Erroneous "The '?' modifier can only be used in TypeScript files"

[Rich-Harris]

Steps to reproduce

With allowJs and checkJs enabled:

const is_morning = new Date().getHours() < 12;

// prettier-ignore
const greeting = ([
  is_morning ? 'good morning' : 'good evening'
]);

Behavior with typescript@5.8

No errors

Behavior with tsgo

Error TS8009:

The '?' modifier can only be used in TypeScript files.

---

Some additional context: we have a TSGo CI check on the Svelte repo, and this check started failing overnight. The real code it's failing on uses a JSDoc annotation, hence the wrapping parens, without which it parses correctly.

[jakebailey]

Most certainly caused by #1732. @ahejlsberg

[ahejlsberg]

Yup, this is #1732. An error is issued during speculative parsing, but our rewind logic doesn't reset JS diagnostics. Will put up a fix.