Should `de Mesnil Juan` be all surname, or family `de Mesnil` plus given `Juan`?

[derek73]

A leading never-given particle makes the whole name a surname, however many pieces follow:

de Mesnil Juan     family='de Mesnil Juan'   given=''
de Mesnil Garcia   family='de Mesnil Garcia' given=''

That is long-standing v1 behaviour in the default order (handle_non_first_name_prefix), and #361 extends it to the family-first orders so all three agree.

The question is whether the fold should take the whole name or stop after the piece the particle attaches to:

                 today                        alternative
de Mesnil Juan   family='de Mesnil Juan'      family='de Mesnil', given='Juan'

The alternative reads more naturally under name_order=FAMILY_FIRST — the caller has declared that the family comes first, so the particle plus its following piece is the family and the remainder is the given name. It is harder to justify in the default given-first order, where "a leading particle means there is no given name" is the rule the fold exists to express.

Why this is a new rule, not a fix

Stopping at the first piece exists in no order today. Implementing it means choosing one of:

• Family-first only — the two orders then disagree for the same string, which is the inverse of the principle behind particles_ambiguous has no effect on the parse under name_order=FAMILY_FIRST #359 ("a never-given particle keeps its particle regardless of name_order").
• Both orders — the default order changes too. That breaks the v1 parity tools/differential protects; the 751-name corpus is currently byte-identical at the 1.4.0, 2.0.0 and 2.1.0 baselines, and each ledger would need re-examining.

Either way it wants its own differential story rather than riding on a fix.

Scope note

de Mesnil (two pieces) is unaffected — family='de Mesnil' under every reading. Only three-or-more-piece names differ, and de Mesnil Garcia is one of the seven corpus names #361 already moves under the family-first orders.

Split out of #361 so that PR stays a fix to the broken order-dependence rather than a redefinition of the fold.

[derek73]

Superseded in substance by #367, which states the general principle this is a consequence of: a particle chain is a surname wherever it appears, whatever name_order was declared.

If that principle holds, the chain in de Mesnil Juan is de Mesnil and it is the surname, so given='Juan' — the "stop at the first piece" reading this issue asks about. Deciding #367 decides this; deciding this in isolation would settle only one shape of the same question.

Keeping it open as the concrete case, but #367 is where the decision belongs.

[derek73]

Correcting my earlier comment on this issue: this is not a consequence of #367, and I was wrong to say so.

de Mesnil Juan groups as [de][Mesnil][Juan] — three separate pieces, no chain. A leading particle deliberately never chains (_group skips index 0 unconditionally, which is what keeps Van Johnson parsing), so what pulls those pieces together is the fold in post_rules rule 1b, not a particle chain.

#367 and its sibling are about a chain that exists and is placed in the wrong field. This issue is about how far the fold reaches when no chain is possible. Independent mechanisms, independent decisions — this one does not need #367 settled first.