Interfaces

• Type: feature
• Status: done (declare/implement/assign/call, is/as/Supports, implicit coercion, identity, nil, inheritance, all 4 targets). The only remaining work — automatic interface refcounting (ARC) — is split to feature-interface-refcounting and deferred (low priority, not needed). See Log.
• Owner: —
• Opened: 2026-06-06 (from todo.md §3 — intentionally deferred)

Priority note (2026-06-16, refined 2026-06-18)

Low priority, but the OOP-family capstone — not "never". Interfaces are largely a COM-era binary-contract mechanism (cross-binary ABI stability: Windows COM, OLE). PXX targets standalone, all-in-one-binary applications, so that primary value does not apply, and the secondary value (inheritance-free polymorphic abstraction + ARC lifetime) is covered by classes, abstract methods, and object (feature-object-reference-type). So it stays low — but it is the last gap in the OOP feature set, and deferring it forever means repeatedly bumping into it. Do it eventually, after the other OOP pieces; pull forward only if a concrete interop need appears.

Effort is MEDIUM, not the scary arc the original note implied — the expensive substrate already exists: VMT + virtual dispatch, the RTTI class registry, ARC refcounts (managed-string IncRef/DecRef), and scope-exit + exception-unwind finalisation. CORBA-style (no refcount, fat-pointer {IMT, instance}, is/as/ Supports via the RTTI walk) is ~2–4 focused days; COM-style ARC is an incremental add reusing the existing ARC + scope-cleanup machinery.

Item 5 is extracted. Class is/as/Supports (the runtime type-walk) is now its own ticket feature-class-is-as — useful independently (class downcasts, the demos) and a prerequisite this ticket reuses.

Motivation (original)

A real language gap, but not active: no current target source requires interfaces, and even a lightweight Linux-native model adds substantial dispatch, ABI, and lifetime surface. Ordered after the LFM arc (reuses its class registry / RTTI) if ever taken up.

Decisions to lock first

• Refcount model: start CORBA-style / no-refcount (pure dispatch); defer COM-style ARC (_AddRef/_Release + try/finally injection) — the hard part.
• Root type: whether to require IInterface/IUnknown; corba can omit.
• GUIDs: parse ['{...}'], ignore initially.

Mechanism (the work)

1. Parse IFoo = interface [guid] <signatures> end;.
2. Class implements: bind each interface method to a class method by name+sig.
3. Per-(class,interface) Interface Method Table (IMT), distinct from VMT. Choose representation: Delphi-style hidden field + Self-adjusting thunks, or fat pointer {IMT, instance}. Document the choice.
4. Interface-typed variable storage + class→interface assignment + call slot[k].
5. is/as/Supports via the class registry.
6. Operators: assign, identity =/<>, params, results.

Synergy: Self-adjusting IMT thunks suit the new inline asm or a small IR op. Out of scope: implements delegation, interface inheritance depth, method resolution clauses, COM ARC.

Acceptance

IFoo/class-implements/interface-call/is/as/Supports covered by tests; self-host fixedpoint holds.

