[Injection] Introduce PlatformSpecificExtensions and PlatformExtensionsClashResolver

This commit introduces the ability to register a PlatformExtensionClashResolver
in a container. Each PlatformExtensionClashResolver has a corresponding
PlatformSpecificExtensions.

If, during container composition, several instances of
PlatformSpecificExtensions were registred, instead of throwing
InvalidCardinalityException, corresponding PlatformExtensionClashResolver
will be asked to resolve clash.

This allows to make injection more composable and less coupled across
different contributors of service, providing a basis for such motivating
cases as composing containers with both JS and JVM services (for analysis
of multiplatform modules).
Previously, that would be impossible:
a) JS would inject default instances for some services which would clash
with non-default JVM services (like SyntheticScopes)
b) Also, there are a very few services for which *both* platforms provide
non-default implementations, so they should be merged manually on
case-by-case basis (e.g., IdentifierChecker)

compiler/container/src/org/jetbrains/kotlin/container/Container.kt

@@ -102,6 +102,11 @@ class StorageComponentContainer(
         return this
     }
 
+    internal fun registerClashResolvers(resolvers: List<PlatformExtensionsClashResolver<*>>): StorageComponentContainer {
+        componentStorage.registerClashResolvers(resolvers)
+        return this
+    }
+
     override fun <T> create(request: Class<T>): T {
         val constructorBinding = request.bindToConstructor(unknownContext)
         val args = constructorBinding.argumentDescriptors.map { it.getValue() }.toTypedArray()

compiler/container/src/org/jetbrains/kotlin/container/Dsl.kt

@@ -50,6 +50,10 @@ fun StorageComponentContainer.useInstanceIfNotNull(instance: Any?) {
     if (instance != null) registerInstance(instance)
 }
 
+fun StorageComponentContainer.useClashResolver(clashResolver: PlatformExtensionsClashResolver<*>) {
+    registerClashResolvers(listOf(clashResolver))
+}
+
 inline operator fun <reified T : Any> ComponentProvider.getValue(thisRef: Any?, desc: KProperty<*>): T {
     return getService(T::class.java)
 }

compiler/container/src/org/jetbrains/kotlin/container/Registry.kt

@@ -70,4 +70,20 @@ internal class ComponentRegistry {
         }
         registrationMap += other.registrationMap
     }
+
+    fun resolveClashesIfAny(container: ComponentContainer, clashResolvers: List<PlatformExtensionsClashResolver<*>>) {
+        /*
+        The idea is to create descriptor, which is very similar to other SingletonDescriptor, but instead of calling
+        constructor we call 'resolveExtensionsClash' with values of clashed components as arguments.
+
+        By mimicking the usual descriptors we get lazy evaluation and consistency checks for free.
+         */
+        for (resolver in clashResolvers) {
+            val clashedComponents = registrationMap[resolver.applicableTo] as? Collection<ComponentDescriptor> ?: continue
+            if (clashedComponents.size <= 1) continue
+
+            val substituteDescriptor = ClashResolutionDescriptor(container, resolver, clashedComponents.toList())
+            registrationMap[resolver.applicableTo] = substituteDescriptor
+        }
+    }
 }

compiler/container/src/org/jetbrains/kotlin/container/Singletons.kt

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.container
 
 import java.io.Closeable
+import java.lang.IllegalStateException
 import java.lang.reflect.Type
 import java.util.*
 
@@ -144,6 +145,29 @@ open class SingletonTypeComponentDescriptor(container: ComponentContainer, val k
     override fun toString(): String = "Singleton: ${klass.simpleName}"
 }
 
+internal class ClashResolutionDescriptor<E : PlatformSpecificExtension<E>>(
+    container: ComponentContainer,
+    private val resolver: PlatformExtensionsClashResolver<E>,
+    private val clashedComponents: List<ComponentDescriptor>
+) : SingletonDescriptor(container) {
+
+    override fun createInstance(context: ValueResolveContext): Any {
+        state = ComponentState.Initializing
+        val extensions = computeArguments(clashedComponents) as List<E>
+        val resolution = resolver.resolveExtensionsClash(extensions)
+        state = ComponentState.Initialized
+        return resolution
+    }
+
+    override fun getRegistrations(): Iterable<Type> {
+        throw IllegalStateException("Shouldn't be called")
+    }
+
+    override fun getDependencies(context: ValueResolveContext): Collection<Type> {
+        throw IllegalStateException("Shouldn't be called")
+    }
+}
+
 class ImplicitSingletonTypeComponentDescriptor(container: ComponentContainer, klass: Class<*>) : SingletonTypeComponentDescriptor(container, klass) {
     override fun toString(): String {
         return "Implicit: ${klass.simpleName}"

