Add brief explanations

Author: zyn0217

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -377,6 +377,9 @@ struct ConvertConstructorToDeductionGuideTransform {
         if (NestedPattern)
           Args.addOuterRetainedLevels(NestedPattern->getTemplateDepth());
         auto [Depth, Index] = getDepthAndIndex(Param);
+        // Depth can still be 0 if FTD belongs to an explicit class template
+        // specialization with an empty template parameter list. In that case,
+        // we don't want the NewDepth to overflow, and it should remain 0.
         assert(Depth ||
                cast<ClassTemplateSpecializationDecl>(FTD->getDeclContext())
                    ->isExplicitSpecialization());