Stricter v2 Decoding

## The three changed defaults `encoding/json/v2` is not a rewrite for its own sake; several of its defaults are the ones the original package would have had if it were written today. Three of them change what a program accepts. ### 1. Duplicate object names are an error Given `{"qty":1,"qty":2}`, `encoding/json` decodes without complaint and the last occurrence wins, so the field ends up 2. `encoding/json/v2` returns an error identifying the duplicated name. This is an interoperability and security fix, not pedantry. RFC 8259 says object names *should* be unique but does not require a parser to reject repeats, and implementations disagree about which one wins -- some take the first, some the last, some concatenate. Any system where one component validates a document and another consumes it can be split by that disagreement: the validator reads the harmless first `role`, the consumer reads the second. Rejecting the document removes the question. Restore the old tolerance with `jsontext.AllowDuplicateNames(true)`. ### 2. Invalid UTF-8 is an error Go strings are arbitrary bytes, not guaranteed UTF-8, while JSON strings are text. `encoding/json` resolves that mismatch by silently replacing each invalid byte with U+FFFD, the Unicode replacement character, when marshaling, and is similarly permissive when decoding. The corruption is irreversible and invisible: nothing in the API tells you it happened. `encoding/json/v2` reports an error instead, so a byte slice that is not valid UTF-8 stops being quietly laundered into replacement characters. Restore the old behaviour with `jsontext.AllowInvalidUTF8(true)`. ### 3. Member names match case-sensitively When v1 binds a document member to a struct field, it first looks for an exact match against the tag name (or field name), and if there is none it falls back to a case-insensitive comparison. That fallback is why `{"USERID": 7}` fills a field tagged `json:"userid"`, and why two fields differing only in case are a coin flip. v2 drops the fallback: names match exactly or not at all. The practical consequence is not an error but a *silence* -- with the default handling of unrecognised members, a case-mismatched name is simply not matched, so the field keeps its zero value. That makes this the most dangerous of the three during a migration, because the other two announce themselves and this one does not. Restore the fallback with `json.MatchCaseInsensitiveNames(true)`. ## Where an option lives tells you what it is Notice the package split. Duplicate names and invalid UTF-8 are properties of the JSON *text*, decidable without knowing the destination type, so their options come from `encoding/json/jsontext`. Case-insensitive matching only means something once there is a Go struct to match against, so its option comes from `encoding/json/v2`. Both are the same kind of value and can be passed to one call: ```go err := json.Unmarshal(data, &v, jsontext.AllowDuplicateNames(true), json.MatchCaseInsensitiveNames(true), ) ``` ## Why per-call options matter more than they look In v1, configuration lived on a `Decoder` or `Encoder` object, so it was only reachable if you were already on the streaming path and it applied to everything that object touched. v2's options are values passed to any call. That is what turns a strictness upgrade from a binary decision into a schedule: one route that talks to a partner known to send duplicate names can pass `jsontext.AllowDuplicateNames(true)` while every other route in the same binary stays strict, and the option sits in the code as a visible, greppable, removable piece of debt rather than a build flag. ## What has *not* changed Unrecognised members are still ignored by default in v2, just as in v1 -- strictness about names is not the same as strictness about *unknown* names, and the two are separate decisions. The old `encoding/json` package also keeps every one of the three behaviours above; you opt into strictness by importing v2 at a call site, not by upgrading the toolchain. ## The summary that answers the interview question Duplicates rejected, invalid UTF-8 rejected, names matched exactly. Two of the three shout when they change behaviour; the third goes quiet, and that is the one to test for.