Typescript: Validate tuple type element positions

[Retsam]

Q	A
Fixed Issues?
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Adds additional validation to the parser functionality added by #8720 and #8805. Per this comment, adds a validation step to the parser which raises syntax errors if a rest param is not at the end of a tuple, or if a mandatory param follows an optional parameter. So the following snippets are correctly considered syntax errors:

const x: [number?, number]; // can't have mandatory member after optional member
const x: [...number[], string]; // rest element must be the last element of the tuple

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/9225/

[danez]

Are they SyntaxErrors? If I try to parse these in the typescript playground, it all parses fine.

[Retsam]

Yeah, they're supposed to be syntax errors:

[danez]

Ah the typescript parser always tries to output something useful even if there are syntax errors in the code. 👍

[danez]

Thanks for your great contributions and helping to improve our typescript support. 🎉