Nested variant parts with a tagged discriminant (case f: T of inside a variant arm)

• Type: bug / feature gap (Pascal frontend — record parser) — Track P (shared parser, A's gate)
• Status: done
• Opened: 2026-07-12, the synsock wall of [[feature-synapse-compile-check]] ("expected field name in variant part").

Symptom

Synapse's ssfpc.inc:357 TVarSin — the sockaddr union every synsock function passes around:

TVarSin = packed record
  case integer of
    0: (AddressFamily: sa_family_t);
    1: (
      case sin_family: sa_family_t of      { <- tagged discriminant, NESTED }
        AF_INET:  (sin_port: word;
                   sin_addr: TInAddr;
                   sin_zero: array[0..7] of byte);
        AF_INET6: (sin6_port: word;
                   sin6_flowinfo: longword;
                   sin6_addr: TInAddr6;
                   sin6_scope_id: longword);
        );
end;

Three combined shapes PXX's variant parser rejects here:

1. a variant ARM whose body is itself a case ... of (nested variant part);
2. a TAGGED discriminant (case sin_family: sa_family_t of — the tag is a real field, not just a selector type);
3. non-integer-literal case labels (AF_INET/AF_INET6 — declared consts).

devdocs/progress/done/ has the flat variant-overlap work (project_variant_record_overlap_done) — this is the recursive/tagged extension of that.

Why it matters

TVarSin is THE parameter type of the whole synsock surface (SizeOfVarSin, bind/connect/accept wrappers) — blcksock/httpsend/smtpsend/pop3send/ftpsend all stop here. With [[bug-cast-deref-as-varparam-arg]] this is one of the two remaining walls in front of the entire Synapse stack.

Acceptance

• Nested variant parts parse recursively; a tagged discriminant declares its tag as an ordinary field at the variant offset; const case labels resolve.
• Layout matches FPC (all arms overlay at the variant start; record size = max arm end; packed respected) — verify with SizeOf(TVarSin) against FPC.
• synsock.pas compiles past line 435; self-host byte-identical.

Log

• 2026-07-12 — resolved, commit 541f8fda.