sx/current/CHECKPOINT-REIFY.md

# CHECKPOINT-REIFY — comptime `type_info` / `reify` (async-first foundation, step 3)

Companion to [PLAN-REIFY.md](PLAN-REIFY.md). Update after every step (one step at a
time, per the cadence rule).

## Last completed step
**Phase 0.2 (green) — Phase 0 COMPLETE.** Implemented `reify(.enum(...))`:
`Lowering.reifyType` (in `lower/nominal.zig`) reads the flat-enum `TypeInfo`
literal off the AST, synthesizes an `ast.EnumDecl`, and feeds it through the
SAME `type_bridge.buildEnumInfo` path source enums use → the minted type is
byte-identical to a hand-written `enum { value: i64; closed; }` and flows
through enum codegen unmodified (**Contract 2 confirmed** — a source enum
exhibits the exact same construct/match behavior, verified by probe). Wired at
the `E :: reify(...)` const-decl hook in `lower/decl.zig`. `examples/0614`
green (`value 3` / `closed`, exit 0); snapshots captured; full suite green
(670 examples, 447 unit). Unsupported reify shapes bail loudly via
`reifyBail` (never a silent default).

Also landed (user-directed, separate commit `feat(parser):`): reserved keyword
as a member name after `.` (`.enum`, `case .enum:`, `x.enum`) — so the reify
example reads `reify(.enum(...))` without a backtick. readme updated.

### Two earlier Phase-0 commits
- **0.1 (xfail).** Added `examples/0614` + empty `.exit` marker → RED.
- **0.0 (lock).** Added the comptime type-metaprogramming surface as the
on-demand module `library/modules/std/meta.sx` (NOT the prelude — see decision
below): `EnumVariant`/`EnumInfo`/`TypeInfo` data types + bodyless `#builtin`
decls `reify` / `type_info` / `field_type`. Each builtin bails LOUDLY when
reached unimplemented (no silent default). Unit test `src/parser.test.zig`
(registered in `src/root.zig`) locks that the decls parse. `zig build test`
green (447/447 unit, 669/669 examples).

## Current state
- `modules/std/meta.sx` declares the surface; the variant uses the backtick raw
  escape `` `enum `` (reads as the keyword, not a mangled `enum_`).
- **Loud bails wired (unimplemented → diagnostic, never a silent type):**
  - `reify(...)` in a `::` type-alias position → `decl.zig` (the `.call`
    const-decl branch) emits "reify is not yet implemented (REIFY Phase 0.2)"
    and poisons the alias to `.unresolved`. This is also where Phase 0.2 will
    hook the real construction.
  - `reify` / `field_type` in any other type position →
    `generic.zig:resolveTypeCallWithBindings` (defense-in-depth).
  - `type_info(...)` in expression position →
    `call.zig:tryLowerReflectionCall`.
- No interpreter-side construction yet — `reify` mints nothing.

## Decision (0.0)
**Meta lives in `modules/std/meta.sx`, not the prelude (`core.sx`).** Declaring
the data types in the always-loaded prelude interns them into every module's
type table and shifts every `.ir` snapshot (broke 37 examples in a trial). An
on-demand import keeps the prelude clean; reify users `#import
"modules/std/meta.sx"`. (User-directed.)

## Next step
**Phase 1 (type-fn → reify identity).** `R :: ($T) -> Type { reify(...) }`; assert
`R(i64)` from two sites is ONE type (assignable / matchable across sites). xfail →
green by registering a reify-returning type-fn's result under the instantiation
mangled name (mirror `generic.zig:1663-1689`). NOTE: Phase 0's `reifyType` is hooked
only at the `E :: reify(...)` const-decl site (`decl.zig`) and reads a LITERAL
`TypeInfo` off the AST; Phase 1 must route a reify call returned from a type-fn body
(and likely generalize the literal-AST reader, or evaluate via the interpreter, for
non-literal `TypeInfo`).

SELF-REFERENCE = Phase 4, API DECIDED (user-directed): explicit **`reserve()` →
`complete()`** (NOT a `reify_rec((self)=>…)` closure). `reserve()` returns a forward
nominal `Type` handle (named from the `::` LHS) usable freely in any later `TypeInfo`
— `*List`, `[]List`, and across types for MUTUAL recursion the one-`self` closure
couldn't express; `complete(handle, info)` fills the body. `reify(info)` stays as the
one-shot sugar. Maps onto existing machinery: reserve = empty nominal slot
(`reserveShadowEnumSlot`-style), complete = `buildEnumInfo` + `updatePreservingKey`,
struct-stub→tagged_union re-key already handled (`internNamedTypeDecl`'s
`adoptsForwardStructStub`). Loud invariants: never-completed reserve = hard error;
by-value self-inclusion rejected (infinite size). Full write-up in PLAN-REIFY Phase 4.

## Known issues
None yet.

## Log
- **0.2 (green) — Phase 0 done.** `reifyType` mints a flat enum from a literal
  `TypeInfo` via the shared `buildEnumInfo` path; `0614` green; Contract 2 confirmed
  (reify'd enum == source enum, same construct/match). Payload-less variant idiom in
  the example is `c : E = .closed;` (enum-literal target), same as a source enum.
- **parser (user-directed).** Keyword as member name after `.` — see `feat(parser)`.
- **0.1 (xfail).** `0614` + empty `.exit` marker → RED.
- **0.0 (lock).** Meta surface in `modules/std/meta.sx` (data types + 3 bodyless
  `#builtin` decls), loud bails at all three reach points, `src/parser.test.zig`
  parse-lock. Two user-directed refinements folded in: variant uses `` `enum ``
  raw escape; surface moved out of the prelude into its own module to avoid
  type-table / `.ir`-snapshot churn. `zig build test` green.
- **Stream carved.** Selected as the first async-first foundation: `reify` gates both
  channel result types (`RecvResult($T)`) and `race`'s synthesized union, is fully
  validated (3 reviewers), and is a self-contained compiler/type-system feature
  testable in isolation (`06xx` comptime). Generic-enum syntax dropped in its favor.