Investigate support for exactOptionalPropertyTypes (on by default from TypeScript 5.9)

[Zaczero]

context

#1347 was closed incorrectly

follow up at #1357

conclusion

The library is incompatible with the compiler standard setting exactOptionalPropertyTypes (as seen in tsc --init)

expected result

1347 gets reopened and merged

---

cc @timostamm

[haines]

It seems like exactOptionalPropertyTypes is on by default from TypeScript 5.9 onwards: microsoft/TypeScript@cd34199#diff-f33350ca9f265f3b380d9d460d4590c2d50640529f31917a352426b4cc4d77b6

[timostamm]

It isn't clear from the issue description, but yes. We need to re-evaluate the situation.

More context:

• https://github.com/microsoft/TypeScript/releases/tag/v5.9.2
• https://devblogs.microsoft.com/typescript/announcing-typescript-5-9/#minimal-and-updated-tsc---init