is / as / Supports — runtime class type-tests

• Type: feature
• Status: done
• Owner: —
• Opened: 2026-06-18 (extracted from feature-interfaces item 5 — useful on its own)
• Resolved: 2026-06-18 (commit e4a4833) — is + as for classes, closed-world VMT-set test, FPC byte-identical on all 4 Linux targets, cross-bootstrap clean. Supports (interface query) deferred into feature-interfaces.

Resolution notes

• Mechanism shipped is closed-world rather than a runtime parent-walk: obj is T ⟺ obj's VMT address ∈ {VMT(T)} ∪ {VMT(descendants)}, the set enumerated at codegen from the compile-time parent chain (whole-program compile, no separate linking). Simpler than a runtime walk and needs no RTTI (which only exists for published classes). New IR primitive IR_VMTADDR; shared IR lowering.
• Required making every class VMT reserve ≥1 slot — classes with no virtual methods previously got 0-byte VMTs that aliased to one address, so any is matched every class.
• Supports not done (interface-only); the class is/as walk it would reuse is now in place. The general unchecked TClass(obj) reinterpret lives in [[feature-general-typename-cast]]; keep checked (as) vs unchecked distinct.
• Pre-existing gap surfaced: (expr).field := x parenthesised-LHS assignment doesn't store (not is/as-specific); logged in devdocs/not-implemented.md.

Motivation

obj is TFoo and obj as TFoo are not implemented even for plain classes today (if a is TB → Expected: then, but got: is). They are the natural downcast/type-test surface for any OO code (and the demos). Extracted from feature-interfaces (which needs the same runtime type-walk for its Supports) because it is independently valuable and much smaller than the full interface arc — and it pre-builds that arc's item 5.

What exists to build on

• RTTI class registry (name / parent / instSize / vmt) is already emitted per class — the parent chain needed for the type-walk is present.
• VMT layout + IR_VIRTUAL_CALL dispatch are done.

The only genuinely new piece is the runtime type-walk: given an instance's class RTTI pointer, walk the parent chain looking for the target class RTTI.

Mechanism

1. Distinct AST nodes. X is T → AN_IS_TEST, X as T → AN_AS_CAST. Resolve in the parser by context+type and tag the node — do not fold into another operator or key behaviour off a global flag (see the and/or history: bitwise-vs-logical broke when keyed off a global; the fix was to decide per-node in the parser. as is currently a contextual ident, not a reserved token — keep the cast-expression use cleanly separate from specialize ... as Name).
2. is → load the instance's class-RTTI pointer, walk parent links; result Boolean (True if target found, False on nil or no match).
3. as → same walk; on match yield the (same-pointer) instance typed as T; on mismatch raise an invalid-cast error (or, until the exception class exists, a clear runtime trap). nil as T = nil.
4. Supports (function form) — same walk, Boolean result; shares the helper.

Emit the walk as a small runtime helper (one per program, like the other managed helpers), called from AN_IS_TEST / AN_AS_CAST codegen, so all targets share one implementation.

Scope boundaries

• Classes only here. Interface is/as/Supports (querying an interface) is feature-interfaces, which reuses this walk.
• No metaclass (TClass) is/as beyond what the registry already gives.

Acceptance

• obj is TDerived / obj is TBase / nil is T give correct Booleans across a small inheritance tree.
• obj as TDerived succeeds on a real derived instance, traps on a bad cast, passes nil through.
• Covered by an oracle test on all four Linux targets; make test green; cross-bootstrap byte-identical.