compiler/container/src/org/jetbrains/kotlin/container/Storage.kt

@@ -19,6 +19,7 @@ package org.jetbrains.kotlin.container
 import com.intellij.util.containers.MultiMap
 import java.io.Closeable
 import java.io.PrintStream
+import java.lang.IllegalStateException
 import java.lang.reflect.Modifier
 import java.lang.reflect.ParameterizedType
 import java.lang.reflect.Type
@@ -37,13 +38,19 @@ internal class InvalidCardinalityException(message: String) : Exception(message)
 
 class ComponentStorage(private val myId: String, parent: ComponentStorage?) : ValueResolver {
     var state = ComponentStorageState.Initial
-    private val registry = ComponentRegistry()
-    init {
-        parent?.let { registry.addAll(it.registry) }
-    }
 
     private val descriptors = LinkedHashSet<ComponentDescriptor>()
     private val dependencies = MultiMap.createLinkedSet<ComponentDescriptor, Type>()
+    private val clashResolvers = ArrayList<PlatformExtensionsClashResolver<*>>()
+    private val registry = ComponentRegistry()
+
+    init {
+        parent?.let {
+            registry.addAll(it.registry)
+            clashResolvers.addAll(it.clashResolvers)
+        }
+    }
+
 
     override fun resolve(request: Type, context: ValueResolveContext): ValueDescriptor? {
         fun ComponentDescriptor.isDefaultComponent(): Boolean =
@@ -107,6 +114,10 @@ class ComponentStorage(private val myId: String, parent: ComponentStorage?) : Va
         return registry.tryGetEntry(request)
     }
 
+    internal fun registerClashResolvers(resolvers: List<PlatformExtensionsClashResolver<*>>) {
+        clashResolvers.addAll(resolvers)
+    }
+
     internal fun registerDescriptors(context: ComponentResolveContext, items: List<ComponentDescriptor>) {
         if (state == ComponentStorageState.Disposed) {
             throw ContainerConsistencyException("Cannot register descriptors in $state state")
@@ -135,6 +146,7 @@ class ComponentStorage(private val myId: String, parent: ComponentStorage?) : Va
 
         val implicits = inspectDependenciesAndRegisterAdhoc(context, descriptors)
 
+        registry.resolveClashesIfAny(context.container, clashResolvers)
         injectProperties(context, descriptors + implicits)
     }
 

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/platform/JvmPlatformConfigurator.kt

@@ -6,9 +6,11 @@
 package org.jetbrains.kotlin.resolve.jvm.platform
 
 import org.jetbrains.kotlin.builtins.jvm.JavaToKotlinClassMap
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
 import org.jetbrains.kotlin.container.StorageComponentContainer
 import org.jetbrains.kotlin.container.useImpl
 import org.jetbrains.kotlin.container.useInstance
+import org.jetbrains.kotlin.load.java.components.SamConversionResolver
 import org.jetbrains.kotlin.load.java.sam.JvmSamConversionTransformer
 import org.jetbrains.kotlin.load.java.sam.SamConversionResolverImpl
 import org.jetbrains.kotlin.resolve.PlatformConfiguratorBase
@@ -18,7 +20,6 @@ import org.jetbrains.kotlin.resolve.jvm.*
 import org.jetbrains.kotlin.resolve.jvm.checkers.*
 import org.jetbrains.kotlin.resolve.jvm.multiplatform.JavaActualAnnotationArgumentExtractor
 import org.jetbrains.kotlin.synthetic.JavaSyntheticScopes
-import org.jetbrains.kotlin.types.DynamicTypesSettings
 import org.jetbrains.kotlin.types.expressions.FunctionWithBigAritySupport
 
 object JvmPlatformConfigurator : PlatformConfiguratorBase(
@@ -73,6 +74,10 @@ object JvmPlatformConfigurator : PlatformConfiguratorBase(
         ExplicitMetadataChecker
     ),
 
