TInterfacedObject does not exist — inheriting from it compiles, then segfaults under ARC

• Type: bug (missing diagnostic + RTL gap — silent runtime crash)
• Track: A — core (unknown base class silently accepted) + B (the RTL type)
• Status: done closure.

Reproduction

program tio2;
{$mode objfpc}{$interfaces com}
type
  IFoo = interface
    procedure Go;
  end;
  TFoo = class(TInterfacedObject, IFoo)   { TInterfacedObject is declared NOWHERE }
    procedure Go;
  end;
procedure TFoo.Go; begin writeln('go'); end;
procedure Use;
var f: IFoo;
begin
  f := TFoo.Create;
  f.Go;
end;
begin
  Use;                       { SIGSEGV on scope exit }
  writeln('survived scope exit');
end.

Compiles clean. f.Go even prints. It dies at scope exit: exit 139.

What is actually wrong — two things

1. There is no TInterfacedObject in the RTL. It is the single most-written base class in FPC/Delphi interface code (it supplies IInterface + _AddRef / _Release / QueryInterface). Every interface test in tree declares its OWN, which is exactly why this was never noticed.
2. An unknown base class name is silently accepted. class(TInterfacedObject, IFoo) with no such type in scope should be a compile error naming it. Instead the heritage resolves to nothing and the class is built with no parent.

Under CORBA interfaces (the default, no refcounting) the damage stays hidden: nothing calls the missing methods, so the program runs. Under {$interfaces com} the ARC path emits _AddRef/_Release calls that dispatch through a parent that does not exist — hence the crash at scope exit, far from the declaration.

Why it matters

This is the shape that hurts: it compiles, it runs, it prints the right thing, and it dies later on a path the author never wrote. Real FPC code opens with class(TInterfacedObject, IFoo) as a matter of course, so a user's first interface program hits it.

Wanted

• Diagnostic: an unknown base-class name in a heritage list is an error, not a silent no-parent class. (Check the interface-name path too — class(TFoo, INoSuchThing).)
• RTL: provide IInterface and TInterfacedObject (refcounted, _AddRef/_Release/ QueryInterface) so the idiom works rather than merely failing loudly.

Gate

make test + self-host byte-identical; the repro above either compiles-and-runs (once the RTL type exists) or fails at COMPILE time — never segfaults.

Log

• 2026-07-14 — resolved, commit 068ef1e1.