From 559fd7ffe7a8cb61b11bec081f14ce3c3ffb210d Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Tue, 27 Jun 2023 14:58:29 +0200
Subject: [PATCH] nix flake check: improve error message if overlay is not a
 lambda (#8582)
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* 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
---
 src/nix/flake.cc      |  6 ++++--
 tests/flakes/check.sh | 12 ++++++++++++
 2 files changed, 16 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);
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
@@ -25,6 +25,18 @@ EOF
 
 (! nix flake check $flakeDir)
 
+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 }: {