IR: support IrExternalOverridabilityCondition in IrOverrideChecker

The code is analogous to OverridingUtil and
ExternalOverridabilityCondition.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/actualizer/ActualFakeOverridesAdder.kt

@@ -37,7 +37,7 @@ internal class ActualFakeOverridesAdder(
     private val diagnosticsReporter: KtDiagnosticReporterWithImplicitIrBasedContext,
     private val typeSystemContext: IrTypeSystemContext
 ) : IrElementVisitorVoid {
-    private val overrideChecker = IrOverrideChecker(typeSystemContext)
+    private val overrideChecker = IrOverrideChecker(typeSystemContext, emptyList())
     private val missingActualMembersMap = mutableMapOf<IrClass, FakeOverrideInfo>()
 
     override fun visitClass(declaration: IrClass) {
@@ -93,7 +93,7 @@ internal class ActualFakeOverridesAdder(
                 @Suppress("UNCHECKED_CAST")
                 val override = klass.declarations.firstOrNull {
                     it is IrOverridableMember &&
-                            overrideChecker.isOverridableByWithoutExternalConditions(
+                            overrideChecker.isOverridableBy(
                                 superMember = memberFromSupertype,
                                 subMember = it,
                                 checkIsInlineFlag = false,

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/actualizer/FakeOverrideRebuilder.kt

@@ -76,7 +76,8 @@ class FakeOverrideRebuilder(
         mangler,
         typeSystemContext,
         friendModules,
-        PartialLinkageSupportForLinker.DISABLED
+        PartialLinkageSupportForLinker.DISABLED,
+        externalOverridabilityConditions = emptyList(), // TODO: KT-61370, KT-61804.
     )
 
     fun rebuildFakeOverrides() {

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/overrides/IrExternalOverridabilityCondition.kt

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.ir.overrides
+
+import org.jetbrains.kotlin.ir.declarations.IrOverridableMember
+
+interface IrExternalOverridabilityCondition {
+    enum class Result {
+        OVERRIDABLE,
+        CONFLICT,
+        INCOMPATIBLE,
+        UNKNOWN
+    }
+
+    enum class Contract {
+        CONFLICTS_ONLY,
+        SUCCESS_ONLY,
+        BOTH
+    }
+
+    /**
+     * Determines whether [superMember] is overridable by [subMember]. The returned result can be one of the following:
+     *
+     *   - [Result.OVERRIDABLE] means that it is definitely overridable, and neither the general override checking algorithm, nor other
+     *     external overridability conditions can refute that.
+     *   - [Result.CONFLICT] means that it is an error to have both declarations available in the same class, so the compiler should report
+     *     an error.
+     *   - [Result.INCOMPATIBLE] means that it is not overridable, but it's OK to have both declarations available in the same class because
+     *     they don't cause a conflict.
+     *   - [Result.UNKNOWN] means that this overridability condition cannot claim anything about the overridability, and the final result
+     *     will be based on the general override checking algorithm and the other external overridability conditions.
+     */
+    fun isOverridable(
+        superMember: IrOverridableMember,
+        subMember: IrOverridableMember,
+    ): Result
+
+    /**
+     * Specifies what values can be returned by [isOverridable]. Used as an optimization to reorder overridability conditions to prevent
+     * redundant calculations.
+     *
+     *   - [Contract.CONFLICTS_ONLY] means that [isOverridable] can return [Result.CONFLICT], [Result.INCOMPATIBLE] or [Result.UNKNOWN].
+     *   - [Contract.SUCCESS_ONLY] means that [isOverridable] can return [Result.OVERRIDABLE] or [Result.UNKNOWN].
+     *   - [Contract.BOTH] means that [isOverridable] can return any result.
+     */
+    val contract: Contract
+}

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/overrides/IrOverrideChecker.kt

@@ -6,6 +6,9 @@
 package org.jetbrains.kotlin.ir.overrides
 
 import org.jetbrains.kotlin.ir.declarations.*
+import org.jetbrains.kotlin.ir.overrides.IrExternalOverridabilityCondition.Contract.CONFLICTS_ONLY
+import org.jetbrains.kotlin.ir.overrides.IrExternalOverridabilityCondition.Contract.SUCCESS_ONLY
+import org.jetbrains.kotlin.ir.overrides.IrExternalOverridabilityCondition.Result.*
 import org.jetbrains.kotlin.ir.types.IrTypeSystemContext
 import org.jetbrains.kotlin.ir.types.IrTypeSystemContextWithAdditionalAxioms
 import org.jetbrains.kotlin.ir.types.createIrTypeCheckerState
@@ -13,7 +16,10 @@ import org.jetbrains.kotlin.resolve.OverridingUtil.OverrideCompatibilityInfo
 import org.jetbrains.kotlin.resolve.OverridingUtil.OverrideCompatibilityInfo.*
 import org.jetbrains.kotlin.types.AbstractTypeChecker
 
-class IrOverrideChecker(private val typeSystem: IrTypeSystemContext) {
+class IrOverrideChecker(
+    private val typeSystem: IrTypeSystemContext,
+    private val externalOverridabilityConditions: List<IrExternalOverridabilityCondition>,
+) {
     fun getBothWaysOverridability(
         overriderDescriptor: IrOverridableMember,
         candidateDescriptor: IrOverridableMember,
@@ -38,11 +44,12 @@ class IrOverrideChecker(private val typeSystem: IrTypeSystemContext) {
         subMember: IrOverridableMember,
         checkIsInlineFlag: Boolean,
     ): OverrideCompatibilityInfo {
-        return isOverridableByWithoutExternalConditions(superMember, subMember, checkIsInlineFlag)
-        // The frontend goes into external overridability condition details here, but don't deal with them in IR (yet?).
+        val basicResult = isOverridableByWithoutExternalConditions(superMember, subMember, checkIsInlineFlag)
+
+        return runExternalOverridabilityConditions(superMember, subMember, basicResult)
     }
 
