fix: Evaluate flake parent source without evaluating its outputs

This requires that we refer to the `sourceInfo` instead of the
`result`. However, `sourceInfo` does not create a chain of basedir
resolution, so we add that back with `flakeDir`.

src/libflake/call-flake.nix

@@ -42,13 +42,20 @@ let
 
       parentNode = allNodes.${getInputByPath lockFile.root node.parent};
 
+      flakeDir =
+        let
+          dir = overrides.${key}.dir or node.locked.path or "";
+          parentDir = parentNode.flakeDir;
+        in
+        if node ? parent then parentDir + ("/" + dir) else dir;
+
       sourceInfo =
         if overrides ? ${key} then
           overrides.${key}.sourceInfo
         else if node.locked.type == "path" && builtins.substring 0 1 node.locked.path != "/" then
           parentNode.sourceInfo
           // {
-            outPath = parentNode.result.outPath + ("/" + node.locked.path);
+            outPath = parentNode.sourceInfo.outPath + ("/" + flakeDir);
           }
         else
           # FIXME: remove obsolete node.info.
@@ -93,6 +100,8 @@ let
           result
         else
           sourceInfo;
+
+      inherit flakeDir sourceInfo;
     }
   ) lockFile.nodes;
 

tests/functional/flakes/relative-paths.sh

@@ -108,3 +108,24 @@ EOF
 [[ $(nix eval "$rootFlake#z") = 90 ]]
 
 fi
+
+# https://github.com/NixOS/nix/pull/10089#discussion_r2041984987
+# https://github.com/NixOS/nix/issues/13018
+mkdir -p "$TEST_ROOT/issue-13018/example"
+(
+  cd "$TEST_ROOT/issue-13018"
+  git init
+  echo '{ outputs = _: { }; }' >flake.nix
+  cat >example/flake.nix <<EOF
+{
+  inputs.parent.url = ../.;
+  outputs = { parent, ... }: builtins.seq parent { ok = null; };
+}
+EOF
+  git add -N .
+  cd example
+  # Important: the error does not trigger for an in-memory lock!
+  nix flake lock
+  # would fail:
+  nix eval .#ok
+)