[RaphaelFakhri]

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

Bug fix, not a new feature.

Problem

With JSONFormatter, a value that resolves to an empty slog group produces invalid JSON:

type empty struct{}

func (empty) LogValue() slog.Value { return slog.GroupValue() }

l := log.NewWithOptions(os.Stdout, log.Options{Formatter: log.JSONFormatter})
l.Info("message", "lv", empty{}, "k", "v")

Before:

{"level":"info","msg":"message","lv":{}"k":"v"}

After:

{"level":"info","msg":"message","lv":{},"k":"v"}

writeSlogValue calls jw.start() for the nested group, which resets the item counter to 0. An empty group leaves the counter at 0, so the next key is written without a comma.

Change

writeSlogValue saves the counter before writing a group and restores it after. Output for non-empty groups is unchanged.

Tests

TestSlogAttr has a new log valuer empty group case. The case checks both the log and the slog paths. It fails without the change and passes with it. go test ./... and golangci-lint run pass.

Fixes #231