SetLength rejects dynamic-array function result

• Type: bug (compiler / codegen)
• Status: backlog
• Owner: — (track A)
• Opened: 2026-06-20
• Relation: surfaced while building lib/rtl/png.pas; blocks idiomatic dynamic-array-returning library code.

Problem

SetLength(Result, n) inside a function returning a dynamic array fails during IR codegen:

pascal26:175: error: SetLength expects a string variable in IR codegen ()

The source shape is normal Pascal and should work for dynamic arrays just as it does for a named local dynamic-array variable.

Reproduction

program repro;

type
  TByteArray = array of Byte;

function MakeBytes(n: Integer): TByteArray;
begin
  SetLength(Result, n);
  if n > 0 then Result[0] := 42;
end;

begin
  writeln(MakeBytes(1)[0]);
end.

The PNG library hits the same shape in:

function BuildRawRGBA(const img: TImage): TByteArray;
begin
  SetLength(Result, img.Height * (1 + img.Width * 4));
  ...
end;

Expected

The compiler should treat the function result lvalue as a dynamic-array variable for SetLength, retain/release it correctly, and allow indexed writes to the resized result.

Actual

The compiler rejects the call as if only string result variables are accepted by the SetLength lowering path.

Notes

Do not work around this in library code by introducing a temporary local solely to satisfy the compiler. The library should remain idiomatic and the compiler should learn this lvalue shape.

Log

• 2026-06-20 — opened from Track B PNG library work. Reproduces with pinned stable when compiling test/lib_png.pas.

RESOLVED 2026-06-20 (Track A)

Function-return-type parsing only set retIsDynArr for a literal array of T result; a NAMED dynamic-array alias (e.g. TByteArray = array of Byte) fell through to the scalar path, so Result was a non-array local and SetLength routed to the string path (-101) -> "expects a string variable". Fix: in the isFunc return-type block (parser.inc), detect a named dyn-array alias via FindArrayType + ArrTypeIsDyn and set retIsDynArr/retElemTk/retElemRec, so Result is allocated as a resizable dyn-array local. SetLength(Result, n) and indexed writes now work for both literal and named-alias dyn-array results. test/test_setlength_dynarray_result.pas added; byte-identical self-host, make test green. (NOTE: inline indexing of a dyn-array-returning CALL — MakeBytes(1)[0] — is a separate parser gap, not this bug.)

Resolved-in: d83fbe9 (finalizing commit)