TComponent / TPersistent (FPC Classes owner-child surface)

• Type: feature (library — RTL Classes parity, Track B)
• Status: DONE 2026-07-04
• Owner: Track B

What / why

FPC/FCL code (custapp, eventlog, and everything descending from TComponent) needs TComponent/TPersistent in Classes. pxx had a reduced TComponent only in classes_lite (the PCL streaming base), not in classes where FPC's uses Classes resolves. Added the FPC-facing surface to lib/rtl/classes.pas.

Done (lib/rtl/classes.pas)

• TPersistent = class(TObject): Assign/AssignTo (virtual; base Assign(Source) calls Source.AssignTo(Self)), GetNamePath.
• TComponent = class(TPersistent) owner-child model: virtual Create(AOwner) (registers with the owner), Destroy (frees owned components), InsertComponent/RemoveComponent, FindComponent (case- insensitive), Notification (virtual hook), + Owner, Components[], ComponentCount, Name, Tag (NativeInt). TComponentClass = class of TComponent, TOperation.

Verified test/test_tcomponent.pas (ownership registration, indexed access, FindComponent, owner-frees-children destructor counting, RemoveComponent detach) = 9/9, on BOTH the fresh and the pinned stable compiler; wired into make lib-test. Re-probing FPC custapp/eventlog now gets past the base type not found: TComponent wall (advances to deeper, separate gaps).

Pin-compatibility note

Initially TComponent.Destroy was virtual-introduce (no inherited) to build on the v173 pin. After re-pinning to v174 (which has this session's implicit-TObject.Destroy override + inherited Destroy no-op), it was made idiomatic: destructor Destroy; override; + inherited Destroy. Verified 9/9 on the v174 pin.

Landmines hit (filed)

• [[bug-forward-class-decl-with-later-base-loses-fields]] — the TComponent = class; … TComponent = class(TPersistent) idiom lost fields; worked around by dropping the forward decl.
• [[bug-downcast-inherited-property-wrong-offset]] — TThing(ref).Tag (downcast to an inherited property) read garbage; test reads via a base ref.
• arr[i].Free does not parse (known); temp-var workaround in Destroy.
• Bare TFoo.Create(owner); constructor-call-as-statement is rejected; assign the result to a temp.

Follow-up

Grow toward more of FPC's Classes as consumers need (TComponent streaming convergence with classes_lite, TCollection, TComponent.Notification wiring on free). Separate tickets.