Building on the merged unit-class_parser.cpp from #5417, add
characterization tests (regression protection for existing behavior, not a
behavior change) for JSON_DIAGNOSTIC_POSITIONS:
- value lifetime: copy ctor copies positions recursively, move ctor resets
the moved-from value to npos, and mutating a parsed document (operator[],
push_back, erase) leaves the parent's stale span and siblings' positions
untouched while new values get npos.
- input adapters: wide-string input positions count transcoded UTF-8 bytes
(not wide characters), BOM-prefixed input's start_pos() reflects the
skipped 3-byte BOM, istringstream/ifstream/iterator-pair inputs report
consistent (non-npos) positions, and binary formats (CBOR, MessagePack,
UBJSON, BSON) always report npos.
- a user-constructed json_sax_dom_parser with no lexer (as used when driving
json::sax_parse() directly) reports npos for every value, since it has no
m_lexer_ref to source positions from.
While characterizing swap(), found that basic_json::swap() (and the friend
swap() that forwards to it) does not swap start_position/end_position,
unlike copy-assignment's operator=(basic_json), which does as part of its
copy-and-swap implementation. This looks like a real inconsistency/bug, but
per the scope of this test-only change it is only pinned (not fixed) here;
see the comment at the "swap() does NOT exchange positions" section.
Fixes#5420
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
03c638d852