In checkOverlay, accept underscored names for final/prev args.

Resolves #4416.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 68bb76742..5855348ac 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -346,10 +346,14 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkOverlay = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
                 state->forceValue(v, pos);
-                if (!v.isLambda() || v.lambda.fun->hasFormals() || std::string(v.lambda.fun->arg) != "final")
+                if (!v.isLambda() || v.lambda.fun->hasFormals() ||
+                    (std::string(v.lambda.fun->arg) != "final" &&
+                        std::string(v.lambda.fun->arg) != "_final"))
                     throw Error("overlay does not take an argument named 'final'");
                 auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);
-                if (!body || body->hasFormals() || std::string(body->arg) != "prev")
+                if (!body || body->hasFormals() ||
+                    (std::string(body->arg) != "prev" &&
+                        std::string(body->arg) != "_prev"))
                     throw Error("overlay does not take an argument named 'prev'");
                 // FIXME: if we have a 'nixpkgs' input, use it to
                 // evaluate the overlay.