Add a note for CWG2982

Author: zyn0217

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -857,6 +857,8 @@ class PackDeductionScope {
       if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(
               TemplateParams->getParam(Index))) {
         if (!NTTP->isExpandedParameterPack())
+          // FIXME: CWG2982 suggests a type-constraint forms a non-deduced
+          // context, however it is not yet resolved.
           if (auto *Expansion = dyn_cast<PackExpansionType>(
                   S.Context.getUnconstrainedType(NTTP->getType())))
             ExtraDeductions.push_back(Expansion->getPattern());