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A string assigned to a record ARRAY ELEMENT is not type-checked

Repro

program TB2;
type
  TR   = record S: string; N: Integer; end;
  TRs  = array of TR;
  TFix = array[0..1] of TR;
var rs: TRs; fx: TFix; s: string;
begin
  SetLength(rs, 2);
  s := 'x';
  rs[1] := s;          { dyn-array element   — ACCEPTED }
  fx[0] := s;          { fixed-array element — ACCEPTED }
  writeln('compiled and ran');
end.



$ pascal26 tb2.pas tb2
ok: tb2  [code=62255B data=2036B bss=42500B procs=129]
$ ./tb2
Segmentation fault (core dumped)

The plain-variable form IS checked, in the same program:

  r := s;   { -> pascal26:11: error: incompatible types: cannot assign AnsiString to record }

FPC rejects every one of them:

typebug.pas(9,12) Error: Incompatible types: got "AnsiString" expected "TR"
typebug.pas(11,8) Error: Incompatible types: got "AnsiString" expected "TR"

Why this is worth more than one diagnostic

It is the shape devdocs/dev/normalise-dont-special-case.md is about: one concept — assigning to an lvalue of record type — reachable through two shapes, with the check on only one of them. The variable arm has it; the element arm does not, for either array kind. The document's own advice applies to the fix: when you add the missing check, grep for the sibling before closing this — a record FIELD (r.Inner := s), a var/out parameter, and a class field are the obvious neighbours and none of them was tested here.

The consequence is the expensive kind rather than the cheap one. It does not fail to compile, it compiles to a byte move of a string HANDLE over a record's first field, and what happens next depends on what that field is. Here it is itself a managed string, so the segfault arrives at scope exit while releasing a handle that was never one.

How it was found, which is the part worth recording

Not by a test for this. The wasm32 lane wrote a bad line into its own slice — rs[1] := o2.Inner.S + '';, a leftover from an edit — expected a type error, and got a working compile and a core dump. A test whose bad line is caught by the compiler teaches nothing; this one was only found because the compiler agreed with it. Nobody was looking at record assignment that day.

Gate

Per CLAUDE.md: make compiler/pascal26 plus the repro above rejecting all four forms. A {%FAIL} conformance case for each shape is the natural regression, since the assertion is that compilation FAILS.

2026-08-30 (frankB) — root cause found; the hole is WIDER than filed, and the fix is Track A

Binary: HEAD fa0aff661, self-host fixedpoint faf762981c3c (= pin v397). Repro confirmed: compiles clean, segfaults, exit 139.

The hole is 5 of 6 forms, not 2

The ticket names the two array-element forms and predicts the neighbours ("a record FIELD, a var/out parameter, and a class field are the obvious neighbours and none of them was tested here"). They were tested here. Every predicted neighbour is also unchecked, plus pointer deref:

# form pxx FPC 3.2.2 (oracle)
1 r := s plain variable rejects rejects
2 rs[1] := s dyn elem accepts rejects
3 fx[0] := s fixed elem accepts rejects
4 r.Inner := s record field accepts rejects
5 c.F := s class field accepts rejects
6 p^ := s deref accepts rejects

FPC rejects all six (Incompatible types: got "AnsiString" expected "TR"); pxx rejects one. Verified non-vacuously: with form 1 deleted, forms 2-6 compile clean — they are not being skipped after a first error.

Root cause: the check never runs, because the destination cannot be typed

The check itself is fine and correctly placed, at the AN_ASSIGN funnel in compiler/ir.inc:9349. Its own comment claims the funnel means "one rule covers for variables, +=, out-param clears and field stores instead of the ~20 sites that build an AN_ASSIGN" — and that claim is what is false. The guard is:

if AssignSideKind(ASTLeft[node], asgDstTk) and
   AssignSideKind(ASTRight[node], asgSrcTk) and
   AssignKindsIncompatible(asgDstTk, asgSrcTk) and ...

AssignSideKind (ir.inc:75) handles only AN_IDENT and literals. There is no case for AN_INDEX, AN_FIELD or AN_DEREF, so it returns False, the and chain short-circuits, and the check is silently skipped. It does not fire and pass — it never runs. So the funnel is real but the typing of the destination is not, and the rule covers exactly the shapes that reach it as a bare identifier.

Note the shape: the bail is deliberate and documented. For an identifier, if Syms[si].IsArray then Exit; carries the comment { the kind is the ELEMENT's }. That comment is also the ingredient a fix needs — for an array symbol Syms[si].TypeKind already IS the element's kind, which is exactly what an AN_INDEX destination wants.

