A class does not inherit from TObject at run time

• Track A (compiler/rtti_emit.inc — the class RTTI blob), surfaced through the Track P Pascal surface (is, InheritsFrom, except on).
• Found 2026-08-20 by an FPC differential probe over the TObject API.

The measurement

fpc -O- -Mobjfpc 3.2.2 vs pxx at 9c309d057. Every row is a silent wrong value — nothing warned, nothing crashed at the point of the defect.

That last row is what hid this for so long. Every link a program writes down worked; only the link nobody writes — the implicit root — was missing. So a class hierarchy behaved correctly right up to the moment something reached the top of it.

And the expensive symptom, which is not a wrong boolean but a lost program:

try
  raise Exception.Create('z');
except
  on E: TObject do writeln('caught ', E.ClassName);
end;

FPC catches. pxx printed Unhandled exception: Exception: z and aborted — the catch-all handler was skipped, because handler matching is the same __pxxInheritsFrom walk. A program whose top-level guard is on E: TObject had no guard at all.

Root cause

RegisterBuiltinTObject (parser.inc:36075) deliberately keeps TObject an implicit parent: a class declared class ... end, and equally one declared class(TObject) end, is registered with parentCi = -1. That is correct and must stay — it is what stops a VMT from relocating for a parent that contributes no fields and no virtual methods.

But rtti_emit.inc wrote the blob's parent word only when UClsParent[ci] >= 0:

if (UClsParent[ci] >= 0) and (UClsRTTIOff[UClsParent[ci]] >= 0) then
  AddDataPtrFix(hdr + 8, UClsRTTIOff[UClsParent[ci]]);

so a root class's PXX_RTTI_PARENT stayed nil, and the runtime walk in __pxxInheritsFrom (builtin.pas) terminated one link before TObject.

The two facts were each locally right and jointly wrong: layout parentage (parentCi) and identity parentage (the RTTI chain) are different questions, and one field was answering both. The fix separates them — the blob links to TObject's RTTI even when the class has no layout parent:

else if (UClsParent[ci] < 0) and (not UClsIsInterface[ci]) then
begin
  tobjCi := FindUClass('TObject');
  if (tobjCi >= 0) and (tobjCi <> ci) and (UClsRTTIOff[tobjCi] >= 0) then
    AddDataPtrFix(hdr + 8, UClsRTTIOff[tobjCi]);
end;

The tobjCi <> ci guard keeps TObject itself from pointing at itself (an infinite walk), and the interface guard keeps an interface out of the class chain — an interface is not a TObject and IFoo.InheritsFrom(TObject) must stay false. Emission is already two-pass (all blobs allocated, then headers filled), so TObject's offset is known when any class's header is written, whatever the declaration order.

Test

test/test_class_inherits_from_tobject.pas, 24 rows, all FPC-verified. It covers the implicit root, the explicit class(TObject) spelling, a two-deep chain, the reflexive TObject.InheritsFrom(TObject), the negatives that must stay false (TRoot.InheritsFrom(TDer), r is TDer), a TObject-typed variable reporting its real class, on E: TObject catching, a specific handler still winning over it, and an interface implementor. Pinned scores two is TObject failures and then aborts on the unhandled exception; the fixed compiler matches FPC 24/24.

Still open — the TObject API surface

The same probe found six TObject members pxx REJECTS outright and FPC accepts: ClassParent, InstanceSize, ClassInfo, ToString, Equals, GetHashCode. Those are loud (a compile error, not a wrong value), so they are filed separately: feature-p-tobject-api-classparent-instancesize-tostring.

Gate

make compiler/pascal26 fixedpoint converged after 2 rounds; tools/gate.sh quick GREEN.