Log

• 2026-06-06 — ticket opened from todo.md §3.
• 2026-06-18 — CORBA vertical slice landed (commit 3a7b0c5). Working end to end on all 4 Linux targets, byte-identical to FPC {$interfaces corba}: declare IFoo = interface ... end, class implements via the parent list, interface var (fat pointer {IMT, instance}, 2*TARGET_PTR_SIZE), class→interface assignment, and interface method dispatch (0-arg + args, procedure-as-statement, Self field read/mutation, same-instance identity). Per-(class,interface) IMT emitted in Data[] and wired via MethodFixups like the VMT; call dispatches through the existing IR_CALL_IND. New: UClsIsInterface, IMT table + FindIMT, IR_IMTADDR, AN_INTF_CALL. make test + cross-bootstrap byte-identical. Mechanism decisions locked: CORBA/no-refcount; fat pointer (not Delphi thunks); GUIDs parsed+ignored; IMT slot = interface method declaration order. Still open (decide/build next): is/as/Supports on interfaces (reuse the feature-class-is-as VMT walk — but interface identity is the IMT, needs a per-instance interface-table lookup or a Supports helper); interface-typed params/results across function calls; interface inheritance; assigning a base-class-typed value (dynamic dispatch through interface); COM ARC; operator =/<> identity. The slice captures the IMT from the RHS static class, so polymorphic class→interface through a base-typed var uses the base's IMT (acceptable v1; revisit with dynamic IMT lookup).
• 2026-06-18 — obj is IFoo landed (commit b8f9289): real implementation query via the closed-world VMT-set (interface target → classes with a matching IMT). All 4 targets; cross-bootstrap byte-identical. Verified already-working: interface params + results (64-bit targets), method dispatch. Narrowed the remaining open items: as IFoo/Supports (checked cast to an interface value), implicit class→interface coercion at call sites, 16-byte interface params on i386 (32-bit aggregate-param ABI), interface inheritance, COM ARC.
• 2026-06-18 — Supports(obj, IFoo) landed (commit 59d5ccc): function form of the interface is-query (same AN_IS_TEST). is + Supports now both work for interfaces. Still open: as IFoo (checked cast to an interface VALUE), implicit class→interface call-arg coercion, i386 16-byte interface params, interface inheritance, COM ARC, identity =/<>.
• 2026-06-19 — all non-COM follow-ups landed (commits 711dd11, 29c9565, 266a750). Done, all four targets byte-identical self-host + cross-bootstrap:
  • obj as IFoo — checked cast to an interface value (IRMaterializeIntfCast: closed-world DYNAMIC IMT lookup, so a base-typed source still picks the derived class's IMT; nil -> null fat pointer; bad cast traps). Wired into IRLowerAST + IRLowerAddress; the parenthesised-postfix . handler now routes (expr).M(args) to AN_INTF_CALL.
  • implicit class→interface coercion at call sites and into a Result (MatchProcCall phase 2c gated on the param being an interface; IRLowerCallArg synthesises the as cast). Param rec ids now persist in ProcParamRecId (pi*16+j) — param sym slots are reused across procs, so Syms[SymIdx].RecName is unreliable at a call site (root cause of the first failed attempt).
  • identity =/<> compares the fat pointer INSTANCE word (not the shared IMT word); iface = nil works; iface := nil zeroes the whole fat pointer.
  • 32-bit fat-pointer param ABI (the mislabelled "i386 16-byte" item — actually an 8-byte fat pointer on 32-bit): interface params are forced by-ref on every target (no-op on 64-bit, where 2*PTR=16 was already >8).
  • interface inheritance IBar = interface(IFoo): parent methods inherited at leading IMT slots; a class implementing IBar also gets ancestor IMTs so is/as/Supports + class→base-interface assignment work; interface widening (foo := bar) and passing a derived interface to a base-interface param both work because inherited methods lead the IMT (base layout is a prefix). Tests: test_interfaces_as / _param / _inherit. Remaining open = COM ARC only (refcounting), plus the out-of-scope implements delegation + method-resolution clauses. The non-COM CORBA surface is complete.

2026-07-14 — CLOSED. The last open item shipped.

The only thing keeping this in done-followup/ was deferred COM/ARC refcounting. That landed: feature-interface-refcounting is in done/, and both test/test_interface_arc.pas and test/test_interface_arc_exc.pas run green at HEAD (create=2 / freed=2, and freed=3 on the exception-unwind path). b337 then settled the ABI on top of it (an interface VALUE is ONE pointer, FPC-compatible).

Nothing remains under this slug.

Spun out — a real gap found while verifying this

class(TInterfacedObject, IFoo) where TInterfacedObject / IInterface are not declared by the user compiles silently and segfaults at runtime. There is no RTL-provided TInterfacedObject; every interface test in tree declares its own, which is exactly why nobody noticed. That is a missing diagnostic AND an RTL gap, and it is filed separately as [[bug-pascal-tinterfacedobject-missing-silent-segfault]] rather than left buried here.

• 2026-07-14 — resolved, commit 20e1232c.