Do not take the obvious patch on trust — the two sources disagree

defs.inc:422 documents AN_INDEX as { Left = base sym idx; Right = index expr }, which would make the index case nearly free. But ir.inc:1544 reads ASTKind[ASTLeft[node]] — treating Left as a node, not a symbol. Both cannot be right for all builders of the node. Anyone fixing this must establish which is true (and whether nested rs[1][2] differs) before writing the case; a wrong reading here produces a check that mis-types a destination, which is worse than the missing check because it would reject valid code.

The IsRef bail on the same function is why the ticket's var/out parameter neighbour is a separate question — a by-ref slot holds an address — and it is not covered by the table above.

Lane: this is a Track A change, filed under P

The defect is a Pascal-frontend symptom but the fix is in compiler/ir.inc, shared core. Per CLAUDE.md that is Track A's file and must not be edited under Track P. Not edited. frankS is concurrently in defs.inc / pasparser_generic.inc / pasparser_decl.inc, so the sole-A guard is not satisfied by inspection. Escalated to the coordinator for the A slot rather than guessed. Diagnosis above is complete enough to hand to whoever holds A.

Gate when it lands

make compiler/pascal26 + all six forms rejected, with a {%FAIL} case per shape. Include form 1 in the regression — it works today and is the arm that proves a fix did not break the path that already worked.

2026-08-30 (frankS) — FIXED, and the two traps frankB flagged were both real

Landed in compiler/ir.inc (+ a corrected defs.inc comment, logged not filed). Self-host fixedpoint b0a33778470d; tools/gate.sh quick GREEN.

The fix

AssignSideKind gains AN_INDEX / AN_FIELD / AN_DEREF, reading the kind off the node (ASTTk), and AssignSideRecOf gains the same three through ResolveNodeRec — otherwise an operator lookup on those shapes is keyed on REC_NONE and stands a declared conversion down. Unset stays silent rather than wrong: ASTTk 0 is tyUnknown, on which AssignKindsIncompatible already stands down, so a shape this cannot type falls back to the old accept.

Trap 1 — defs.inc:422 was the false source, and it was the tempting one

{ Left = base sym idx } would have made a symbol walk nearly free. Swept all 196 builders of the three node kinds in compiler/*.inc — AN_INDEX 52, AN_FIELD 88, AN_DEREF 56 — and every one assigns a NODE: node, idn, tmpN, CloneAST(target), RMakeIdent(sym), PyMakeIdent(sym), GenMakeIdent(sym, tk). Not one assigns a bare symbol index; the *MakeIdent helpers exist precisely to wrap one in a node. ir.inc:1544 and ResolveNodeRec are right. frankwasm corroborated it from the other end (ASTStrElemTkOf dispatches on ASTKind[ASTLeft[node]], nonsense on a symbol index). Comment corrected in the same commit, logged to LOGBOOK.md.

Trap 2 — the false reject frankB predicted, found by measuring not reasoning

An interface is spelled tyRecord (a 16-byte fat pointer {IMT, instance}), so (dstTk = tyRecord) <> (srcTk = tyRecord) would refuse ptr := o.I, which FPC accepts. PXXDBG=a.ast on the probe prints the AN_FIELD for rr.I as kind=11 tk=5 against a tk=17 destination — the record-XOR pair, firing. The AN_IDENT arm already carried this bail for the same measured reason (one regression in a 625-pair FPC differential). Without it the fix would have converted a false accept into a false reject on every interface field and interface array element.

Result — 12 rows, and the oracle agrees line for line

test/test_assign_lvalue_shapes_fail.pas: all six shapes, plus the mirror direction and the scalar/handle confusions on the same shapes, plus rs[0].N := s (a field OF an element — two node kinds at once). pxx rejects 12; fpc 3.2.2 rejects the same 12 lines in the same directions. The count is the Makefile assertion, so recovery is proven too. Form 1 is in the file, as the ticket required.

test/test_assign_lvalue_shapes_ok.pas is the half that matters more: it compiles and runs, and its stdout matches fpc's byte for byte. Interfaces, class-typed fields, sets in records, array-typed fields, ShortString/AnsiString interchange, Char-into-string in every shape.

Wider evidence: the self-host build is ~40k lines of dense Pascal and converged; the three widestring tests frankwasm named still match .expected; the pre-existing assign fail/ok pair is unchanged (13 rows, same output); four Rust and two Zig samples, plus examples/chess and examples/raytracer, compile clean. What that swept and what it did not: Rust and Zig are under this guard (C and NilPy are excluded by the guard's own not CProgramMode and not PyProgramMode), and six samples is a sample, not the population — the matrix is Track T's.

NOT fixed, deliberately

var / out parameters. frankB was right that it is a separate question: a by-ref slot holds an address, and AssignSideKind's IsRef bail is what stands down there. It did not fall out of this fix and no gate here claims it.

Log