Decide: `{$ASSERTIONS}` and the assertion message format

Type: decision — Track U
Status: open
Opened: 2026-08-05
Raised by: Track A, closing compat-pascal-assert-halts-instead-of-raising-eassertionfailed. The catchability half is fixed and verified; these two remainders are parity calls that were deliberately left rather than folded in.

Two divergences, both deliberate remainders

1. Assertions are always evaluated

FPC compiles Assert out unless -Sa or {$ASSERTIONS ON}. pxx always evaluates it. That is the LAX direction — we run a check FPC skipped — so it produces no wrong value, but it means:

a release build carries assertion cost FPC would have removed;
an assertion with a SIDE EFFECT (Assert(Advance(x) > 0)) runs here and not there, which is a behavioural difference, not just a performance one.

2. The message omits the source position

FPC : caught: EAssertionFailed: boom (a.pas, line 6)
pxx : caught: EAssertionFailed: boom

Text only, no behavioural consequence — but any test comparing assertion output against FPC diverges on every line.

Why they are one ticket

Implementing {$ASSERTIONS} means touching the Assert lowering, and that is the same place the file/line suffix would have to come from (the call site's position is known there and nowhere later). Doing them separately means opening that lowering twice.

Options

Both — full FPC parity: {$ASSERTIONS} / -Sa gating plus the (file, line N) suffix. Most compatible; costs a new directive and a position argument threaded into __pxxAssert.
Message only — cheap, removes the visible text divergence, leaves the always-evaluated semantics (and its side-effect difference) in place.
Neither — declare always-on assertions a deliberate dialect choice and write it down in docs/language/**. Defensible: an assertion that always runs is arguably the safer default, and pxx has chosen its own dialect before.

Recommendation

1, but low urgency — nothing is silently wrong today. If only one is wanted, prefer the {$ASSERTIONS} gating over the message: the side-effect difference is behavioural, the message is cosmetic.

Note the ordering: __pxxAssert lives in compiler/builtin/, so this lands under Track A's gate. Today's hook change there did NOT need a repin (selfhost_fixedpoint.sh converged) — worth measuring rather than assuming for the next one.

DECIDED 2026-08-06 — option 1: both, full FPC parity

User's call. Adopt {$ASSERTIONS} / -Sa gating and the (file, line N) message suffix.

Consistent with the two other calls made in the same review (scope hiding by default, dynamic arrays matching FPC): where FPC sets a clear precedent and there is no argument for diverging, follow it rather than accumulate a private dialect. Low urgency — nothing is silently wrong today — but not contentious.

Implementation: [[feature-p-assertions-directive-and-position]].

Decide: {$ASSERTIONS} and the assertion message format