encoding/json/v2: report error when unmarshaling into ambiguous set of fields

When MatchCaseInsensitiveNames is specified, an incoming JSON name
may match multiple Go struct fields to prospectively unmarshal into.
The v1 default behavior is to select the first field declared.
This change makes it such that the v2 default is to report an error
if multiple candidate fields exist.
The ReportErrorsWithLegacySemantics options opts into v1 behavior.

Updates #79733

Change-Id: Idcfca4fe0a21c7675c61ee836c51da1b1a719c7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/792780
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
6 files changed
tree: 9c8d72a522c12480c66283d6856c8648cf703772
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. codereview.cfg
  11. CONTRIBUTING.md
  12. go.env
  13. LICENSE
  14. PATENTS
  15. README.md
  16. SECURITY.md
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