Precompilation is not the same as generation.

Honestly its likely not a very documented topic. Also I am pretty certain TS does an awful lot of caching regardless of if you are making and using loads of utility types. Best thing is letting instantiation and type counts plus tracing guide you

Its also worth checking both instatiations and type count. They are both useful, when both go down its a win-win. But if one goes up and the other goes down, it can probably vary which is faster

But its not always the case though. Sometimes you want A and B to be instantiated lazily, when they are used. And you only want to do the work for A and not B only when A is used.

Its probably the caching. If you know both A and B are always used in the majority of cases then having a shared util type which does the work once can definitely help

Now is the time to design for typescript and write plenty of types

Just think how many more types we can write now

Also feels less natural to me

Mapped types feel too verbose for this purpose

In the end. Its about 'rich types', adding precise types where it matters most

Or should it be the most suitable type? Or are we saying 'type-safe' stuff just cares about TypeScript. If so, that is sad.

I just see them as if expressions

Types are pure functions. There are no side effects possible, like ever. The language is also quite small. I personally find this combination makes it easy to reason

I think the designer of those languages has a better understanding of what language to choose for a code base they know very well than maybe anybody on the outside

There are some footguns which I have learnt to avoid regarding performance. If the compiler is 10 times faster then definitely some quirks become way less noticable. I wonder if things like conflicting properties with intersections might still be noticable.

I use mapped types when i want to transform an object or tuple

I prefer to use distributive conditionals to transform unions