Clarify naming around inference fork points

^KT-43296 In Progress

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/ConstraintSystemCompleter.kt

@@ -62,8 +62,7 @@ class ConstraintSystemCompleter(components: BodyResolveComponents, private val c
     ) {
         val topLevelTypeVariables = topLevelType.extractTypeVariables()
 
-        // NB: it's called in ConstraintSystemForks resolution stage by FE 1.0
-        processForkConstraints()
+        resolveForkPointsConstraints()
 
         completion@ while (true) {
             // TODO: This is very slow

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/NewConstraintSystem.kt

@@ -25,10 +25,29 @@ interface NewConstraintSystem {
 
     fun asConstraintSystemCompleterContext(): ConstraintSystemCompletionContext
     fun asPostponedArgumentsAnalyzerContext(): PostponedArgumentsAnalyzerContext
-    fun processForkConstraints()
+    fun resolveForkPointsConstraints()
 
     fun getEmptyIntersectionTypeKind(types: Collection<KotlinTypeMarker>): EmptyIntersectionTypeInfo?
 }
 
-typealias ForkPointData = List<ConstraintsFromSingleFork>
-typealias ConstraintsFromSingleFork = Set<Pair<TypeVariableMarker, Constraint>>
+/**
+ * In some cases we're not only adding constraints linearly to the system, but sometimes we need to consider several variants of constraints
+ *
+ * For example, from smartcast we've got a value of a type A<Int, String> & A<E, F> that we'd like to pass as an argument to the parameter
+ * of type A<Xv, Yv> (where Xv and Yv are the type variables of the current call)
+ *
+ * So, we've got a subtyping constraint
+ * A<Int, String> & A<E, F> <: A<Xv, Yv>
+ *
+ * And we might go with the first intersection component, having the following variables constraint set: {Xv=Int,Yv=String}
+ * Or, if we'd consider the second component it would be {Xv=E, Yv=F}
+ *
+ * And all existing and future constraints might work differently depending on which option we've chosen.
+ * Thus, ideally we need to create two versions of the constraint system and try to resolve each of them.
+ * But that lead to exponential complexity, so we only use some set of heuristics for that
+ *
+ * Lately, we call such situation a "fork point" and each of the options a "fork point branch"
+ * Each branch is defined by the set of constraints that need to be added to the system if we choose the particular branch.
+ */
+typealias ForkPointData = List<ForkPointBranchDescription>
+typealias ForkPointBranchDescription = Set<Pair<TypeVariableMarker, Constraint>>

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/components/ConstraintInjector.kt

@@ -8,7 +8,7 @@ package org.jetbrains.kotlin.resolve.calls.inference.components
 import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.resolve.calls.inference.ConstraintSystemOperation
-import org.jetbrains.kotlin.resolve.calls.inference.ConstraintsFromSingleFork
+import org.jetbrains.kotlin.resolve.calls.inference.ForkPointBranchDescription
 import org.jetbrains.kotlin.resolve.calls.inference.ForkPointData
 import org.jetbrains.kotlin.resolve.calls.inference.model.*
 import org.jetbrains.kotlin.resolve.calls.inference.model.ConstraintKind.*
@@ -46,7 +46,7 @@ class ConstraintInjector(
             constraints: MutableList<Pair<TypeVariableMarker, Constraint>>
         )
 
-        fun processForkConstraints()
+        fun resolveForkPointsConstraints()
     }
 
     fun addInitialSubtypeConstraint(c: Context, lowerType: KotlinTypeMarker, upperType: KotlinTypeMarker, position: ConstraintPosition) {
@@ -141,7 +141,7 @@ class ConstraintInjector(
         processConstraints(c, typeCheckerState, skipProperEqualityConstraints = false)
     }
 
-    fun processForkConstraints(
+    fun processGivenForkPointBranchConstraints(
         c: Context,
         constraintSet: Collection<Pair<TypeVariableMarker, Constraint>>,
         position: IncorporationConstraintPosition
@@ -166,7 +166,7 @@ class ConstraintInjector(
 
