JVM_IR KT-48945 generate special bridge with unsubstituted signature

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/SpecialBridgeMethods.kt

@@ -24,6 +24,7 @@ data class SpecialMethodWithDefaultInfo(
     val defaultValueGenerator: (IrSimpleFunction) -> IrExpression,
     val argumentsToCheck: Int,
     val needsGenericSignature: Boolean = false,
+    val needsUnsubstitutedBridge: Boolean = false
 )
 
 class BuiltInWithDifferentJvmName(
@@ -78,11 +79,11 @@ class SpecialBridgeMethods(val context: CommonBackendContext) {
         makeDescription(StandardNames.FqNames.list, "lastIndexOf", 1) to
                 SpecialMethodWithDefaultInfo(::constMinusOne, 1),
         makeDescription(StandardNames.FqNames.map, "getOrDefault", 2) to
-                SpecialMethodWithDefaultInfo(::getSecondArg, 1, needsGenericSignature = true),
+                SpecialMethodWithDefaultInfo(::getSecondArg, 1, needsGenericSignature = true, needsUnsubstitutedBridge = true),
         makeDescription(StandardNames.FqNames.map, "get", 1) to
-                SpecialMethodWithDefaultInfo(::constNull, 1, needsGenericSignature = true),
+                SpecialMethodWithDefaultInfo(::constNull, 1, needsGenericSignature = true, needsUnsubstitutedBridge = true),
         makeDescription(StandardNames.FqNames.mutableMap, "remove", 1) to
-                SpecialMethodWithDefaultInfo(::constNull, 1, needsGenericSignature = true)
+                SpecialMethodWithDefaultInfo(::constNull, 1, needsGenericSignature = true, needsUnsubstitutedBridge = true)
     )
 
     private val specialProperties = mapOf(

compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/BridgeLowering.kt

@@ -265,18 +265,35 @@ internal class BridgeLowering(val context: JvmBackendContext) : FileLoweringPass
                                 irClass.addSpecialBridge(superBridge, superTarget)
                             }
                         }
-                        else -> irFunction
+                        else -> {
+                            irFunction
+                        }
                     }
 
                     blacklist += bridgeTarget.jvmMethod
                 }
 
-                if (irClass.functions.any { !it.isFakeOverride && it.name == irFunction.name && specialBridge.signature == it.jvmMethod }) {
-                    return irFunction
+                // Add special bridge with unsubstituted signature (as generated by old JVM back-end) if required.
+                // According to KT-40277 we generate special bridges for 'get', 'getOrDefault', and 'remove' with substituted return type.
+                // However, this introduces performance regressions and changes in behavior if cases like KT-48945
+                // (where a Kotlin class implements a read-only collection interface and extends a Java collection class).
+                val unsubstitutedSpecialBridge = specialBridge.unsubstitutedSpecialBridge
+                if (unsubstitutedSpecialBridge != null &&
+                    irClass.functions.none { it.isClashingWithPotentialBridge(irFunction.name, unsubstitutedSpecialBridge.signature) }
+                ) {
+                    blacklist += unsubstitutedSpecialBridge.signature
+                    // TODO what should be the special bridge target if we CAN NOT add a substituted special bridge
+                    //  (below, clashes with existing method),
+                    //  but CAN add an unsubstituted special bridge (here)?
+                    irClass.addSpecialBridge(unsubstitutedSpecialBridge, bridgeTarget)
                 }
 
-                blacklist += specialBridge.signature
-                return irClass.addSpecialBridge(specialBridge, bridgeTarget)
+                if (irClass.functions.none { it.isClashingWithPotentialBridge(irFunction.name, specialBridge.signature) }) {
+                    blacklist += specialBridge.signature
+                    return irClass.addSpecialBridge(specialBridge, bridgeTarget)
+                }
+
+                return irFunction
             }
 
             val specialBridgeTarget = getSpecialBridgeTargetAddingExtraBridges()
@@ -339,6 +356,10 @@ internal class BridgeLowering(val context: JvmBackendContext) : FileLoweringPass
             .forEach { irClass.addBridge(it, bridgeTarget) }
     }
 
+    private fun IrSimpleFunction.isClashingWithPotentialBridge(name: Name, signature: Method): Boolean =
+        (!this.isFakeOverride || this.modality == Modality.FINAL) && this.name == name && this.jvmMethod == signature
+
+
     // Returns the special bridge overridden by the current methods if it exists.
     private val IrSimpleFunction.specialBridgeOrNull: SpecialBridge?
         get() = context.bridgeLoweringCache.computeSpecialBridge(this)

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/SpecialBridge.kt

@@ -42,4 +42,9 @@ data class SpecialBridge(
     val isFinal: Boolean = true,
     val isSynthetic: Boolean = false,
     val isOverriding: Boolean = true,
+    // 'true' if we also should produce a synthetic bridge with unsubstituted signature.
+    // NB this is passed down the hierarchy to the point where 'unsubstitutedSpecialBridge' is created,
+    // see BridgeLoweringCache::computeSpecialBridge
+    val needsUnsubstitutedBridge: Boolean = false,
+    val unsubstitutedSpecialBridge: SpecialBridge? = null
 )

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/caches/BridgeLoweringCache.kt

@@ -54,7 +54,10 @@ class BridgeLoweringCache(private val context: JvmBackendContext) {
         val specialMethodInfo = specialBridgeMethods.getSpecialMethodInfo(function)
         if (specialMethodInfo != null)
             return SpecialBridge(
-                function, computeJvmMethod(function), specialMethodInfo.needsGenericSignature, methodInfo = specialMethodInfo
+                overridden = function, signature = computeJvmMethod(function),
+                needsGenericSignature = specialMethodInfo.needsGenericSignature,
+                methodInfo = specialMethodInfo,
+                needsUnsubstitutedBridge = specialMethodInfo.needsUnsubstitutedBridge
             )
 
         val specialBuiltInInfo = specialBridgeMethods.getBuiltInWithDifferentJvmName(function)
@@ -87,10 +90,22 @@ class BridgeLoweringCache(private val context: JvmBackendContext) {
                 parent = function.parent
             }
 
+            val substitutedOverrideSignature = computeJvmMethod(substitutedOverride)
+            val unsubstitutedSpecialBridge =
+                when {
+                    specialBridge.unsubstitutedSpecialBridge != null ->
+                        specialBridge.unsubstitutedSpecialBridge
+                    specialBridge.needsUnsubstitutedBridge && specialBridge.signature != substitutedOverrideSignature ->
+                        specialBridge.copy(isSynthetic = true)
+                    else ->
+                        null
+                }
+
             return specialBridge.copy(
-                signature = computeJvmMethod(substitutedOverride),
+                signature = substitutedOverrideSignature,
                 substitutedParameterTypes = substitutedParameterTypes,
-                substitutedReturnType = function.returnType
+                substitutedReturnType = function.returnType,
+                unsubstitutedSpecialBridge = unsubstitutedSpecialBridge
             )
         }