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, 12 insertions, 0 deletions

diff --git a/tests/flakes/check.sh b/tests/flakes/check.sh
index 34b82c61c..0433e5335 100644
--- a/tests/flakes/check.sh
+++ b/tests/flakes/check.sh
@@ -27,6 +27,18 @@ EOF
 
 cat > $flakeDir/flake.nix <<EOF
 {
+  outputs = { self, ... }: {
+    overlays.x86_64-linux.foo = final: prev: {
+    };
+  };
+}
+EOF
+
+checkRes=$(nix flake check $flakeDir 2>&1 && fail "nix flake check --all-systems should have failed" || true)
+echo "$checkRes" | grepQuiet "error: overlay is not a function, but a set instead"
+
+cat > $flakeDir/flake.nix <<EOF
+{
   outputs = { self }: {
     nixosModules.foo = {
       a.b.c = 123;