fix: make the Exact helper compile on TypeScript 7.1 - #1293

Symptom

Every generated file that uses useExactTypes (the default) fails to compile on TypeScript 7.1 (checked with 7.1.0-beta and 7.1.0-dev.20261020):

error TS2344: Type 'I[K]' does not satisfy the constraint 'P[K]'.
  Type 'I' is not assignable to type 'P'.
    'I' is only assignable to the non-distributed 'P', but 'P' has been distributed here.
error TS2536: Type 'K' cannot be used to index type 'I'.

TypeScript 5.9 and 7.0 accept the same code.

Cause

type Exact<P, I extends P> = P extends Builtin ? P
  : P & { [K in keyof P]: Exact<P[K], I[K]> } & { [K in Exclude<keyof I, KeysOfUnion<P>>]: never };

The conditional distributes over P, so in the false branch P is a single union member, while the constraint I extends P still refers to the whole union. Since microsoft/TypeScript#64237 (merged for 7.1) the compiler distinguishes the distributed P from the non-distributed one, so I[K] (with K extends keyof <distributed P>) is no longer considered a valid index into I, and I[K] no longer satisfies P[K].

This is intentional rather than a beta bug: the old alias was unsound (I extends Child | undefined does not guarantee that I has the keys of Child), and that PR uses the same shape, type Issue<A, B extends A> = A extends unknown ? Show<A, B> : never, as its example of a newly reported error. microsoft/TypeScript#64673 and its fix #64307 concern a separate regression from the same change (identical generic signatures no longer relating) and do not cover this pattern; the latest 7.1 nightly still rejects it.

Fix

type Exact<P, I> = P extends Builtin ? P
  : P & { [K in keyof P as Extract<K, keyof I>]: Exact<P[K], I[K & keyof I]> } & { [K in Exclude<keyof I, KeysOfUnion<P>>]: never };

• I is no longer constrained by the alias. fromPartial/create already constrain I extends Exact<DeepPartial<T>, I>, so nothing changes for callers.
• The mapped type keeps only the keys of P that I actually has, which lets I[K & keyof I] type-check. This produces the same checks as before: for keys missing from I the old I[K] resolved to unknown, i.e. no excess-property check for that key, and P & still contributes every property. Union fields (oneof=unions), Builtins, useUnknownForValue and $type behave as before.

Alternatives that were tried and rejected: K extends keyof I ? I[K] : unknown breaks the nested Child.fromPartial(object.child) calls inside generated code (deferred conditional types no longer relate across the generic Is), and any (I & X)[K] form produces "circularly references itself" errors.

Testing

• New type tests (@ts-expect-error for unknown properties at the top level, in nested messages, in repeated fields and inside oneof cases, plus compiling valid partials) in integration/simple, integration/oneof-unions and integration/value-unknown.
• yarn proto2ts re-generated all integration output; yarn tsc:check and yarn test pass on TypeScript 5.9.3.
• integration/**/*.ts and integration/**/*-test.ts type-check on [email protected] (with the removed esModuleInterop option dropped from the tsconfig). The only remaining 7.1 errors are the pre-existing new DataLoader(...) calls on a namespace import in batching-with-context and lower-case-svc-methods, which are unrelated to this change.