fix: align RpcPromise construction with result elision

.changeset/rpc-promise-ctor-elision.md

@@ -0,0 +1,5 @@
+---
+"capnweb": patch
+---
+
+The `RpcPromise` constructor now applies the same stub elision as method result types: wrapping a `Promise<RpcStub<T>>` produces the same `RpcPromise<T>` a method declared to return that stub would, plain-interface stub payloads keep their stub type, and promises resolving to inline object literals with methods now infer correctly.

__type-tests__/rpc-base-cases.test.ts

@@ -161,6 +161,26 @@ expectType<RpcPromise<PointTarget>>(new RpcPromise<PointTarget>(Promise.reject(n
 const promisedFromStub = new RpcPromise(Promise.resolve(pointStub))
 type _PromisedFromStubInfersTarget = Expect<Equal<typeof promisedFromStub, RpcPromise<PointTarget>>>
 
+// An inline object literal with a method is context-sensitive (the method's return type must be
+// inferred), which is only compatible with the constructor's plain-promise overload; it must
+// infer without an explicit type argument. Its methods' return types stay un-widened (`1`, not
+// `number`), hence assignable-to rather than exactly-equal-to the widened shape.
+const promisedFromInline = new RpcPromise(Promise.resolve({ value: 1, next() { return 1 } }))
+expectAssignable<RpcPromise<{ value: number, next(): number }>>(promisedFromInline)
+
+// The same shape predeclared widens normally and infers exactly.
+const predeclaredShape = { value: 1, next() { return 1 } }
+expectType<RpcPromise<{ value: number, next(): number }>>(
+    new RpcPromise(Promise.resolve(predeclaredShape)))
+
+// An explicit type argument combines with a stub payload (the constructor's fallback overload).
+expectType<RpcPromise<PointTarget>>(new RpcPromise<PointTarget>(Promise.resolve(pointStub)))
+
+// A promise for a union of the target and its stub still infers the target type.
+declare const targetOrStubPromise: Promise<PointTarget | RpcStub<PointTarget>>
+const promisedFromUnion = new RpcPromise(targetOrStubPromise)
+type _PromisedFromUnionInfersTarget = Expect<Equal<typeof promisedFromUnion, RpcPromise<PointTarget>>>
+
 async function assertAwaitedConstructedPromiseShapes() {
   const target = await new RpcPromise(Promise.resolve(new PointTarget()))
   expectType<RpcStub<PointTarget>>(target)

__type-tests__/stub-elision.test.ts

@@ -1,7 +1,7 @@
 // Declared stub returns/properties (`Promise<RpcStub<T>>`, `RpcStub<T>`) must produce the same
-// `RpcPromise<T>` as returning the payload directly (`Promise<T>`). Plain-interface stubs are
-// the exception: they are NOT elided, because `RpcPromise<U>` only awaits back to a stub when
-// `U` is Stubable.
+// `RpcPromise<T>` as returning the payload directly (`Promise<T>`), matching what
+// `new RpcPromise(Promise.resolve(stub))` produces. Plain-interface stubs are the exception:
+// they are NOT elided, because `RpcPromise<U>` only awaits back to a stub when `U` is Stubable.
 import { RpcPromise, RpcStub, RpcTarget } from "../src/index.js"
 import type { Stubable } from "../src/types.js"
 import { expectAssignable, expectType, type Equal, type Expect } from "./helpers.js"
@@ -76,8 +76,29 @@ type _CallableStubElides = Expect<Equal<typeof fnViaStub, typeof fnViaTarget>>
 type _AwaitedFnStub = Expect<Equal<Awaited<typeof fnViaStub>, RpcStub<Formatter>>>
 expectAssignable<Promise<string>>(fnViaStub(4))
 
-// 7. Union payloads distribute: `Promise<RpcStub<T> | null>` returns elide the stub arm.
+// 7. Constructor/method equivalence: wrapping a promised stub yourself produces exactly the
+// same type as a method declared to return the stub, for every payload shape.
+declare const counterStub: RpcStub<Counter>
+const constructed = new RpcPromise(Promise.resolve(counterStub))
+type _ConstructorMatchesMethodReturn = Expect<Equal<typeof constructed, typeof viaStub>>
+
+// 7b. Callable stubs elide in the constructor too. (An explicit type argument with a stub
+// payload is covered in rpc-base-cases.test.ts.)
+declare const formatterStub: RpcStub<Formatter>
+const constructedFn = new RpcPromise(Promise.resolve(formatterStub))
+type _CallableCtorMatchesMethodReturn = Expect<Equal<typeof constructedFn, typeof fnViaStub>>
+
+// 7c. Plain-interface stubs are not elided in either form, and the two forms agree.
+declare const plainStub: RpcStub<PlainApi>
+const constructedPlain = new RpcPromise(Promise.resolve(plainStub))
+const plainViaMethod = api.getApi()
+type _PlainCtorMatchesMethodReturn = Expect<Equal<typeof constructedPlain, typeof plainViaMethod>>
+
+// 7d. Union payloads distribute identically in both forms.
+declare const maybePromise: Promise<RpcStub<Counter> | null>
+const constructedMaybe = new RpcPromise(maybePromise)
 const maybeViaMethod = api.maybeStub()
+type _UnionCtorMatchesMethodReturn = Expect<Equal<typeof constructedMaybe, typeof maybeViaMethod>>
 api.consumeMaybe(maybeViaMethod)
 
