[llvm] [Support] Extend ExtensibleRTTI utility to support basic multiple inheritance. (PR #112643)

[David Blaikie via llvm-commits]

================
@@ -110,21 +106,36 @@ class RTTIRoot {
 ///   static char ID;
 /// };
 ///
-template <typename ThisT, typename ParentT>
-class RTTIExtends : public ParentT {
+template <typename ThisT, typename... ParentTs>
+class RTTIExtends : public ParentTs... {
 public:
   // Inherit constructors from ParentT.
-  using ParentT::ParentT;
+  using ParentTs::ParentTs...;
 
   static const void *classID() { return &ThisT::ID; }
 
   const void *dynamicClassID() const override { return &ThisT::ID; }
 
+  /// Check whether this instance is a subclass of QueryT.
+  template <typename QueryT> bool isA() const { return isA(QueryT::classID()); }
+
   bool isA(const void *const ClassID) const override {
-    return ClassID == classID() || ParentT::isA(ClassID);
+    return ClassID == classID() || parentsAreA<ParentTs...>(ClassID);
----------------
dwblaikie wrote:

Might be able to skip the `parentsAreA` helper:
```
return ClassId == classId() || (ParentTs::isA(ClassID) || ...);
```
(here's at least a little proof of concept I did: https://godbolt.org/z/vGca7jYbT )

https://github.com/llvm/llvm-project/pull/112643