kotlin-fork/docs/fir/inference.md

Inference

Currently, this document contains some basic terms that are common for different specific inference types. Lately, it might be extended to include some basic description of how inference works.

Glossary

TV = type variable

To avoid confusion with type parameter types, we mostly use Tv for referencing type variable based on a type parameter named T

CS = Constraint system

• Mostly, it’s just a collection of TVs and the constraints for them in a form of Xi <: SomeType or SomeType <: Xi or Xi = SomeType
• Represented as an instance of org.jetbrains.kotlin.resolve.calls.inference.model.NewConstraintSystemImpl

Related TVs

See TypeVariableDependencyInformationProvider for details.

Shallow relation

Two variables Xv and Yv are shallowly related if there is some chain of a constraints Xv ~ a_1 ~ .. ~ a_n ~ Yv where (~ is either <: or >:)

Deep relation

Two variables Xv and Yv are deeply related if there is some chain of a constraints a_1 ~ .. ~ a_n where (~ is either <: or >:) and a_1 contains Xv while a_n contains Yv.

Call-tree

A tree of calls, in which constraint systems are joined and solved(completed) together

Proper constraint

A constraint that doesn't reference any type variables

Input types of a lambda

• Receiver type
• Value parameter types

Completion is a process that for a given call, its CS and postponed atoms (lambdas and callable references) tries to infer some TV and analyze some lambdas

Completion mode

Completion mode (ConstraintSystemCompletionMode) defines how actively/forcefully the given call is being completed

• PARTIAL — used for calls that are other calls arguments
  • It prevents from fixing TV that are related somehow to return types because their actual result depends on the chosen outer call
  • Otherwise, it tries to infer as much as possible, but doesn’t lead to reporting not-inferred-type-parameter or running PCLA lambdas
  • On one hand, we might’ve not run that kind of completion at all, and just accumulate nested argument calls into an outer as is,
  • but inferring something allows disambiguating outer candidates

  • val x: Int = 1
    fun main() {
        x.plus(run { x })
    }
    

• FULL — used for top statement-level calls and receivers
  • It tries to fix/analyze everything we have
  • And start PCLA analysis if necessary
  • Reports errors if for some TV there’s no enough information to infer them
• UNTIL_FIRST_LAMBDA — used for OverloadByLambdaReturnType
  • Similar to FULL, but stops after first lambda analyzed
• PCLA_POSTPONED_CALL — see pcla.md.

Inference session

A set of callbacks related to inference that being called during function body transformations. See FirInferenceSession.