+    additionalClashResolvers = listOf(
+        PlatformExtensionsClashResolver.FallbackToDefault(SamConversionResolver.Empty, SamConversionResolver::class.java)
+    ),
+
     identifierChecker = JvmSimpleNameBacktickChecker,
 
     overloadFilter = JvmOverloadFilter,
@@ -98,6 +103,6 @@ object JvmPlatformConfigurator : PlatformConfiguratorBase(
         container.useImpl<JvmTypeSpecificityComparator>()
         container.useImpl<JvmDefaultSuperCallChecker>()
         container.useImpl<JvmSamConversionTransformer>()
-        container.useInstance(FunctionWithBigAritySupport.LANGUAGE_VERSION_DEPENDENT)
+        container.useInstance(FunctionWithBigAritySupport.LanguageVersionDependent)
     }
 }

compiler/frontend/src/org/jetbrains/kotlin/resolve/IdentifierChecker.kt

@@ -17,12 +17,14 @@
 package org.jetbrains.kotlin.resolve
 
 import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
+import org.jetbrains.kotlin.container.PlatformSpecificExtension
 import org.jetbrains.kotlin.diagnostics.DiagnosticSink
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtSimpleNameExpression
 
 @DefaultImplementation(impl = IdentifierChecker.Default::class)
-interface IdentifierChecker {
+interface IdentifierChecker : PlatformSpecificExtension<IdentifierChecker> {
     fun checkIdentifier(simpleNameExpression: KtSimpleNameExpression, diagnosticHolder: DiagnosticSink)
     fun checkDeclaration(declaration: KtDeclaration, diagnosticHolder: DiagnosticSink)
 
@@ -31,3 +33,18 @@ interface IdentifierChecker {
         override fun checkDeclaration(declaration: KtDeclaration, diagnosticHolder: DiagnosticSink) {}
     }
 }
+
+class IdentifierCheckerClashesResolver : PlatformExtensionsClashResolver<IdentifierChecker>(IdentifierChecker::class.java) {
+    override fun resolveExtensionsClash(extensions: List<IdentifierChecker>): IdentifierChecker = CompositeIdentifierChecker(extensions)
+}
+
+// Launches every underlying checker
+class CompositeIdentifierChecker(private val identifierCheckers: List<IdentifierChecker>) : IdentifierChecker {
+    override fun checkIdentifier(simpleNameExpression: KtSimpleNameExpression, diagnosticHolder: DiagnosticSink) {
+        identifierCheckers.forEach { it.checkIdentifier(simpleNameExpression, diagnosticHolder) }
+    }
+
+    override fun checkDeclaration(declaration: KtDeclaration, diagnosticHolder: DiagnosticSink) {
+        identifierCheckers.forEach { it.checkDeclaration(declaration, diagnosticHolder) }
+    }
+}

compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt

@@ -6,12 +6,9 @@
 package org.jetbrains.kotlin.resolve
 
 import org.jetbrains.kotlin.builtins.PlatformToKotlinClassMap
-import org.jetbrains.kotlin.container.StorageComponentContainer
-import org.jetbrains.kotlin.container.composeContainer
-import org.jetbrains.kotlin.container.useImpl
-import org.jetbrains.kotlin.container.useInstance
-import org.jetbrains.kotlin.resolve.calls.checkers.*
 import org.jetbrains.kotlin.container.*
+import org.jetbrains.kotlin.resolve.calls.checkers.*
+import org.jetbrains.kotlin.resolve.calls.results.TypeSpecificityComparator
 import org.jetbrains.kotlin.resolve.checkers.*
 import org.jetbrains.kotlin.resolve.lazy.DelegationFilter
 import org.jetbrains.kotlin.types.DynamicTypesSettings
@@ -55,6 +52,20 @@ private val DEFAULT_CLASSIFIER_USAGE_CHECKERS = listOf(
 )
 private val DEFAULT_ANNOTATION_CHECKERS = listOf<AdditionalAnnotationChecker>()
 
