Bring typeof switch behaviour inline with if

[Jack Williams (jack-williams)]

This PR has now morphed into something that bundles a couples of changes to essentially bring switch narrowing inline with if, which was recently updated in two keys ways:

1. unknown narrowing for "function" and "object"
2. narrowing aswell as filtering for union types.

I don't know if this PR should be split in separate requests tackling different issues. This can probably be resolved after the release of 3.1 when 3.2 is more relevant.

Fixes#27180
Fixes#27335
Adds tests for #27181

[Jack Williams (jack-williams)]

I'm tempted to re-open this PR with a fresh one to tidy up all the history into a single commit. Just give me a thumbs up / response and I'll sort it out, link to the new one, and close this.

[Justin Grant (justingrant)]

Jack Williams (@jack-williams)Ryan Cavanaugh (@RyanCavanaugh) - will this PR remove the compile error from the code below? Or will it only affect switch (typeof) and not other kinds of switch expressions? I assume the latter, but wanted to check before opening a dupe issue.

// compiled with strictNullChecksfunctionrepro(x: string[]|null){switch(x&&x.length){case1: console.log(x[0]);// compiler error: Object is possibly nullbreak;default: console.log('invalid x');break;}if(x&&x.length===1){console.log(x[0]);// no compiler error}else{console.log('invalid x');}}

[Jack Williams (jack-williams)]

Justin Grant (@justingrant)

I assume the latter, but wanted to check before opening a dupe issue.

You're right, this PR will only affect switch (typeof x) I'm afraid.

[Justin Grant (justingrant)]

OK thanks Jack Williams (@jack-williams). After some more searching, it looks like the problem I ran into was #24091.

[Jack Williams (jack-williams)]

Ryan Cavanaugh (@RyanCavanaugh)

Closing in favour of #27680