bug: nested `{ }` comments break the FPC idiom `{ ... '{' ... }`

Type: bug
Status: done
Track: A
Opened: 2026-06-23
Closed: 2026-06-24
Breaks: lib/rtl/json.pas (lib-test gate), and any source with a lone brace character inside a { } comment.

Summary

After done/bug-nested-brace-comments, { } comments nest. But standard Pascal / FPC { } comments do not nest — a { } comment ends at the first }. Nesting them silently breaks the extremely common idiom of mentioning a brace inside a comment:

{ consume '{' }
writeln('after');     { <- swallowed: outer comment never closes }

The inner { opens a nested level; the single } closes only that level, leaving the outer comment open, so it eats subsequent source until the next } (or EOF). FPC compiles this fine.

Repro (compiler only)

program t;
begin
  { consume '{' }
  writeln('after comment');
end.

pinned t.pas → error: undefined variable (interface) (the comment swallowed writeln(...) and beyond). Expected: compiles, prints after comment.

Found in lib/rtl/json.pas:491 ({ consume '{' }), which fails to compile under v44 → examples/json/jsondemo.pas breaks → make lib-test red.

Expected (FPC semantics)

{ } comments do NOT nest; (* *) do not nest either; only the (* { *) / { (* } cross pairs and {$...} are special. A { inside a { } comment is plain text and must not open a nested comment. If nesting is wanted at all it must be opt-in (a directive/mode), never the default — it is an FPC-compat break.

Note

done/bug-nested-brace-comments should be re-examined: it likely over-reached. Real FPC behaviour is non-nesting { }. Track B hit this only now because the v44 re-pin is the first stable to carry the nesting change together with libs that mention braces in comments.

Acceptance

The repro compiles and prints after comment; make lib-test passes jsondemo again; make test (Track A self-host) stays green.

Log

2026-06-23 — filed from Track B (surfaced running make lib-test after the v44 re-pin). Not worked around — json.pas left idiomatic. Grep marker: bug-nested-comment-breaks-fpc-brace.
2026-06-24 — FIXED (Track A). Root cause: done/bug-nested-brace-comments flipped NestedComments default to True; FPC default is non-nesting. Reverted lexer.inc:537 default to False (nesting now opt-in via {$NESTEDCOMMENTS ON}, which compiler/compiler.pas:10 already sets, so self-host unaffected and stays byte-identical). {$...} directive open-brace still raises depth so prose can mention directives. Repro compiles + prints after comment; make test green (exit 0), self-host byte-identical. Annotated the over-reached done ticket. Stabilized + pinned for Track B.

CORRECTION (2026-07-02) — this revert's FPC claim was wrong

Re-verified against real FPC 3.2.2: FPC does nest { { } } by default (default AND objfpc modes), and FPC rejects { consume '{' } exactly like nested-pxx does — the claim above that "FPC compiles this fine" was incorrect (never reproduced against a real FPC). The lone-brace idiom is not FPC-legal; json.pas's comment was the bug, not the nesting. Default re-flipped to ON in v148 ({$mode delphi} = off, directive overrides), json.pas comment made FPC-legal, whole tree lexes byte-identically under nesting. This ping-pong (ON 06-23 → OFF 06-24 → ON 07-02) is settled by the empirical FPC matrix in test/test_nested_comments.pas.

bug: nested { } comments break the FPC idiom { ... '{' ... }