+private val DEFAULT_CLASH_RESOLVERS = listOf<PlatformExtensionsClashResolver<*>>(
+    IdentifierCheckerClashesResolver(),
+
+    /**
+     * We should use NONE for clash resolution, because:
+     * - JvmTypeSpecificityComparator covers cases with flexible types and primitive types loaded from Java, and all this is irrelevant for
+     *   non-JVM modules
+     * - JsTypeSpecificityComparator covers case with dynamics, which are not allowed in non-JS modules either
+     */
+    PlatformExtensionsClashResolver.FallbackToDefault(TypeSpecificityComparator.NONE, TypeSpecificityComparator::class.java),
+
+    PlatformExtensionsClashResolver.FallbackToDefault(DynamicTypesSettings(), DynamicTypesSettings::class.java)
+)
+
 
 abstract class PlatformConfiguratorBase(
     private val dynamicTypesSettings: DynamicTypesSettings? = null,
@@ -63,6 +74,7 @@ abstract class PlatformConfiguratorBase(
     additionalTypeCheckers: List<AdditionalTypeChecker> = emptyList(),
     additionalClassifierUsageCheckers: List<ClassifierUsageChecker> = emptyList(),
     additionalAnnotationCheckers: List<AdditionalAnnotationChecker> = emptyList(),
+    additionalClashResolvers: List<PlatformExtensionsClashResolver<*>> = emptyList(),
     private val identifierChecker: IdentifierChecker? = null,
     private val overloadFilter: OverloadFilter? = null,
     private val platformToKotlinClassMap: PlatformToKotlinClassMap? = null,
@@ -76,6 +88,7 @@ abstract class PlatformConfiguratorBase(
     private val classifierUsageCheckers: List<ClassifierUsageChecker> =
         DEFAULT_CLASSIFIER_USAGE_CHECKERS + additionalClassifierUsageCheckers
     private val annotationCheckers: List<AdditionalAnnotationChecker> = DEFAULT_ANNOTATION_CHECKERS + additionalAnnotationCheckers
+    private val clashResolvers: List<PlatformExtensionsClashResolver<*>> = DEFAULT_CLASH_RESOLVERS + additionalClashResolvers
 
     override val platformSpecificContainer = composeContainer(this::class.java.simpleName) {
         useInstanceIfNotNull(dynamicTypesSettings)
@@ -84,6 +97,7 @@ abstract class PlatformConfiguratorBase(
         typeCheckers.forEach { useInstance(it) }
         classifierUsageCheckers.forEach { useInstance(it) }
         annotationCheckers.forEach { useInstance(it) }
+        clashResolvers.forEach { useClashResolver(it) }
         useInstanceIfNotNull(identifierChecker)
         useInstanceIfNotNull(overloadFilter)
         useInstanceIfNotNull(platformToKotlinClassMap)

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/PlatformDiagnosticSuppressor.kt

@@ -17,11 +17,13 @@
 package org.jetbrains.kotlin.resolve.checkers
 
 import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
+import org.jetbrains.kotlin.container.PlatformSpecificExtension
 import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
 import org.jetbrains.kotlin.descriptors.VariableDescriptor
 
 @DefaultImplementation(PlatformDiagnosticSuppressor.Default::class)
-interface PlatformDiagnosticSuppressor {
+interface PlatformDiagnosticSuppressor : PlatformSpecificExtension<PlatformDiagnosticSuppressor>{
     fun shouldReportUnusedParameter(parameter: VariableDescriptor): Boolean
 
     fun shouldReportNoBody(descriptor: CallableMemberDescriptor): Boolean
@@ -32,3 +34,18 @@ interface PlatformDiagnosticSuppressor {
         override fun shouldReportNoBody(descriptor: CallableMemberDescriptor): Boolean = true
     }
 }
+
+class CompositePlatformDiagnosticSuppressor(private val suppressors: List<PlatformDiagnosticSuppressor>) : PlatformDiagnosticSuppressor {
+    override fun shouldReportUnusedParameter(parameter: VariableDescriptor): Boolean =
+        suppressors.all { it.shouldReportUnusedParameter(parameter) }
+
+    override fun shouldReportNoBody(descriptor: CallableMemberDescriptor): Boolean =
+        suppressors.all { it.shouldReportNoBody(descriptor) }
+}
+
+class PlatformDiagnosticSuppressorClashesResolver : PlatformExtensionsClashResolver<PlatformDiagnosticSuppressor>(
+    PlatformDiagnosticSuppressor::class.java
+) {
+    override fun resolveExtensionsClash(extensions: List<PlatformDiagnosticSuppressor>): PlatformDiagnosticSuppressor =
+        CompositePlatformDiagnosticSuppressor(extensions)
+}

compiler/frontend/src/org/jetbrains/kotlin/resolve/deprecation/deprecationUtil.kt

