proposal: strconv: NumError implementing Unwrap

[JOT85]

The strconv.NumError struct (https://golang.org/pkg/strconv/#NumError) contains the error which caused the conversion to fail in the Err field. It seems like it would be nice for NumError to implement the Unwrap() error method so that, for example, errors.Is(err, strconv.ErrRange) could be used. By exposing the struct field it is already part of the API and implementing the Unwrap method is a more convenient and standard way to use it.

[ianlancetaylor]

CC @jba @neild

[neild]

SGTM. Pretty sure I just overlooked it when adding Unwrap methods in 1.13.

[JOT85]

I've just realised, this change was added in commit 0adc89a to master which hasn't yet been put in to a release. It's tagged for go1.14beta1. Sorry about that.

[smasher164]

Closing since this is already in 1.14.