update comments

Author: a-tarasyuk

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -4694,6 +4694,13 @@ bool Sema::InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
                                       ParmVarDecl *Param) {
   assert(Param->hasUninstantiatedDefaultArg());
 
+  // FIXME: We don't track member specialization info for non-defining
+  // friend declarations, so we will not be able to later find the function
+  // pattern. As a workaround, don't instantiate the default argument in this
+  // case. This is correct per wording and only an error recovery issue, as per
+  // [dcl.fct.default]p4:
+  //   if a friend declaration D specifies a default argument expression,
+  //   that declaration shall be a definition.
   if (FD->getFriendObjectKind() != Decl::FOK_None &&
       !FD->getTemplateInstantiationPattern())
     return true;