Use TypeScript assert condition signature for invariant(condition, message)

[PowerKiKi]

This allow TypeScript to know that an assertion was made and that
types can be narrowed down after a call to invariant().

For more details see microsoft/TypeScript#32695

[PowerKiKi]

This PR is required if we want to solve apollographql/apollo-client#6073

[benjamn]

@PowerKiKi Do you know if using this syntax is safe for applications that use an older version of TypeScript?

[SimenB]

It is not, I broke some people on an older version of TS by adding this to Jest. Seems worth it to make a breaking change though, so invariant works also at the type level. And just tell people to use at least TS@3.7

[PowerKiKi]

Correct, we now require TypeScript 3.7. I have no idea how tests did not fail in my local install... I rebased and forced push a new commit including versions increase.

I am wondering whether this should also be declared as peerDependencies in packages/ts-invariant/package.json ?

PS: also notice that I simplified the README, as it is no longer true. And we could argue whether InvariantError should still be part of public API or not. But since apollo-client make use of that, it's probably best to keep it as is for the time being.

[PowerKiKi]

@benjamn would you have time to have another look at this?

[PowerKiKi]

@benjamn it seems you were interested in this PR earlier this year. And I fixed the issues raised in the meantime. I still think it would add a lot of value to this lib. Would you have time to review/merge it ?

[benjamn]

@PowerKiKi Thanks for the ping. I am checking with my team (@hwillson and a few others) to see if we can raise the minimum to 3.7 at this point, and I will report back (hopefully today).

[SimenB]

🎉