Add compatibility warning for SAM conversions

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

@@ -63,6 +63,11 @@ class CallableReferenceCandidate(
     val diagnostics: List<KotlinCallDiagnostic>
 ) : Candidate {
     override val resultingApplicability = getResultApplicability(diagnostics)
+
+    override fun addCompatibilityWarning(other: Candidate) {
+        // TODO: now only arguments can be converted, so CR candidates shouldn't be affected
+    }
+
     override val isSuccessful get() = resultingApplicability.isSuccess
 
     var freshSubstitutor: FreshVariableNewTypeSubstitutor? = null

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

@@ -249,3 +249,9 @@ class CandidateChosenUsingOverloadResolutionByLambdaAnnotation : KotlinCallDiagn
         reporter.onCall(this)
     }
 }
+
+class CompatibilityWarning : KotlinCallDiagnostic(RESOLVED) {
+    override fun report(reporter: DiagnosticReporter) {
+        reporter.onCall(this)
+    }
+}

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

@@ -165,6 +165,12 @@ class KotlinResolutionCandidate(
             return maxOf(currentApplicability, systemApplicability, variableApplicability)
         }
 
+    override fun addCompatibilityWarning(other: Candidate) {
+        if (this !== other) {
+            addDiagnostic(CompatibilityWarning())
+        }
+    }
+
     override fun toString(): String {
         val descriptor = DescriptorRenderer.COMPACT.render(resolvedCall.candidateDescriptor)
         val okOrFail = if (currentApplicability.isSuccess) "OK" else "FAIL"

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

@@ -35,6 +35,8 @@ interface Candidate {
     val isSuccessful: Boolean
 
     val resultingApplicability: ResolutionCandidateApplicability
+
+    fun addCompatibilityWarning(other: Candidate)
 }
 
 interface CandidateFactory<out C : Candidate> {
@@ -310,9 +312,25 @@ class TowerResolver {
 
         override fun getSuccessfulCandidates(): Collection<C>? {
             if (!isSuccessful) return null
-            val firstGroupWithResolved = candidateGroups.firstOrNull {
-                it.any(::isSuccessfulCandidate)
-            } ?: return null
+            var compatibilityCandidate: C? = null
+            var firstGroupWithResolved: Collection<C>? = null
+            outer@ for (group in candidateGroups) {
+                for (candidate in group) {
+                    if (isSuccessfulCandidate(candidate)) {
+                        firstGroupWithResolved = group
+                        break@outer
+                    }
+
+                    if (compatibilityCandidate == null && isSuccessfulPreserveCompatibility(candidate)) {
+                        compatibilityCandidate = candidate
+                    }
+                }
+            }
+
+            if (firstGroupWithResolved == null) return null
+            if (compatibilityCandidate != null) {
+                firstGroupWithResolved.forEach { it.addCompatibilityWarning(compatibilityCandidate) }
+            }
 
             return firstGroupWithResolved.filter(::isSuccessfulCandidate)
         }
@@ -322,6 +340,9 @@ class TowerResolver {
                     || candidate.resultingApplicability == ResolutionCandidateApplicability.RESOLVED_WITH_ERROR
         }
 
+        private fun isSuccessfulPreserveCompatibility(candidate: C): Boolean =
+            candidate.resultingApplicability == ResolutionCandidateApplicability.RESOLVED_NEED_PRESERVE_COMPATIBILITY
+
         override fun pushCandidates(candidates: Collection<C>) {
             val thereIsSuccessful = candidates.any { it.isSuccessful }
             if (!isSuccessful && !thereIsSuccessful) {