Implicit (sloppy) local variables behind a switch — {$IMPLICITVARS ON} / --auto-locals

• Type: feature (language / parser) — Track A
• Status: DONE 2026-07-04
• Opened: 2026-06-30
• Origin: the original --auto-locals idea (feature-implicit-identifier-binding-strictness-switch, "the originally-imagined feature"), correctly scoped after a design pass as an opt-in, non-standard layer. The typed/standard layer is [[feature-inline-loop-var-rio]].

Idea

Under an explicit opt-in switch, an assignment to a previously-undeclared name creates an inferred local (no var, no type):

{$IMPLICITVARS ON}      { or  pxx --auto-locals  }
begin
  i := 0;               { implicit local, inferred Integer }
  s := 'abc';           { implicit local, inferred AnsiString }
  for i := 0 to 9 do ...
end;

This is "sloppy mode" — Python/BASIC ergonomics in Pascal. pxx's Nil-Python and BASIC frontends already do exactly this, via the same tyAuto (defs.inc:540) inference the inline-var path uses, so the machinery exists.

Why / why behind a switch

• Why: eliminate boilerplate for scratch variables; ease porting Python/BASIC and quick scripts.
• Why opt-in (NEVER default): it masks typos — cont := 0 silently makes a new variable instead of erroring on the misspelling of count. Pascal-the-language is declare-before-use; this is a deliberate departure.

Behaviour

• Default (off): an undeclared i := 0 is an error (today's behaviour — the decl-order gating fix, feature-implicit-identifier-binding-strictness-switch, already does this correctly). Unchanged.
• {$IMPLICITVARS ON} / --auto-locals: assignment to an undeclared name declares a tyAuto routine-local, type inferred from the RHS (reuse the inline-var := expr resolve at parser.inc ~7550). Emit a warning (implicit variable 'i') so it is visible (suppressible).
• --strict / {$DECLORDER ON} strict family: force it off (hard error) even if requested, so strict builds stay declare-before-use.

Implementation notes

• The hook is the lvalue-resolution failure in the assignment-statement path: where FindSym(name) < 0 for the LHS today raises "undefined variable", under the switch instead AllocVar(name, tyAuto) (routine-local) and continue, then let the RHS inference fill the type — the exact path the inline-var statement already uses (parser.inc ~7512/7550). Guard with the new switch global + EnableAutoVar.
• A tyAuto local that is read before any assignment must still error (use of auto variable before type is inferred, parser.inc ~4223 already exists) — so a bare read of an undeclared name stays an error even in sloppy mode; only assignment creates one.

Acceptance

• Off by default: undeclared i := 0 errors (unchanged). With --auto-locals / {$IMPLICITVARS ON}: it declares + infers, runs, and warns. --strict forces the error regardless. Self-host byte-identical (switch off in the self-build). Tests: off-errors, on-infers-and-warns, strict-overrides, read-before-assign still errors.

Log

• 2026-07-04 — DONE (Track A). Opt-in switch landed exactly as specced.
  • ImplicitVars global (defs.inc), default off; reset in PasInitDefines (runs before CLI parse). --auto-locals (compiler.pas) and {$IMPLICITVARS ON|OFF} (lexer.inc, next to {$DECLORDER}) both set it.
  • Assignment hook (parser.inc, plain-lvalue path after idx := FindSym(name)): when on + name undeclared + not unit-qualified + not a routine + the very next token is :=, create a routine-local tyAuto var (AllocVar(name, tyAuto)); the existing tyAuto-on-assignment block infers its type from the RHS + allocates the frame slot. Tight gate → obj.f :=, a[i] :=, a bare read, and calls all keep the normal path.
  • For-counter hook (parser.inc, for-header else branch): an undeclared counter is auto-created — counted for i := a to b → Integer, for x in c → tyAuto (element type backfilled by the for-in variant), mirroring the for var i inline form. Loops are the primary sloppy-mode use.
  • WarnImplicitVar (lexer.inc, writeln-based like WarnStackFrame, -Werror-aware): implicit variable 'name' on each creation, so a typo'd name becoming a new var is visible.
  • Read of an undeclared name still errors (only assignment/for-counter creates one) — the ParseFactor rvalue path is untouched.
  • Off by default → self-host byte-identical (build vs verify cmp clean, procs 1530→1531 from the new WarnImplicitVar routine only). make test green. Regression test/test_auto_locals.pas (int / string(managed) / counted-for / for-in-array, total ok 4 / 4) wired into test-core both ways: passes with --auto-locals, and the same file WITHOUT the flag is asserted to fail with undefined variable.
  • Not done (out of original scope): a hard --strict family override that forces the error even when requested — no --strict flag exists in the compiler today; {$IMPLICITVARS OFF} / omitting the flag is the off path. File a follow-up if a strict-mode veto is wanted.