 // 8. map() over a declared `RpcStub<T>[]` return: the callback placeholder is `T`-shaped,

src/index.ts

@@ -6,8 +6,8 @@ import { RpcTarget as RpcTargetImpl, RpcStub as RpcStubImpl, RpcPromise as RpcPr
 import { serialize, deserialize, EncodingLevel } from "./serialize.js";
 import { RpcTransport, RpcTransportWithCustomEncoding, AnyRpcTransport, RpcSession as RpcSessionImpl, RpcSessionOptions } from "./rpc.js";
 import { RpcLimits, DEFAULT_LIMITS, DEFAULT_MAX_DEPTH } from "./serialize.js";
-import { RpcTargetBranded, RpcCompatible, Stub, type RpcPromise as RpcPromiseType,
-         __RPC_TARGET_BRAND } from "./types.js";
+import { RpcTargetBranded, RpcCompatible, Stub, ElideStub, PayloadOrStub,
+         type RpcPromise as RpcPromiseType, __RPC_TARGET_BRAND } from "./types.js";
 import { newWebSocketRpcSession as newWebSocketRpcSessionImpl,
          newWorkersWebSocketRpcResponse, WebSocketTransport } from "./websocket.js";
 import { newHttpBatchRpcSession as newHttpBatchRpcSessionImpl,
@@ -70,7 +70,19 @@ export const RpcStub: {
  */
 export type RpcPromise<T extends RpcCompatible<T>> = RpcPromiseType<T>;
 export const RpcPromise: {
-  new <T extends RpcCompatible<T>>(value: Promise<T | Stub<T>>): RpcPromise<T>;
+  // The return type applies `ElideStub` — the same transformation `Result` applies to a
+  // declared stub return — so constructing from a promised stub produces exactly the type a
+  // method returning that stub would. See `PayloadOrStub` for what the promise may resolve to.
+  //
+  // Two overloads, for inference reasons. A context-sensitive argument — e.g.
+  // `Promise.resolve({f() { ... }})`, where the method's return type must be inferred — is
+  // contextually typed against the first overload only, and a contextual type containing a
+  // `Stub` arm collapses such an argument's inference. The first overload therefore keeps its
+  // parameter a plain `Promise<T>`. Since `PayloadOrStub`'s stub arm is `NoInfer` anyway, both
+  // overloads infer identically; the second one matters only when `T` is explicitly annotated
+  // and the payload is a stub, e.g. `new RpcPromise<Counter>(promiseOfStub)`.
+  new <T extends RpcCompatible<T>>(value: Promise<T>): RpcPromiseType<ElideStub<T>>;
+  new <T extends RpcCompatible<T>>(value: Promise<PayloadOrStub<T>>): RpcPromiseType<ElideStub<T>>;
 } = <any>RpcPromiseImpl;
 
 /**

src/types.d.ts

@@ -184,11 +184,21 @@ export type RpcPromise<T> =
 // stubified record. The payload check is deliberately non-distributive (`[U] extends [...]`).
 // `Stub<any>` is not elided either: `[any] extends [Stubable]` is true, so without the `IsAny`
 // guard an `any`-payload stub would lose its stub surface.
+// Also used by the `RpcPromise` constructor signature (index.ts), which applies exactly this
+// transformation so constructing from a promised stub matches the method-return type.
 export type ElideStub<T> =
   T extends StubBase<infer U>
     ? (IsAny<U> extends true ? T : [U] extends [Stubable] ? U : T)
     : T;
 
+// What the promise given to `new RpcPromise<T>(...)` may resolve to: the payload itself, or —
+// for stubable payloads — a stub of it. `NoInfer` keeps the stub arm out of inference, so an
+// inferred `T` is always the promise's own resolution type; the arm only matters when `T` is
+// explicitly annotated (`new RpcPromise<Counter>(promiseOfStub)`). Stubs of non-stubable
+// payloads are deliberately rejected: `ElideStub` wouldn't elide those, so accepting one would
+// claim the promise awaits to a stubified record while the runtime resolves to a stub.
+export type PayloadOrStub<T> = T | NoInfer<Stub<Extract<T, Stubable>>>;
+
 // Type for method return or property on an RPC interface.
 // - Stubable types are replaced by stubs.
 // - RpcCompatible types are passed by value, with stubable types replaced by stubs