                 // During completion, we start processing fork constrains immediately
                 if (c.atCompletionState) {
-                    c.processForkConstraints()
+                    c.resolveForkPointsConstraints()
                 }
             }
         }
@@ -275,8 +275,8 @@ class ConstraintInjector(
         private var possibleNewConstraints: MutableList<Pair<TypeVariableMarker, Constraint>>? = null
 
         private var forkPointsData: MutableList<ForkPointData>? = null
-        private var stackForConstraintsSetsFromCurrentForkPoint: Stack<MutableList<ConstraintsFromSingleFork>>? = null
-        private var stackForCurrentConstraintSetFromSingleFork: Stack<MutableList<Pair<TypeVariableMarker, Constraint>>>? = null
+        private var stackForConstraintsSetsFromCurrentForkPoint: Stack<MutableList<ForkPointBranchDescription>>? = null
+        private var stackForConstraintSetFromCurrentForkPointBranch: Stack<MutableList<Pair<TypeVariableMarker, Constraint>>>? = null
 
         override val isInferenceCompatibilityEnabled = languageVersionSettings.supportsFeature(LanguageFeature.InferenceCompatibility)
 
@@ -294,9 +294,9 @@ class ConstraintInjector(
         fun addPossibleNewConstraint(variable: TypeVariableMarker, constraint: Constraint) {
             val constraintsSetsFromCurrentFork = stackForConstraintsSetsFromCurrentForkPoint?.lastOrNull()
             if (constraintsSetsFromCurrentFork != null) {
-                val currentConstraintSetForFork = stackForCurrentConstraintSetFromSingleFork?.lastOrNull()
-                require(currentConstraintSetForFork != null) { "Constraint has been added not under fork {...} call " }
-                currentConstraintSetForFork.add(variable to constraint)
+                val currentConstraintSetForForkPointBranch = stackForConstraintSetFromCurrentForkPointBranch?.lastOrNull()
+                require(currentConstraintSetForForkPointBranch != null) { "Constraint has been added not under fork {...} call " }
+                currentConstraintSetForForkPointBranch.add(variable to constraint)
                 return
             }
 
@@ -333,7 +333,7 @@ class ConstraintInjector(
                 )
                 return true
             } else if (constraintSets.size == 1) {
-                processForkConstraints(
+                processGivenForkPointBranchConstraints(
                     c,
                     constraintSets.single(),
                     position,
@@ -347,17 +347,17 @@ class ConstraintInjector(
             var anyForkSuccessful = false
 
             override fun fork(block: () -> Boolean) {
-                if (stackForCurrentConstraintSetFromSingleFork == null) {
-                    stackForCurrentConstraintSetFromSingleFork = SmartList()
+                if (stackForConstraintSetFromCurrentForkPointBranch == null) {
+                    stackForConstraintSetFromCurrentForkPointBranch = SmartList()
                 }
 
-                stackForCurrentConstraintSetFromSingleFork!!.add(SmartList())
+                stackForConstraintSetFromCurrentForkPointBranch!!.add(SmartList())
 
                 block().also { anyForkSuccessful = anyForkSuccessful || it }
 
                 stackForConstraintsSetsFromCurrentForkPoint!!.last()
                     .addIfNotNull(
-                        stackForCurrentConstraintSetFromSingleFork?.popLast()?.takeIf { it.isNotEmpty() }?.toSet()
+                        stackForConstraintSetFromCurrentForkPointBranch?.popLast()?.takeIf { it.isNotEmpty() }?.toSet()
                     )
             }
         }

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/components/ConstraintSystemCompletionContext.kt

