Forward class decl + full decl that adds a base loses the class's fields

• Type: bug (compiler — parser / class registration, Track A)
• Status: done
• Opened: 2026-07-04 (found writing TComponent — the TComponent = class; + TComponent = class(TPersistent) pattern)

Symptom

A forward class declaration TFoo = class; followed by a full declaration that adds a base class makes the full class's fields invisible:

type
  TBase = class end;
  TFoo = class;                { forward — no base }
  TFooClass = class of TFoo;
  TFoo = class(TBase)          { full — adds a base }
  private
    FVal: Integer;
  public
    constructor Create;
  end;
constructor TFoo.Create;
begin
  FVal := 7;                   { pascal26: error: undefined variable (FVal) }
end;

Without the forward decl (declare TFoo = class(TBase) directly), it works. A forward decl whose full form has NO base also works — the trigger is the forward stub (rootless) + a full decl that introduces inheritance.

Impact

Blocks the standard TFoo = class; TFooClass = class of TFoo; TFoo = class(TBase) metaclass-before-decl idiom for any inherited class. Worked around in lib/rtl/classes.pas by dropping the forward decl and moving TComponentClass = class of TComponent AFTER the full declaration.

Direction

The full declaration must attach the base + fields to the SAME class entry the forward stub created; today the fields appear to land on a different entry (or the stub's rootless layout wins). Fix the merge of forward stub → full decl when a base is introduced.

Acceptance

Repro compiles + runs (FVal=7); regression .pas; self-host byte-identical.

Resolution (2026-07-04)

FIXED. Root cause exactly as filed: the full decl called AddUClass again, minting a shadowed duplicate — FindUClass returns the FIRST match, so lookups (incl. the class-of alias) stayed on the empty rootless stub while base+fields went to the invisible second entry. Fix: new UClsForward flag (defs.inc, cleared in AddUClass); bare class; without heritage marks the stub; the full decl reuses that entry, re-anchoring UClsFBase/MBase/PBase to the current tails (stub counts are 0) and resetting VirtCount before applying the parent. class(TBase); remains the FPC empty-body shorthand. Regression test/test_forward_class_base.pas (6/6: fields, inherited field, is-base, metaclass alias, mutual refs, virtual override through a forward) in test-core. Self-host byte-identical, make test green. lib/rtl/classes.pas workaround (forward dropped, class-of moved after) can revert to the idiom after the next re-pin.