Revert pattern matching improvements introducing regressions in 3.6.0

[WojciechMazur]

Reverts #21000 (introducing regression #21569) from 3.6.0
Reverts #18377 single commit 9045834 (introducing regression #21218) from 3.6.0
Done in the same way, as it was done to the LTS branch in #21573 and 3.5.2 in #21734

[dwijnand]

Why are we reverting this from 3.6.0?

[dwijnand]

Why?

[tgodzik]

I think that's due to #21569 regression.

[WojciechMazur]

Introduces regression increasing compilation time in #21569 original project from few minutes to almost 4 hours. Personally I'd like to keep all the improvements, but this level of slowdown is unacceptable even though the codebase is quite specific.

[dwijnand]

The fix is correct, and the code that was already slow and is now insanely slow is some pretty crazy code by itself. So I can understand dropping it from a bugfix release, but we shouldn't revert it for a minor release.

[WojciechMazur]

I've tried to collect some numbers from the Open CB execution times.
For the majority of projects the compilation time has not changed. There's less than 10 minutes to total OpenCB execution time, which with ~1600 tested takes ~30h, so can be treated as a statical error.
However, I've found out that another project is affected by this change and was unnoticed until now.

https://github.com/andimiller/cats-parse-interpolator was previously compiling in 1m 36s, now it takes 4h 5m.
https://github.com/fommil/jzon was previously taking 2m 55s, now it's 4h as well

[WojciechMazur]

I suggest we give the revert of these changes under the vote of today's core meeting.

[WojciechMazur]

During the core meeting, we all agreed that #21000 solved multiple, real problems and decided we should not revert it in 3.6.0.

Both of the affected projects were to some extent abusing the Scala type system (via making everything inlined or creating an abnormally large type signature). Instead, we should work on introducing a mechanism that would trigger a warning when pattern match analysis (for reachability or exhaustivity) is taking too much time.
We'd check if we can mitigate the problems of too-long compilation by adjusting the codebase of affected projects.

[dwijnand]

I'm working on that warning now, which we can backport to 3.6.1, or maybe if we have a 3.6.0-RC2.