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Niels Lohmann
0ec4bdb8fa
json_sax_dom_callback_parser::key() unconditionally overwrote the object slot for a key with a `discarded` placeholder as soon as the key was accepted by the parser callback. For a duplicate key (legal JSON), this destroyed the pre-existing value from an earlier occurrence of the same key before the new value was even parsed. If the new value was then rejected by the callback, remove_discarded_value() erased the member entirely instead of leaving the original value in place, contradicting the documented behavior that a discarded value behaves as if it was never read. Add a small stash of (slot pointer, previous value) pairs so that when key() overwrites an existing member with the discarded placeholder, the previous value can be restored later if the corresponding value (scalar, object, or array) is rejected, instead of being erased. The stash entry is dropped without restoring once the new value is definitively accepted (in handle_value() for scalars, end_object()/end_array() for containers), so a duplicate key whose new value is accepted still keeps the last value as before. Non-duplicate keys are unaffected: rejecting their value still removes the member entirely, since there is nothing to restore. Signed-off-by: Niels Lohmann <mail@nlohmann.me>