-    fun isOverridableByWithoutExternalConditions(
+    private fun isOverridableByWithoutExternalConditions(
         superMember: IrOverridableMember,
         subMember: IrOverridableMember,
         checkIsInlineFlag: Boolean,
@@ -122,6 +129,48 @@ class IrOverrideChecker(private val typeSystem: IrTypeSystemContext) {
 
         return success()
     }
+
+    private fun runExternalOverridabilityConditions(
+        superMember: IrOverridableMember,
+        subMember: IrOverridableMember,
+        basicResult: OverrideCompatibilityInfo,
+    ): OverrideCompatibilityInfo {
+        var wasSuccess = basicResult.result == OverrideCompatibilityInfo.Result.OVERRIDABLE
+
+        for (externalCondition in externalOverridabilityConditions) {
+            // Do not run CONFLICTS_ONLY while there was no success
+            if (externalCondition.contract == CONFLICTS_ONLY) continue
+            if (wasSuccess && externalCondition.contract == SUCCESS_ONLY) continue
+            val result =
+                externalCondition.isOverridable(superMember, subMember)
+            when (result) {
+                OVERRIDABLE -> wasSuccess = true
+                CONFLICT -> return conflict("External condition failed")
+                INCOMPATIBLE -> return incompatible("External condition")
+                UNKNOWN -> {}
+            }
+        }
+
+        if (!wasSuccess) return basicResult
+
+        // Search for conflicts from external conditions
+        for (externalCondition in externalOverridabilityConditions) {
+            // Run all conditions that was not run before (i.e. CONFLICTS_ONLY)
+            if (externalCondition.contract != CONFLICTS_ONLY) continue
+            val result =
+                externalCondition.isOverridable(superMember, subMember)
+            when (result) {
+                CONFLICT -> return conflict("External condition failed")
+                INCOMPATIBLE -> return incompatible("External condition")
+                OVERRIDABLE -> error(
+                    "Contract violation in ${externalCondition.javaClass} condition. It's not supposed to end with success"
+                )
+                UNKNOWN -> {}
+            }
+        }
+
+        return success()
+    }
 }
 
 private val IrSimpleFunction?.hasExtensionReceiver: Boolean

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/overrides/IrOverridingUtil.kt

@@ -33,9 +33,10 @@ import org.jetbrains.kotlin.utils.memoryOptimizedMapNotNull
 
 class IrOverridingUtil(
     private val typeSystem: IrTypeSystemContext,
-    private val fakeOverrideBuilder: FakeOverrideBuilderStrategy
+    private val fakeOverrideBuilder: FakeOverrideBuilderStrategy,
+    private val externalOverridabilityConditions: List<IrExternalOverridabilityCondition>,
 ) {
-    private val overrideChecker = IrOverrideChecker(typeSystem)
+    private val overrideChecker = IrOverrideChecker(typeSystem, externalOverridabilityConditions)
     private val originals = mutableMapOf<IrOverridableMember, IrOverridableMember>()
     private val IrOverridableMember.original get() = originals[this] ?: error("No original for ${this.render()}")
     private val originalSuperTypes = mutableMapOf<IrOverridableMember, IrType>()

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrUtils.kt

@@ -1219,7 +1219,7 @@ fun IrClass.addFakeOverrides(
     implementedMembers: List<IrOverridableMember> = emptyList(),
     ignoredParentSymbols: List<IrSymbol> = emptyList()
 ) {
-    IrOverridingUtil(typeSystem, FakeOverrideBuilderForLowerings())
+    IrOverridingUtil(typeSystem, FakeOverrideBuilderForLowerings(), emptyList())
         .buildFakeOverridesForClassUsingOverriddenSymbols(this,
                                                           implementedMembers = implementedMembers,
                                                           compatibilityMode = false,

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/overrides/FakeOverrides.kt

@@ -29,6 +29,7 @@ import org.jetbrains.kotlin.ir.builders.declarations.buildTypeParameter
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.linkage.partial.IrUnimplementedOverridesStrategy.ProcessAsFakeOverrides
 import org.jetbrains.kotlin.ir.overrides.FakeOverrideBuilderStrategy
+import org.jetbrains.kotlin.ir.overrides.IrExternalOverridabilityCondition
 import org.jetbrains.kotlin.ir.overrides.IrOverridingUtil
 import org.jetbrains.kotlin.ir.symbols.IrPropertySymbol
 import org.jetbrains.kotlin.ir.symbols.IrSimpleFunctionSymbol
@@ -81,7 +82,8 @@ class FakeOverrideBuilder(
     val platformSpecificClassFilter: FakeOverrideClassFilter = DefaultFakeOverrideClassFilter,
     private val fakeOverrideDeclarationTable: DeclarationTable = FakeOverrideDeclarationTable(mangler) { builder, table ->
         IdSignatureSerializer(builder, table)
-    }
+    },
+    externalOverridabilityConditions: List<IrExternalOverridabilityCondition> = emptyList(),
 ) : FakeOverrideBuilderStrategy(
     friendModules = friendModules,
     unimplementedOverridesStrategy = if (partialLinkageSupport.isEnabled)
@@ -91,7 +93,7 @@ class FakeOverrideBuilder(
 ) {
     private val haveFakeOverrides = mutableSetOf<IrClass>()
 
-    private val irOverridingUtil = IrOverridingUtil(typeSystem, this)
+    private val irOverridingUtil = IrOverridingUtil(typeSystem, this, externalOverridabilityConditions)
     private val irBuiltIns = typeSystem.irBuiltIns
 
     // TODO: The declaration table is needed for the signaturer.