pearu commented on PR #50146:
URL: https://github.com/apache/arrow/pull/50146#issuecomment-4668734736

   Two out-of-scope discoveries made while working on this, recorded here 
rather than folded into the PR to keep it minimal:
   
   1. `MultipleParsersTimestampValueDecoder::Decode` (pre-existing, 
`csv/converter.cc`) declares its `zone_offset_present` flag once outside the 
parser loop. The built-in parsers happen to write the out-parameter on every 
call (the strptime parser unconditionally, the ISO-8601 parser resets it to 
`false` before scanning), so this is currently harmless — but `TimestampParser` 
is a public interface, and a user-implemented parser that only writes the flag 
when an offset is found could observe a stale value from a previous loop 
iteration. The new decoder in this PR declares the flag per-iteration; the 
timestamp decoder could get the same two-line treatment as a MINOR follow-up.
   
   2. The "Timestamp inference/parsing" section of the C++ CSV user guide 
(`docs/source/cpp/csv.rst`) does not mention 
`ConvertOptions::timestamp_parsers` at all — custom timestamp parsing was 
undocumented in the user guide before the date/time subsection added here. A 
short paragraph there could be a docs follow-up.


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