Add compatibility warning for reference adaptation

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

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.resolve.calls.components.CreateFreshVariablesSubstit
 import org.jetbrains.kotlin.resolve.calls.inference.ConstraintSystemOperation
 import org.jetbrains.kotlin.resolve.calls.inference.components.FreshVariableNewTypeSubstitutor
 import org.jetbrains.kotlin.resolve.calls.inference.model.ArgumentConstraintPosition
+import org.jetbrains.kotlin.resolve.calls.inference.model.LowerPriorityToPreserveCompatibility
 import org.jetbrains.kotlin.resolve.calls.model.*
 import org.jetbrains.kotlin.resolve.calls.tasks.ExplicitReceiverKind
 import org.jetbrains.kotlin.resolve.calls.tasks.ExplicitReceiverKind.DISPATCH_RECEIVER
@@ -60,12 +61,17 @@ class CallableReferenceCandidate(
     val explicitReceiverKind: ExplicitReceiverKind,
     val reflectionCandidateType: UnwrappedType,
     val callableReferenceAdaptation: CallableReferenceAdaptation?,
-    val diagnostics: List<KotlinCallDiagnostic>
+    initialDiagnostics: List<KotlinCallDiagnostic>
 ) : Candidate {
+    private val mutableDiagnostics = initialDiagnostics.toMutableList()
+    val diagnostics: List<KotlinCallDiagnostic> = mutableDiagnostics
+
     override val resultingApplicability = getResultApplicability(diagnostics)
 
     override fun addCompatibilityWarning(other: Candidate) {
-        // TODO: now only arguments can be converted, so CR candidates shouldn't be affected
+        if (this !== other) {
+            mutableDiagnostics.add(CompatibilityWarning())
+        }
     }
 
     override val isSuccessful get() = resultingApplicability.isSuccess
@@ -207,6 +213,10 @@ class CallableReferencesCandidateFactory(
             callComponents.builtIns
         )
 
+        if (needCompatibilityWarning(callableReferenceAdaptation)) {
+            diagnostics.add(LowerPriorityToPreserveCompatibility)
+        }
+
         if (callableReferenceAdaptation != null &&
             callableReferenceAdaptation.defaults != 0 &&
             !callComponents.languageVersionSettings.supportsFeature(LanguageFeature.FunctionReferenceWithDefaultValueAsOtherType)
@@ -251,6 +261,15 @@ class CallableReferencesCandidateFactory(
         )
     }
 
+    private fun needCompatibilityWarning(callableReferenceAdaptation: CallableReferenceAdaptation?): Boolean {
+        if (callableReferenceAdaptation == null) return false
+
+        return callableReferenceAdaptation.defaults != 0 ||
+                callableReferenceAdaptation.suspendConversionStrategy != SuspendConversionStrategy.NO_CONVERSION ||
+                callableReferenceAdaptation.coercionStrategy != CoercionStrategy.NO_COERCION ||
+                callableReferenceAdaptation.mappedArguments.values.any { it is ResolvedCallArgument.VarargArgument }
+    }
+
     private enum class VarargMappingState {
         UNMAPPED, MAPPED_WITH_PLAIN_ARGS, MAPPED_WITH_ARRAY
     }

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

@@ -106,7 +106,13 @@ class CallableReferenceResolver(
                 )
             }
             diagnosticsHolder.addDiagnosticIfNotNull(diagnostic)
-            chosenCandidate.diagnostics.forEach { diagnosticsHolder.addDiagnostic(it) }
+            chosenCandidate.diagnostics.forEach {
+                val transformedDiagnostic = when (it) {
+                    is CompatibilityWarning -> CompatibilityWarningOnArgument(argument)
+                    else -> it
+                }
+                diagnosticsHolder.addDiagnostic(transformedDiagnostic)
+            }
             chosenCandidate.freshSubstitutor = toFreshSubstitutor
         } else {
             if (candidates.isEmpty()) {

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

@@ -254,4 +254,10 @@ class CompatibilityWarning : KotlinCallDiagnostic(RESOLVED) {
     override fun report(reporter: DiagnosticReporter) {
         reporter.onCall(this)
     }
+}
+
+class CompatibilityWarningOnArgument(val argument: CallableReferenceKotlinCallArgument) : KotlinCallDiagnostic(RESOLVED) {
+    override fun report(reporter: DiagnosticReporter) {
+        reporter.onCallArgument(argument, this)
+    }
 }