NI: Support forking inference with heuristics

Mostly, it only affects FIR

It partially allows to consider several variance of constraints like
A<Int> & A<T> <: A<X_var> that are mostly brought by smart casts

^KT-49542 Fixed
^KT-50489 Relates

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CallKind.kt

@@ -19,6 +19,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckDslScopeViolation,
         CheckLowPriorityInOverloadResolution,
         PostponedVariablesInitializerResolutionStage,
+        ConstraintSystemForks
     )
 
     object SyntheticSelect : CallKind(
@@ -27,7 +28,8 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CreateFreshTypeVariableSubstitutorStage,
         CollectTypeVariableUsagesInfo,
         CheckArguments,
-        EagerResolveOfCallableReferences
+        EagerResolveOfCallableReferences,
+        ConstraintSystemForks,
     )
 
     object Function : CallKind(
@@ -47,6 +49,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         EagerResolveOfCallableReferences,
         CheckLowPriorityInOverloadResolution,
         PostponedVariablesInitializerResolutionStage,
+        ConstraintSystemForks,
     )
 
     object DelegatingConstructorCall : CallKind(
@@ -62,6 +65,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckDslScopeViolation,
         CheckArguments,
         EagerResolveOfCallableReferences,
+        ConstraintSystemForks,
     )
 
     object CallableReference : CallKind(
@@ -84,7 +88,8 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CreateFreshTypeVariableSubstitutorStage,
         CollectTypeVariableUsagesInfo,
         CheckArguments,
-        EagerResolveOfCallableReferences
+        EagerResolveOfCallableReferences,
+        ConstraintSystemForks,
     )
 
     internal class CustomForIde(vararg resolutionSequence: ResolutionStage) : CallKind(*resolutionSequence)

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ResolutionStages.kt

@@ -7,14 +7,19 @@ package org.jetbrains.kotlin.fir.resolve.calls
 
 import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.descriptors.Modality
-import org.jetbrains.kotlin.fir.*
+import org.jetbrains.kotlin.fir.FirVisibilityChecker
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.utils.*
 import org.jetbrains.kotlin.fir.expressions.*
+import org.jetbrains.kotlin.fir.languageVersionSettings
+import org.jetbrains.kotlin.fir.matchingParameterFunctionType
 import org.jetbrains.kotlin.fir.references.FirSuperReference
 import org.jetbrains.kotlin.fir.resolve.directExpansionType
 import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
-import org.jetbrains.kotlin.fir.resolve.inference.*
+import org.jetbrains.kotlin.fir.resolve.inference.ConeTypeParameterBasedTypeVariable
+import org.jetbrains.kotlin.fir.resolve.inference.ResolvedCallableReferenceAtom
+import org.jetbrains.kotlin.fir.resolve.inference.csBuilder
+import org.jetbrains.kotlin.fir.resolve.inference.hasBuilderInferenceAnnotation
 import org.jetbrains.kotlin.fir.resolve.toSymbol
 import org.jetbrains.kotlin.fir.scopes.FirTypeScope
 import org.jetbrains.kotlin.fir.scopes.FirUnstableSmartcastTypeScope
@@ -25,6 +30,7 @@ import org.jetbrains.kotlin.fir.symbols.SyntheticSymbol
 import org.jetbrains.kotlin.fir.symbols.ensureResolved
 import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.types.*
+import org.jetbrains.kotlin.fir.visibilityChecker
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
@@ -527,3 +533,15 @@ internal object CheckDeprecatedSinceKotlin : ResolutionStage() {
         }
     }
 }
+
+internal object ConstraintSystemForks : ResolutionStage() {
+    override suspend fun check(candidate: Candidate, callInfo: CallInfo, sink: CheckerSink, context: ResolutionContext) {
+        if (candidate.system.hasContradiction) return
+
+        candidate.system.processForkConstraints()
+
+        if (candidate.system.hasContradiction) {
+            sink.yieldDiagnostic(candidate.system.errors.firstOrNull()?.let(::InferenceError) ?: InapplicableCandidate)
+        }
+    }
+}

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

@@ -125,4 +125,7 @@ object ConeConstraintSystemUtilContext : ConstraintSystemUtilContext {
 
     override val isForcedConsiderExtensionReceiverFromConstrainsInLambda: Boolean
         get() = true
+
+    override val isForcedAllowForkingInferenceSystem: Boolean
+        get() = true
 }

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

@@ -50,6 +50,8 @@ class ConstraintSystemCompleter(private val components: BodyResolveComponents, p
     ) = with(c) {
         val topLevelTypeVariables = candidateReturnType.extractTypeVariables()
 
+        c.processForkConstraints()
+
         completion@ while (true) {
             val postponedArguments = getOrderedNotAnalyzedPostponedArguments(topLevelAtoms) // TODO: This is very slow