Proof-of-Concept: Excess property checking for nested intersection and union

[jack-williams]

Proof-of-concept excess property checking for nesting intersection and union (independent of discriminant properties). See tests/cases/compiler/nestedExcessPropertyChecks.ts for some examples. Just putting this out there incase it is of interest to anyone.

Please feel free to check it out and have a go, though the error messages are not good and the code is slow and ugly.

[mattmccutchen]

This is intended to fix #13813 and #20863, right? Adding references.

[jack-williams]

Yes thats the aim and currently it does fix those cases.

Though I will stress that I don't intend this PR to be the fix. The issue is not a community milestone and I don't know if it's already being worked on by the TS team. Also, the code is still far away from being in a reviewable state (there is one bug I've found already).

I was mainly just hoping it might be of interest or use to someone. If there is desire to try and integrate it then I'm happy to put more work into it.

[typescript-bot]

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