Record why open universes and value/stringification collisions are
rejected, the `Member<T, K>` inversion, and the findings from the
rejected open-universe attempt so they are not re-run.
A `boolean` / `null` / `undefined` discriminant cannot key a handler
map, so the tagged-union matcher now routes it through the
`PatternKey` / `PatternParam` projection the primitive-union matcher
already used. Both matchers share the projection from
`matcher-shared.ts`; `MapTaggedUnion`, `Handlers`, `HandledMembers`
and `MustBePartial` key off the projected form and invert it to
recover the real member.
Covers exhaustive and fallback dispatch, the redundant-fallback
guard, and the LSP popup offering the tags by name.
`getMatcher` / `getMatcherW` become `getPrimitiveUnionMatcher` /
`getPrimitiveUnionMatcherW`; `src/primitive.ts` and its test move to
`src/primitive-union.*`. The `Union` suffix mirrors `getTaggedUnionMatcher`.
String(shape) defeats V8's numeric-key fast path (measured ~2x on
number-keyed dispatch). JS already coerces boolean/null/undefined to the
same property key, so assert shape to string | number and index directly.
The assertion is TS2538-only; the facade keeps it sound. Needs a source
no-unsafe-type-assertion disable, next to the existing one.
State the end-user limits once, in README § Caveats: a member colliding
with its stringification, the unsupported symbol/bigint, and NaN/-0. Drop
the duplicated known-issue prose from development/library.md and the
source comment; link to the README instead.
Extend the matcher universe beyond string | number so a union can carry
boolean, null and undefined members. true|false, null and undefined are
not property keys, so handler keys are their stringification while the
callback still receives the real member. Symbols are rejected (a brand
is compile-time only) and bigint is not a property key.
A fallback paired with a handler map that already covers `T` was still
accepted, even though its remainder is empty. Fold the guard into
`Handled`'s own (F-bounded) constraint so it is checked after inference;
a conditional in the fallback parameter is evaluated while `Handled` is
still its constraint and rejects every partial map whose handler callbacks
need contextual typing.
Rewrite development/library.md around the new shape: the fallback is a second
argument (so it can receive the remainder), `R` needs an inference site in the
handler map, `Exact` restores the excess-property check, and the overload order
is load-bearing. Record this branch's rejected alternatives (single-object `_`,
currying, `this`/HKT, variance/`const`/`NoInfer`/brands) and the redundant-
fallback known issue. Add the changelog note and check off the backlog task.
Drop the strict/widened aliases from src/index.ts so the factories are
exported under their own names, and align the type tests, the autocomplete
probe (which now imports the public surface from ./index.ts) and library.md
with them.
Also correct the widened-overload comment: the excess-property check does not
apply to a generic P, so MatcherWidening's keyof guard — not the closed
constraint — is what rejects keys outside T/_, as library.md already
documented.
The matcher is now two three-overload factories; library.md documents why
the union merge, inferred universe, conditional RequireKeys and cases-first
paths were rejected, and lists the two open issues (the fallback sees all of
T; a redundant _ is still accepted). testing.md records the language server
as the autocomplete oracle, and CONTRIBUTING points the exception at the
matcher's own test file.
The backlog marks the design-doc and autocomplete groundwork done alongside
the adoption.
Same decisions, rationale, rejected alternatives and known issues, said with
less padding: ~5,530 -> ~4,730 words (-15%). Every fact from the first draft is
kept; only the wording, duplicated lead-ins and restated context are cut.
Category files under development/ replace the decision prose that was
scattered through README.md and CONTRIBUTING.md. Each decision is a block with
Decision (YYYY-MM) / Why / Rejected / Known issue; the new library.md records
the public API contract and its limitations. AGENTS.md and backlog.tasks now
point at the new home.
tmu