@@ -8,6 +8,8 @@ package org.jetbrains.kotlin.resolve.deprecation
 import com.intellij.psi.PsiElement
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
+import org.jetbrains.kotlin.container.PlatformSpecificExtension
 import org.jetbrains.kotlin.descriptors.annotations.AnnotationDescriptor
 import org.jetbrains.kotlin.diagnostics.Diagnostic
 import org.jetbrains.kotlin.diagnostics.Errors
@@ -61,7 +63,7 @@ internal fun createDeprecationDiagnostic(
 }
 
 @DefaultImplementation(CoroutineCompatibilitySupport::class)
-class CoroutineCompatibilitySupport private constructor(val enabled: Boolean) {
+class CoroutineCompatibilitySupport private constructor(val enabled: Boolean) : PlatformSpecificExtension<CoroutineCompatibilitySupport>{
     @Suppress("unused")
     constructor() : this(true)
 

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/DoubleColonExpressionResolver.kt

@@ -11,6 +11,8 @@ import org.jetbrains.kotlin.builtins.functions.FunctionInvokeDescriptor
 import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
+import org.jetbrains.kotlin.container.PlatformSpecificExtension
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.descriptors.impl.AnonymousFunctionDescriptor
@@ -820,14 +822,19 @@ class DoubleColonExpressionResolver(
     }
 }
 
-// By default, function types with big arity are supported. On platforms where they are not supported by default (e.g. JVM),
+/**
-// LANGUAGE_VERSION_DEPENDENT should be used which makes the code check if the corresponding language feature is enabled.
+ * By default, function types with big arity are supported. On platforms where they are not supported by default (e.g. JVM),
-@DefaultImplementation(FunctionWithBigAritySupport::class)
+ * [LanguageVersionDependent] should be used which makes the code check if the corresponding language feature is enabled.
-class FunctionWithBigAritySupport private constructor(val shouldCheckLanguageVersionSettings: Boolean) {
+ */
-    constructor() : this(false)
+@DefaultImplementation(FunctionWithBigAritySupport.Enabled::class)
+interface FunctionWithBigAritySupport {
+    val shouldCheckLanguageVersionSettings: Boolean
 
-    companion object {
+    object Enabled : FunctionWithBigAritySupport {
-        @JvmField
+        override val shouldCheckLanguageVersionSettings: Boolean = false
-        val LANGUAGE_VERSION_DEPENDENT = FunctionWithBigAritySupport(true)
     }
-}
+
+    object LanguageVersionDependent : FunctionWithBigAritySupport {
+        override val shouldCheckLanguageVersionSettings: Boolean = true
+    }
+}

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/results/FlatSignature.kt

@@ -17,6 +17,8 @@
 package org.jetbrains.kotlin.resolve.calls.results
 
 import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
+import org.jetbrains.kotlin.container.PlatformSpecificExtension
 import org.jetbrains.kotlin.descriptors.CallableDescriptor
 import org.jetbrains.kotlin.descriptors.MemberDescriptor
 import org.jetbrains.kotlin.descriptors.ValueParameterDescriptor
@@ -30,7 +32,7 @@ interface SpecificityComparisonCallbacks {
 }
 
 @DefaultImplementation(impl = TypeSpecificityComparator.NONE::class)
-interface TypeSpecificityComparator {
+interface TypeSpecificityComparator : PlatformSpecificExtension<TypeSpecificityComparator> {
     fun isDefinitelyLessSpecific(specific: KotlinTypeMarker, general: KotlinTypeMarker): Boolean
 
     object NONE : TypeSpecificityComparator {

core/descriptors.jvm/src/org/jetbrains/kotlin/load/java/components/SamConversionResolver.kt

@@ -17,11 +17,13 @@
 package org.jetbrains.kotlin.load.java.components
 
 import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
+import org.jetbrains.kotlin.container.PlatformSpecificExtension
 import org.jetbrains.kotlin.load.java.descriptors.JavaClassDescriptor
 import org.jetbrains.kotlin.types.SimpleType
 
