> Go 1.21 now defines that if a goroutine is panicking and recover was called directly by a deferred function, the return value of recover is guaranteed not to

> Go 1.21 now defines that if a goroutine is panicking and recover was called directly by a deferred function, the return value of recover is guaranteed not to be nil. To ensure this, calling panic with a nil interface value (or an untyped nil) causes a run-time panic of type *runtime.PanicNilError. > To support programs written for older versions of Go, nil panics can be re-enabled by setting GODEBUG=panicnil=1. This setting is enabled automatically when compiling a program whose main package is in a module with that declares go 1.20 or earlier. An explanation of why this change was implemented is contained in the description of the implementing change, https://go-review.googlesource.com/c/go/+/461956 https://go-review.googlesource.com/c/go/+/461956. In short, calling `panic(nil)` before would make `recover` return nil, which is what's returned when there is no panic. `recover` doesn't have a `panicArg, ok = recover()` return value variant, so they fixed it this way.