Add compatibility warning for SAM conversions

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -761,6 +761,7 @@ public interface Errors {
     DiagnosticFactory1<PsiElement, KotlinType> REIFIED_TYPE_UNSAFE_SUBSTITUTION = DiagnosticFactory1.create(WARNING);
     DiagnosticFactory0<KtElement> CANDIDATE_CHOSEN_USING_OVERLOAD_RESOLUTION_BY_LAMBDA_ANNOTATION = DiagnosticFactory0.create(WARNING);
 
+    DiagnosticFactory0<KtElement> COMPATIBILITY_WARNING = DiagnosticFactory0.create(WARNING);
 
     DiagnosticFactory3<KtReferenceExpression, ClassifierDescriptor, WrongResolutionToClassifier, String> RESOLUTION_TO_CLASSIFIER =
             DiagnosticFactory3.create(ERROR);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -898,6 +898,8 @@ public class DefaultErrorMessages {
         MAP.put(TYPE_PARAMETERS_NOT_ALLOWED, "Type parameters are not allowed here");
         MAP.put(CANDIDATE_CHOSEN_USING_OVERLOAD_RESOLUTION_BY_LAMBDA_ANNOTATION, "Candidate was choosen only by @OverloadResolutionByLambdaReturnType annotation");
 
+        MAP.put(COMPATIBILITY_WARNING, "Candidate was chosen to preserve compatibility");
+
         MAP.put(TYPE_PARAMETER_OF_PROPERTY_NOT_USED_IN_RECEIVER, "Type parameter of a property must be used in its receiver type");
 
         MAP.put(SUPERTYPES_FOR_ANNOTATION_CLASS, "Annotation class cannot have supertypes");

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/DiagnosticReporterByTrackingStrategy.kt

@@ -80,6 +80,9 @@ class DiagnosticReporterByTrackingStrategy(
             CandidateChosenUsingOverloadResolutionByLambdaAnnotation::class.java -> {
                 trace.report(CANDIDATE_CHOSEN_USING_OVERLOAD_RESOLUTION_BY_LAMBDA_ANNOTATION.on(psiKotlinCall.psiCall.callElement))
             }
+            CompatibilityWarning::class.java -> {
+                trace.report(COMPATIBILITY_WARNING.on(psiKotlinCall.psiCall.callElement))
+            }
         }
     }
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/tower/NewResolutionOldInference.kt

@@ -422,6 +422,10 @@ class NewResolutionOldInference(
             getResultApplicability(diagnostics)
         }
 
+        override fun addCompatibilityWarning(other: Candidate) {
+            // Only applicable for new inference
+        }
+
         override val isSuccessful = getResultApplicability(eagerDiagnostics).isSuccess
     }