Fix ensure

[travisbrown]

In the [0.10.0] release we introduced a new Decoder#ensure method that was semantically kind of weird and included some behavior that (when used together with error accumulation) was definitely buggy:

scala> import io.circe.{ Decoder, Json }
import io.circe.{Decoder, Json}

scala> val decodePos: Decoder[Int] = Decoder[Int].ensure(_ > 0, "not positive")
decodePos: io.circe.Decoder[Int] = io.circe.Decoder$$anon$25@5b9070e3

scala> decodePos.accumulating(Json.Null.hcursor)
res0: io.circe.AccumulatingDecoder.Result[Int] = Invalid(NonEmptyList(DecodingFailure(Int, List()), DecodingFailure(not positive, List())))

Thanks to @ylaurent for noticing and reporting this.

The tests would have caught this bug, but the relevant test was itself buggy, containing a check for oddness that said e.g. -1 was not odd.

I've changed the documentation for the previous version of ensure to state explicitly that dec.ensure(p1, "p1").ensure(p2, "p2") will only provide the first failure, and have added a new, more powerful ensure method that allows the user to provide multiple errors that depend on the decoded value.

@pfcoperez, I was thinking we might be able to add a similar HCursor => List[String] variant for validate that would accommodate the use case supported by your #1020, and that would keep these two methods consistent. What do you think?

[pfcoperez]

@pfcoperez, I was thinking we might be able to add a similar HCursor => List[String] variant for validate that would accommodate the use case supported by your #1020, and that would keep these two methods consistent. What do you think?

@travisbrown I think it is a great idea! I'll update my PR with that variant. That is, I'll replace the HCursor => Boolean, HCursor => String with HCursor => List[String] and implement the legacy method def validate(pred: HCursor => Boolean, message: => String) with it.

[travisbrown]

Thanks, @pfcoperez!

[codecov-io]

Codecov Report

Merging #1025 into master will decrease coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1025      +/-   ##
==========================================
- Coverage   85.39%   85.34%   -0.06%     
==========================================
  Files          75       75              
  Lines        2383     2388       +5     
  Branches      168      170       +2     
==========================================
+ Hits         2035     2038       +3     
- Misses        348      350       +2

Impacted Files	Coverage Δ
.../core/shared/src/main/scala/io/circe/Decoder.scala	92.37% <66.66%> (-0.82%)	⬇️
...re/shared/src/main/scala/io/circe/JsonNumber.scala	91.13% <0%> (-1.27%)	⬇️
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	90.96% <0%> (+0.6%)	⬆️
...les/core/shared/src/main/scala/io/circe/Json.scala	79.61% <0%> (+0.63%)	⬆️

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