Add compatibility warning for SAM conversions on Kotlin functions

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/components/samConversionUtils.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.ParameterDescriptor
 import org.jetbrains.kotlin.incremental.record
+import org.jetbrains.kotlin.resolve.calls.inference.model.LowerPriorityToPreserveCompatibility
 import org.jetbrains.kotlin.resolve.calls.model.*
 import org.jetbrains.kotlin.resolve.sam.SAM_LOOKUP_NAME
 import org.jetbrains.kotlin.resolve.sam.getFunctionTypeForPossibleSamType
@@ -86,6 +87,10 @@ object SamTypeConversions : ParameterTypeConversion {
             SamConversionDescription(convertedTypeByOriginal, convertedTypeByCandidate!!)
         )
 
+        if (needCompatibilityResolve(candidate, expectedParameterType)) {
+            candidate.addDiagnostic(LowerPriorityToPreserveCompatibility)
+        }
+
         val samDescriptor = originalExpectedType.constructor.declarationDescriptor
         if (samDescriptor is ClassDescriptor) {
             callComponents.lookupTracker.record(candidate.scopeTower.location, samDescriptor, SAM_LOOKUP_NAME)
@@ -93,4 +98,13 @@ object SamTypeConversions : ParameterTypeConversion {
 
         return convertedTypeByCandidate
     }
+
+    private fun needCompatibilityResolve(candidate: KotlinResolutionCandidate, typeToConvert: UnwrappedType): Boolean {
+        // fun interfaces is a new feature with a new modifier, so no compatibility resolve is needed
+        val descriptor = typeToConvert.constructor.declarationDescriptor
+        if (descriptor is ClassDescriptor && descriptor.isFun) return false
+
+        // now conversions for Kotlin candidates are possible, so we have to perform compatibility resolve
+        return !candidate.callComponents.samConversionOracle.isJavaApplicableCandidate(candidate.resolvedCall.candidateDescriptor)
+    }
 }

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

@@ -19,8 +19,7 @@ package org.jetbrains.kotlin.resolve.calls.inference.model
 import org.jetbrains.kotlin.descriptors.TypeParameterDescriptor
 import org.jetbrains.kotlin.resolve.calls.model.*
 import org.jetbrains.kotlin.resolve.calls.tower.ResolutionCandidateApplicability
-import org.jetbrains.kotlin.resolve.calls.tower.ResolutionCandidateApplicability.INAPPLICABLE
-import org.jetbrains.kotlin.resolve.calls.tower.ResolutionCandidateApplicability.INAPPLICABLE_WRONG_RECEIVER
+import org.jetbrains.kotlin.resolve.calls.tower.ResolutionCandidateApplicability.*
 import org.jetbrains.kotlin.resolve.scopes.receivers.DetailedReceiver
 import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.model.KotlinTypeMarker
@@ -123,3 +122,5 @@ class ConstrainingTypeIsError(
     val constraintType: KotlinTypeMarker,
     val position: IncorporationConstraintPosition
 ) : ConstraintSystemCallDiagnostic(INAPPLICABLE)
+
+object LowerPriorityToPreserveCompatibility : ConstraintSystemCallDiagnostic(RESOLVED_NEED_PRESERVE_COMPATIBILITY)

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/tower/ImplicitScopeTower.kt

@@ -86,6 +86,7 @@ fun getResultApplicability(diagnostics: Collection<KotlinCallDiagnostic>) =
 enum class ResolutionCandidateApplicability {
     RESOLVED, // call success or has uncompleted inference or in other words possible successful candidate
     RESOLVED_WITH_ERROR, // call has error, but it is still successful from resolution perspective
+    RESOLVED_NEED_PRESERVE_COMPATIBILITY, // call resolved successfully, but using new features that changes resolve
     RESOLVED_LOW_PRIORITY,
     CONVENTION_ERROR, // missing infix, operator etc
     MAY_THROW_RUNTIME_ERROR, // unsafe call or unstable smart cast
@@ -120,6 +121,7 @@ class UsedSmartCastForDispatchReceiver(val smartCastType: KotlinType) : Resoluti
 object ErrorDescriptorDiagnostic : ResolutionDiagnostic(RESOLVED) // todo discuss and change to INAPPLICABLE
 object LowPriorityDescriptorDiagnostic : ResolutionDiagnostic(RESOLVED_LOW_PRIORITY)
 object DynamicDescriptorDiagnostic : ResolutionDiagnostic(RESOLVED_LOW_PRIORITY)
+object ResolvedUsingNewFeatures : ResolutionDiagnostic(RESOLVED_NEED_PRESERVE_COMPATIBILITY)
 object UnstableSmartCastDiagnostic : ResolutionDiagnostic(RESOLVED_WITH_ERROR)
 object HiddenExtensionRelatedToDynamicTypes : ResolutionDiagnostic(HIDDEN)
 object HiddenDescriptor : ResolutionDiagnostic(HIDDEN)