Dynamic-array `a + b` concat silently miscompiles (compiles, no output)

Type: bug (parser/codegen — silent miscompile) — Track A
Status: DONE (2026-06-30, Track A) — silent miscompile fixed (now a clean error); concat-as-feature deferred
Opened: 2026-06-30
Found by: feature-dynarray-torture-test.

Symptom

var a, b, c: array of Integer;
begin
  SetLength(a,2); a[0]:=1; a[1]:=2; SetLength(b,2); b[0]:=3; b[1]:=4;
  c := a + b;                 { compiles with NO error }
  writeln('len=', Length(c)); { prints NOTHING — program produces no output, exit 0 }
end.

The compile succeeds (no diagnostic), but the resulting binary produces no output at all — the c := a + b statement corrupts the program (the following writeln never runs / is mis-emitted). A silent miscompile is worse than an honest "operator + not supported for dynamic arrays".

Expected

Either implement + as element concat (FPC {$modeswitch arrayoperators}: c = [a..., b...], Length(c) = Length(a)+Length(b)), or reject it at parse time with a clear error. Do not silently emit a broken binary.

Fix sketch

Find where a binary + over two dynarray operands is lowered (it currently slips through to some path that emits nothing useful). Minimum viable: detect dynarray + operands and Error('operator + not supported for dynamic arrays'). Better: lower to a concat helper (allocate Length(a)+Length(b), copy both, element-type aware — same generic-over-T problem as [[feature-copy-intrinsic]]).

Acceptance

c := a + b either concatenates correctly (oracle: 4 / 1 / 4) or errors at compile time; never a silent no-output binary. Regression test.

Fixed (2026-06-30, Track A)

a + b (and -/*//) with a dynamic-array operand fell through the AN_BINOP lowering to the integer/pointer IR_BINOP, which added the two array handles → wild pointer → runtime SIGSEGV. Now rejected at IR lowering with arithmetic operator not supported for dynamic arrays (...) (ir.inc AN_BINOP, gated on NodeDynDepth(operand) > 0 + an arithmetic op), so it is a clean compile-time error instead of a silent crashing binary. Integer + and string concat unchanged; self-host byte-identical; make test green (test/test_dynarray_concat_rejected.pas, a negative test).