the concrete function that satisfies that requirement.
     // We always want to favor a concrete method over an interface
@@ -1334,16 +1362,12 @@ int SemanticsVisitor::CompareLookupResultItems(
     // directly (it is only visible through the requirement witness
     // information for inheritance declarations).
     //
-    auto leftDeclRefParent = getParentDeclRef(left.declRef);
-    auto rightDeclRefParent = getParentDeclRef(right.declRef);
     bool leftIsInterfaceRequirement = isInterfaceRequirement(left.declRef.getDecl());
     bool rightIsInterfaceRequirement = isInterfaceRequirement(right.declRef.getDecl());
     if (leftIsInterfaceRequirement != rightIsInterfaceRequirement)
         return int(leftIsInterfaceRequirement) - int(rightIsInterfaceRequirement);
 
     // Prefer non-extension declarations over extension declarations.
-    bool leftIsExtension = as<ExtensionDecl>(leftDeclRefParent.getDecl()) != nullptr;
-    bool rightIsExtension = as<ExtensionDecl>(rightDeclRefParent.getDecl()) != nullptr;
     if (leftIsExtension != rightIsExtension)
     {
         // Add a special case for constructors, where we prefer the one that is not synthesized,