@@ -11,7 +11,9 @@ import org.jetbrains.kotlin.resolve.calls.inference.ConstraintSystemBuilder
 import org.jetbrains.kotlin.resolve.calls.inference.model.*
 import org.jetbrains.kotlin.resolve.calls.model.PostponedAtomWithRevisableExpectedType
 import org.jetbrains.kotlin.resolve.calls.model.PostponedResolvedAtomMarker
-import org.jetbrains.kotlin.types.model.*
+import org.jetbrains.kotlin.types.model.KotlinTypeMarker
+import org.jetbrains.kotlin.types.model.TypeConstructorMarker
+import org.jetbrains.kotlin.types.model.TypeVariableMarker
 
 abstract class ConstraintSystemCompletionContext : VariableFixationFinder.Context, ResultTypeResolver.Context {
     abstract val allTypeVariables: Map<TypeConstructorMarker, TypeVariableMarker>
@@ -33,7 +35,7 @@ abstract class ConstraintSystemCompletionContext : VariableFixationFinder.Contex
     abstract fun fixVariable(variable: TypeVariableMarker, resultType: KotlinTypeMarker, position: FixVariableConstraintPosition<*>)
 
     abstract fun couldBeResolvedWithUnrestrictedBuilderInference(): Boolean
-    abstract fun processForkConstraints()
+    abstract fun resolveForkPointsConstraints()
 
     fun <A : PostponedResolvedAtomMarker> analyzeArgumentWithFixedParameterTypes(
         languageVersionSettings: LanguageVersionSettings,

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt

@@ -357,7 +357,11 @@ class NewConstraintSystemImpl(
             return storage.constraintsFromAllForkPoints
         }
 
-    override fun processForkConstraints() {
+    /**
+     * This function tries to find the solution (set of constraints) that is consistent with some branch of each fork
+     * And those constraints are being immediately applied to the system
+     */
+    override fun resolveForkPointsConstraints() {
         if (constraintsFromAllForkPoints.isEmpty()) return
         val allForkPointsData = constraintsFromAllForkPoints.toList()
         constraintsFromAllForkPoints.clear()
@@ -369,7 +373,7 @@ class NewConstraintSystemImpl(
         // 1. {Xv=Int} – is a one-element set (but potentially there might be more constraints in the set)
         // 2. {Xv=T} – second constraints set
         for ((position, forkPointData) in allForkPointsData) {
-            if (!processForkPointData(forkPointData, position)) {
+            if (!applyConstraintsFromFirstSuccessfulBranchOfTheFork(forkPointData, position)) {
                 addError(NoSuccessfulFork(position))
             }
         }
@@ -392,7 +396,7 @@ class NewConstraintSystemImpl(
         var result: ConstraintSystemError? = null
         runTransaction {
             for ((position, forkPointData) in allForkPointsData) {
-                if (!processForkPointData(forkPointData, position)) {
+                if (!applyConstraintsFromFirstSuccessfulBranchOfTheFork(forkPointData, position)) {
                     result = NoSuccessfulFork(position)
                     break
                 }
@@ -409,20 +413,20 @@ class NewConstraintSystemImpl(
     /**
      * @return true if there is a successful constraints set for the fork
      */
-    private fun processForkPointData(
+    private fun applyConstraintsFromFirstSuccessfulBranchOfTheFork(
         forkPointData: ForkPointData,
         position: IncorporationConstraintPosition
     ): Boolean {
-        return forkPointData.any { constraintSetForSingleFork ->
+        return forkPointData.any { constraintSetForForkBranch ->
             runTransaction {
-                constraintInjector.processForkConstraints(
+                constraintInjector.processGivenForkPointBranchConstraints(
                     this@NewConstraintSystemImpl.apply { checkState(State.BUILDING, State.COMPLETION, State.TRANSACTION) },
-                    constraintSetForSingleFork,
+                    constraintSetForForkBranch,
                     position,
                 )
 
                 if (constraintsFromAllForkPoints.isNotEmpty()) {
-                    processForkConstraints()
+                    resolveForkPointsConstraints()
                 }
 
                 !hasContradiction