A PromoInt parameter takes only a variable

• Type: bug — Track A (promotable-int parameter passing)
• Status: done
• Opened: 2026-08-05
• Found by: Track A, while verifying the fix for bug-a-promoint-function-result-crashes. Both shapes were needed to write that ticket's own test and neither compiles.

Repro

function viaOp(n: PromoInt): PromoInt; begin Result := n + 0; end;
function mk: PromoInt; begin Result := 12; end;
var p: PromoInt;
begin
  p := mk;
  writeln(viaOp(p));    { OK — a variable }
  writeln(viaOp(12));   { error: no overload of viaOp matches these arguments
                                 argument types: (Integer) }
  writeln(viaOp(mk));   { error: by-reference argument must be a variable }
end.

Both are ordinary calls. f(12) and f(g()) work for Integer, Int64, Double — every other numeric parameter type — so a promo parameter is the odd one out, and the two errors say different things about what is really one gap.

Why

A promo parameter is by-REFERENCE (the value is a {tag, payload} slot and the callee needs its address), so:

• a literal has no slot, and nothing materialises one; and the overload matcher does not consider Integer→PromoInt a conversion, hence the "no overload" wording rather than a by-ref complaint;
• a call result has no slot the caller owns at the point of the call, hence the by-ref complaint.

Both want the same thing: a caller-side temp holding the promo value, exactly as IRLowerCallArg already materialises for a by-value record argument. That machinery exists; the promo path does not reach it.

Why it matters

Arbitrary-precision arithmetic is most useful at the boundary — Fact(20), Pow(base, 64), f(g(x)). Requiring a named variable for every promo argument makes the type awkward exactly where it should be natural, and pushes callers to spell out temporaries the compiler should mint.

Related

• bug-a-promoint-function-result-crashes — the RESULT half, fixed (promo joins RetViaHiddenDest). This is the ARGUMENT half and is untouched by it.
• bug-a-promoint-parameter-32bit-by-ref-indirection-hangs — a different promo-parameter defect on 32-bit.
• decide-promoint-rvalue-representation — the design note behind "a promo rvalue is the slot address", which is what makes both cases need a temp.

Resolution (2026-08-05) — one gap, two faces

The two errors said different things, which is why this read as two limitations. It is one gap seen from opposite sides, and it took three small pieces:

1. TypesCompatible (symtab.inc) already knew promo ARGUMENT → integer PARAMETER — added so a recursive def fib(n: int) could call fib(n-1) — but not the reverse, so an integer literal never matched a promo formal. Added the mirror direction, Boolean excluded on the same grounds as the existing rule.
2. The lvalue check (parser.inc) exempts by-value records and const Variant precisely because IRLowerCallArg copies them into a hidden temp. It does the same for promo via PXXPromoCopy; the exemption simply never listed promo. Added.
3. The boxing (ir.inc) — an ordinal argument to a promo parameter now goes through PXXPromoFromInt into a promo temp, the identical lowering p := 12 uses. Without this the call would RESOLVE and then hand the callee a machine int where it expects a {tag, payload} slot address — the exact failure the neighbouring pointer-parameter arm carries a long warning about.

Verified against Python

Literal (f(12)), negative literal, expression (f(3+4)), Int64 variable, nesting (f(f(3))), call result (f(fact(20))), and fact(25) — 25!, well past 2^63. Every value matches math.factorial exactly.

Deliberately included a plain Integer / string overload pair in the test: a too-permissive compatibility rule would silently rebind existing calls, and that is the kind of regression that shows up as wrong behaviour rather than a build failure. Both still resolve correctly.

testmgr --tier native 1164/1164 pass. Locked in as test/test_promoint_arg_literal_and_result.pas.

The promo type is now usable at call boundaries in both directions

Third promo ticket closed today: results return through a hidden dest (bug-a-promoint-function-result-crashes), 32-bit parameters work (bug-a-promoint-parameter-32bit-by-ref-indirection-hangs), and arguments now accept what every other numeric type accepts.

Log

• 2026-08-05 — resolved, commit c283b3a3a.