 @DefaultImplementation(impl = SamConversionResolver.Empty::class)
-interface SamConversionResolver {
+interface SamConversionResolver : PlatformSpecificExtension<SamConversionResolver> {
     object Empty : SamConversionResolver {
         override fun resolveFunctionTypeIfSamInterface(classDescriptor: JavaClassDescriptor): SimpleType? = null
     }

core/descriptors/src/org/jetbrains/kotlin/types/dynamicTypes.kt

@@ -18,6 +18,8 @@ package org.jetbrains.kotlin.types
 
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.container.PlatformExtensionsClashResolver
+import org.jetbrains.kotlin.container.PlatformSpecificExtension
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.renderer.DescriptorRenderer
 import org.jetbrains.kotlin.renderer.DescriptorRendererOptions
@@ -25,7 +27,7 @@ import org.jetbrains.kotlin.types.model.DynamicTypeMarker
 import org.jetbrains.kotlin.types.typeUtil.builtIns
 
 @DefaultImplementation(impl = DynamicTypesSettings::class)
-open class DynamicTypesSettings {
+open class DynamicTypesSettings : PlatformSpecificExtension<DynamicTypesSettings> {
     open val dynamicTypesAllowed: Boolean
         get() = false
 }

core/util.runtime/src/org/jetbrains/kotlin/container/DefaultImplementation.kt

@@ -29,5 +29,7 @@ import kotlin.reflect.KClass
  *
  * Such configurations may arise, for example, for multiplatform modules: consider analyzing JVM+JS module, where JS contributes
  * default implementation of some particular service, and JVM contributes non-default.
+ *
+ * If you need more fine-grained control of clashes resolution, consider using [PlatformExtensionsClashResolver]
  **/
 annotation class DefaultImplementation(val impl: KClass<*>)

core/util.runtime/src/org/jetbrains/kotlin/container/PlatformSpecificExtension.kt

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.container
+
+/**
+ * This is a marker-interface for components which are needed for common resolve
+ * facilities (like resolve, or deserialization), but are platform-specific.
+ *
+ * PlatformSpecificExtensions has to be present in the container in exactly one
+ * instance (hence common pattern with providing no-op DEFAULT/EMPTY implementation
+ * in the corresponding interface)
+ *
+ * In multiplatform modules such components require special treatment. Namely,
+ * if several components of the same type are provided, then it's not an illegal state;
+ * rather, we have to carefully resolve clash on case-by-case basis.
+ * See also [PlatformExtensionsClashResolver].
+ */
+interface PlatformSpecificExtension<S : PlatformSpecificExtension<S>>
+
+/**
+ * Allows to specify which [PlatformSpecificExtension] should be used if there were two or more registrations
+ * for [applicableTo] class in the container.
+ *
+ * [PlatformExtensionsClashResolver] should be registred in the container via [useClashResolver]-extension.
+ *
+ * NB. YOU DON'T NEED this mechanism for the most popular case of "one or several default vs.
+ * zero or one non-default". Just use [DefaultImplementation], and default instances will be automatically
+ * discriminated (see respective KDoc).
+ * Use [PlatformExtensionsClashResolver] only for cases when you need more invloved logic.
+ *
+ * Example: [org.jetbrains.kotlin.resolve.IdentifierChecker]. It is used in platform-agnostic code,
+ * which resolves and checks identifiers for correctness. Each platform has it's own rules
+ * regarding identifier correctness. In MPP modules we can't choose only one IdentifierChecker;
+ * instead, we have to provide a "composite" IdentifierChecker which will launch checks of *each*
+ * platform.
+ *
+ */
+abstract class PlatformExtensionsClashResolver<E : PlatformSpecificExtension<E>>(val applicableTo: Class<E>) {
+    abstract fun resolveExtensionsClash(extensions: List<E>): E
+
+    class FallbackToDefault<E : PlatformSpecificExtension<E>>(
+        private val defaultValue: E,
+        applicableTo: Class<E>
+    ) : PlatformExtensionsClashResolver<E>(applicableTo) {
+
+        override fun resolveExtensionsClash(extensions: List<E>): E = defaultValue
+    }
+}
+