A pointer dereference p^ is rejected as a var/by-ref argument

• Type: bug (parser / argument binding)
• Status: backlog (Track A)
• Owner: — (Track A — compiler/**)
• Opened: 2026-06-24
• Found-by: [[feature-synapse-compile-check]] — building lib/rtl/sockets.pas (fpSelect wanted to pass readfds^ / exceptfds^ to the FD_* helpers).

Symptom

Passing a dereferenced pointer p^ as a var (by-reference) argument errors:

type TR = record b: LongWord; end; PR = ^TR;
procedure setit(var r: TR); begin r.b := 5; end;
var rr: TR; p: PR;
begin p := @rr; setit(p^); end.

→ error: by-reference argument must be a variable

p^ is a valid l-value — FPC accepts it as a var argument. Assigning through the deref (p^ := x, p^.field := x) works; only using it as a by-ref argument fails.

Impact / workaround

Idiomatic FPC code passes record-pointer derefs to var-param helpers freely. In lib/rtl/sockets.pas fpSelect this was worked around by reading the record fields directly off the pointer (readfds^.bits[i]) instead of calling fpFD_ISSET(.., readfds^), and inlining fpFD_ZERO(exceptfds^). Equivalent and acceptable there, but the general gap will bite other RTL/Synapse code.

Fix

In argument binding, treat a pointer dereference p^ (and p^.field, p^[i]) as an addressable l-value eligible for a var/out/untyped parameter — the same l-value test that already allows arr[i] and rec.field.

Done when

• setit(p^) compiles; the var-arg l-value set includes pointer derefs.
• Regression test under make test.
• Self-host fixedpoint byte-identical.

Resolution (2026-06-25, Track A)

Fixed. Two by-ref l-value checks in parser.inc (the Delphi-bare-proc call path ~4738 and the statement-call path ~7973) accepted only AN_IDENT/AN_INDEX/ AN_FIELD; added AN_DEREF. IsASTLValue (ir.inc:952) and the arg-lowering addressing path already handled AN_DEREF, so no codegen change was needed — just the parser gate relaxation. Covers p^, p^.field, p^[i].

Test: test/test_ptr_deref_vararg.pas (in make test), prints 5 7 7. Verified x86-64 + i386/aarch64/arm32 (all 5 7 7). Self-host byte-identical.