Consider call `foo(bar())` where bar() returns some type variable `T`;
We had a contract that call `bar` can be completed without completion
of foo (type variables can be inferred from the current context) if `T`
has at least one proper lower constraint (ProperType <: T).
Indeed, new constraints can be added only as upper ones, so there is
no need to grow constraint system.
Unfortunately, we have Exact annotation that is used on return type of
elvis. Now, consider the following situation:
```
fun foo(a: Any) {}
fun bar(e: T): @Exact T
foo(bar("str"))
```
Here, because of Exact annotation, constraint with `Any`-type will be
added as an equal one => our prerequisite that there will be no new
lower constraints is false. `bar("str")` is inferred to Any in OI,
this seems conceptually wrong, but it's another topic of discussion.
In NI we can't just grow constraint system to use outer call because
of another important use-case:
```
fun <T> generic(i: Inv<T>) {}
fun test(a: Inv<*>?, b: Inv<*>) {
generic(a ?: b)
}
```
Common constraint system for these two calls can't be solved
(fundamentally) for this example, only if (a ?: b) and generic(result)
are computed separately.
So, to mitigate initial issue, we'll grow constraint system only if
there is at least one non-proper constraint.
#KT-31969 Fixed
It's enough to have at least one good constraint.
Note that the whole algorithm can be a bit more general:
we could check also Out<T>, In<T> and verify that T has good only
lower constraint or upper constraint, but there are questions for
types like Inv<Out<T>>, where T should have lower and upper constraints
#KT-31514 Fixed
There is added a new service named `SubstitutingScopeProvider`, that
provides factory that creates captured types and approximator for them.
In OI they are the same as before commit, for NI they are empty, because
that approximation interferes with NI algorithm
That service is injected into function descriptors and property descriptors
and used for creating `SubstitutingScope` with correct services
Also there is changed time when we approximate captured types in NI
(after all call checkers)
#KT-25290 Fixed
Mostly, these optimisations are picked from the old inference.
Also, remove exponential complexity for flexible types in approximation,
note that more correct fix for this would be to introduce new types
that corresponds just to platform types to avoid nullability problems,
but due to complexity it will be done later
#KT-31415 Fixed
Also that commit removes usages of builtins inside
effect system and starts refactoring of functor
composition via composition instead of inheritance.
There are some changes in testdata related to inference of recursive
functions with implicit return types.
After this commit they all are marked as unresolved. It happens because
those functions have DeferredType as return type, and computing this
type produces recursive exception, which provides “typechecker
recursive problem” diagnostic.
Before this commit, function call was completed successfully, because
call completer didn’t computed that type, and computation of DeferredType
were triggered only in `DataFlowAnalyzer.checkType`.
Now, effect system tries to compute that type while wrapping KotlinTypes
into ESTypes, and effect system itself is triggered in in call completer,
so, call completion doesn’t finish and function call is marked as unresolved.
#KT-31364
Expression will be checked against expected type later.
Theoretically, this is not very good, but it aligns with the old
inference, plus it helps avoiding multiple type mismatch diagnostics.
Consider common supertype of `S` and `Nothing`, where `S` has nullable
upper bound or it's flexible. Before the fix, result was `S?`, which
is correct but too conservative. Now, we'll preserve nullability of
resulting type if it's already nullable.
This happened because we were failing to find path of not-nullable
types from `Nothing` to `S`, which should obviously exists by
semantics of Nothing
Mikhail Zarechenskiy