std.member does not handle strings correctly

[smcgivern]

Regular jsonnet defines std.member on strings in terms of std.findSubstr: https://github.com/google/jsonnet/blob/813c7412d1c7a42737724d011618d0fd7865bc69/stdlib/std.jsonnet#L236-L241

This means that if the first argument is a string, the second argument can be a string of any length.

The native version added in #638 only allows the second argument to be a single-character string. That is, with this input:

{
  'foo': std.member('foobar', 'foo'),
}

go-jsonnet 0.18.0 returns:

{
   "foo": true
}

But 0.19 and later returns:

{
   "foo": false
}

Doing std.member('foobar', 'o') will return true, however.

[salvacorts]

I just opened a PR fixing this: #657

[smcgivern]

Thanks!

[anguslees]

Request: can we get a 0.19.x, containing this fix?

It's subtle, is a regression, and yet silently does the wrong thing. I just lost a few hours to debugging something that turned out to be this - and it would be nice if we can help other folks avoid this experience.

[sparkprime]

Yes this makes sense and we should have done a release much earlier