Improve variance measurement

[Anders Hejlsberg (ahejlsberg)]

This PR improves variance measurement for generic types. Previously we'd default to covariance for recursive references to the same type during variance measurement. Now we instead assume recursive references to be "possibly related" (signaled using Ternary.Maybe), similarly to what happens when we hit the recursion depth limiter. This effectively means we measure variance only from type parameter occurrences that aren't nested in recursive instantiations of the generic type.

Previously, a type like the following:

interface Foo<T> {
  f: (x: T) => void;
  g: Foo<Array<T>>;
}

would be measured as invariant in T: We would correctly measure the reference to T in the f property as contravariant, but then the recursive reference to Foo in the g property would default to covariance and we'd end up with a combined invariant result. We now correctly measure the type as contravariant in T:

declare const f1: Foo<string>;
const f2: Foo<'a'> = f1;  // Ok
const f3: Foo<unknown> = f1;  // Error

We furthermore now correctly measure unwitnessed type parameters in recursive references as independent:

interface Bar<T> {
  g: Bar<Array<T>>;
}

declare const b1: Bar<string>;
const b2: Bar<number> = b1;  // Ok

We could potentially further improve variance measurement by structurally exploring recursive references to some depth other than zero, but we already know that the limit of 5 imposed by the recursion depth limiter is too high to prevent runaway recursion in some of our test suites.

Fixes #35805.

[Anders Hejlsberg (ahejlsberg)]

TypeScript Bot (@typescript-bot) test this
TypeScript Bot (@typescript-bot) user test this
TypeScript Bot (@typescript-bot) run dt

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at b453606. You can monitor the build here. It should now contribute to this PR's status checks.

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized community code test suite on this PR at b453606. You can monitor the build here. It should now contribute to this PR's status checks.

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at b453606. You can monitor the build here. It should now contribute to this PR's status checks.

[Wesley Wigham (weswigham)]

Seems straightforward to me.

[TypeScript Bot (typescript-bot)]

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

[Anders Hejlsberg (ahejlsberg)]

Community test failures appear to be preexisting conditions. Other tests are clean.