tsgo typechecks slower than TS6

[XiNiHa]

Against the same code, tsgo typechecks 5x+ slower than TS6.

Repro: https://github.com/XiNiHa/kysely-tsgo-slower

[ahejlsberg]

This is caused by #3445, which is a change from TS6. TS6 often bails out early of analyzing type relationships because it thinks it is dealing with an infinitely expanding type. TS7 is better able to recognize non-recursive types and therefore goes deeper in its analysis. That's what's happening here. Specifically, TS6 performs 601,143 type instantiations when compiling your repro, whereas TS7 performs 9,795,115 because of the deeper analysis.

I haven't deeply looked at the actual types in question. I wouldn't be surprised if they can be simplified to reduce the number of instantiations.

[XiNiHa]

Do you mean the behavior is expected/intended? Should it be fixed from the library side?

[ahejlsberg]

@XiNiHa Yes, the behavior is expected. TS6 would similarly slow down if the changes in #3445 were applied to that codebase. The core problem is the complexity and expense of the types.