Clang: Fix elementtype attribute mismatch in thunk generation

Previously, CurrentThisTy was assigned from FnInfo, which could contain
type information that was not updated. This caused a type mismatch between
the function's parameter type and the elementtype attribute, specially
in multiple-inheritance cases.

Now we extract CurrentThisTy directly from the actual thunk function parameter,
so that they are both consistent. We need to make sure that the thunk function
has arguments, otherwise there can be issues accessing their type.
Only typed pointers can be handled this way because for opaque pointers,
the getPointerElementType() is depricated. Therefore, they get handled in the
previous way in which we assigned CurrentThisTy

Author: Andres Madrid Ucha

clang/lib/CodeGen/CGVTables.cpp

@@ -576,7 +576,24 @@ llvm::Constant *CodeGenVTables::maybeEmitThunk(GlobalDecl GD,
   llvm::Function *ThunkFn = cast<llvm::Function>(Thunk->stripPointerCastsSafe());
 
   int thisIndex = ThunkFn->hasStructRetAttr() && !FnInfo.getReturnInfo().isSRetAfterThis() ? 1 : 0;
-  llvm::Type* CurrentThisTy = CGM.getTypes().ConvertTypeForMem(cast<PointerType>(FnInfo.arg_begin()->type)->getPointeeType());
+
+  // CurrentThisTy is the value used for elementtype attribute
+  // Extract the pointee type from the actual parameter to ensure consistency
+  // between the function signature and the elementtype attribute
+  llvm::Type* CurrentThisTy = nullptr;
+  
+  if (thisIndex < ThunkFn->arg_size()) {
+    llvm::Argument* ThisArg = ThunkFn->getArg(thisIndex);
+    llvm::Type* ActualParamType = ThisArg->getType();
+    auto* PtrTy = dyn_cast<llvm::PointerType>(ActualParamType);
+    // The getPointerElementType function is depricated for Opaque pointers
+    if (PtrTy && !PtrTy->isOpaque())
+      CurrentThisTy = PtrTy->getPointerElementType();
+  }
+
+  if (CurrentThisTy == nullptr)
+    CurrentThisTy = CGM.getTypes().ConvertTypeForMem(cast<PointerType>(FnInfo.arg_begin()->type)->getPointeeType());
+
 
   if (!ThunkFn->hasParamAttribute(thisIndex, llvm::Attribute::ElementType))
     ThunkFn->addParamAttr(thisIndex, llvm::Attribute::get(CGM.getLLVMContext(), llvm::Attribute::ElementType, CurrentThisTy));