nix flake check: improve error message if overlay is not a lambda (#8582)

* nix flake check: improve error message if overlay is not a lambda

Suppose you have an overlay like this

    {
      inputs = { /* ... */ };
      outputs = { flake-utils, ... }: flake-utils.lib.eachDefaultSystem
        (system: {
          overlays.default = final: prev: {

          };
        });
    }

then `nix flake check` (correctly) fails because `overlays` are supposed
to have the structure `overlays.<name> = final: prev: exp`. However, the
error-message is a little bit counter-intuitive:

    error: overlay does not take an argument named 'final'

While one might guess where the error actually comes from because the
trace above says `… while checking the overlay 'overlays.x86_64-linux'`
this is still pretty confusing because it complains about an argument
not being named `final` even though that's evidently the case.

With this change, the error-message actually makes it clear what's
wrong:

    [ma27@carsten:~/Projects/nix/tmp]$ nix flake check --extra-experimental-features 'nix-command flakes' path:$(pwd)
    error:
           … while checking flake output 'overlays'

             at /nix/store/clgblnxx003hyrq8qkz5ab6kgqkck6qc-source/flake.nix:4:5:

                3|   outputs = { ... }: {
                4|     overlays.x86_64-linux.snens = final: prev: {
                 |     ^
                5|       kek = throw "snens";

           … while checking the overlay 'overlays.x86_64-linux'

             at /nix/store/clgblnxx003hyrq8qkz5ab6kgqkck6qc-source/flake.nix:4:5:

                3|   outputs = { ... }: {
                4|     overlays.x86_64-linux.snens = final: prev: {
                 |     ^
                5|       kek = throw "snens";

           error: overlay is not a lambda, but a set instead

1 files changed, 4 insertions, 2 deletions

diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 8d39de389..b5f5d0cac 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -386,8 +386,10 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkOverlay = [&](const std::string & attrPath, Value & v, const PosIdx pos) {
             try {
                 state->forceValue(v, pos);
-                if (!v.isLambda()
-                    || v.lambda.fun->hasFormals()
+                if (!v.isLambda()) {
+                    throw Error("overlay is not a function, but %s instead", showType(v));
+                }
+                if (v.lambda.fun->hasFormals()
                     || !argHasName(v.lambda.fun->arg, "final"))
                     throw Error("overlay does not take an argument named 'final'");
                 auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);