[core] Proof of concept for solving the .d.ts generation issue

[oliviertassinari]

One part of #24112.

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against 919a160

[eps1lon]

Let me take a look at that. This is only fixing symptoms but not the underlying issue.

[HofmannZ]

@eps1lon Anything we can help with?

[oliviertassinari]

@eps1lon Agree, let's hold on for some time. I have tried to find the root issue but gave up after an hour. If we can figure it out, that would be better, in case we hit the problem in another case in the future.

If we don't, I think that we could live without it too. The proposed approach 1. wrecks the types with withStyles, but we are moving to styled anyway. 2. forces us to manually type the classes which seems to help with the migration to styled.

@HofmannZ During the end of year holidays, things might move slowly. Feel free to spend time exploring what's wrong. The closest I could find was microsoft/TypeScript#24599 which was relative to a fs case sensibility issue in the core of TypeScript.

[eps1lon]

@eps1lon Anything we can help with?

My intuition tells me we need a change at the tsconfig/tsc level. The current approach doesn't guarantee that it doesn't regress in the future.

Alternatively we get a smoke test for the build output (started in #23166) that catches these regressions.

Either way I want to make sure this doesn't get lost over the holidays since it's important to get right before v5. I rather block it in this instance to keep visibility up instead of merging a quick fix that'll just re-surface later.

[eps1lon]

The generated declaration from tsc is just so broken in this instance:

1. inlines type imports
2. uses path imports relative to root dir instead of built file layout
3. removes WithStyles<typeof styles> intersection

It makes sense because it's pretty hard to generate a sensible type in this particular instance but tsc should throw instead of garbling this particular declaration file.