FIR: take lower priority into account during references EagerResolve

#KT-48158 Fixed

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/FirCallResolver.kt

@@ -339,7 +339,7 @@ class FirCallResolver(
     fun resolveCallableReference(
         constraintSystemBuilder: ConstraintSystemBuilder,
         resolvedCallableReferenceAtom: ResolvedCallableReferenceAtom,
-    ): Boolean {
+    ): Pair<CandidateApplicability, Boolean> {
         val callableReferenceAccess = resolvedCallableReferenceAtom.reference
         val lhs = resolvedCallableReferenceAtom.lhs
         val coneSubstitutor = constraintSystemBuilder.buildCurrentSubstitutor() as ConeSubstitutor
@@ -383,7 +383,7 @@ class FirCallResolver(
                     callableReferenceAccess.source
                 )
                 resolvedCallableReferenceAtom.resultingReference = errorReference
-                return false
+                return applicability to false
             }
             reducedCandidates.size > 1 -> {
                 if (resolvedCallableReferenceAtom.hasBeenPostponed) {
@@ -393,10 +393,10 @@ class FirCallResolver(
                         callableReferenceAccess.source
                     )
                     resolvedCallableReferenceAtom.resultingReference = errorReference
-                    return false
+                    return applicability to false
                 }
                 resolvedCallableReferenceAtom.hasBeenPostponed = true
-                return true
+                return applicability to true
             }
         }
 
@@ -417,7 +417,7 @@ class FirCallResolver(
         resolvedCallableReferenceAtom.resultingReference = reference
         resolvedCallableReferenceAtom.resultingTypeForCallableReference = chosenCandidate.resultingTypeForCallableReference
 
-        return true
+        return applicability to true
     }
 
     fun resolveDelegatingConstructorCall(

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ResolutionStages.kt

@@ -29,6 +29,7 @@ import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.resolve.calls.tasks.ExplicitReceiverKind.*
+import org.jetbrains.kotlin.resolve.calls.tower.CandidateApplicability
 import org.jetbrains.kotlin.types.AbstractNullabilityChecker
 
 abstract class ResolutionStage {
@@ -179,8 +180,15 @@ internal object EagerResolveOfCallableReferences : CheckerStage() {
         if (candidate.postponedAtoms.isEmpty()) return
         for (atom in candidate.postponedAtoms) {
             if (atom is ResolvedCallableReferenceAtom) {
-                if (!context.bodyResolveComponents.callResolver.resolveCallableReference(candidate.csBuilder, atom)) {
+                val (applicability, success) =
+                    context.bodyResolveComponents.callResolver.resolveCallableReference(candidate.csBuilder, atom)
+                if (!success) {
                     sink.yieldDiagnostic(InapplicableCandidate)
+                } else when (applicability) {
+                    CandidateApplicability.RESOLVED_NEED_PRESERVE_COMPATIBILITY ->
+                        candidate.addDiagnostic(LowerPriorityToPreserveCompatibilityDiagnostic)
+                    else -> {
+                    }
                 }
             }
         }

compiler/testData/diagnostics/tests/callableReference/unitAdaptationForReferenceCompatibility.fir.kt

@@ -20,6 +20,6 @@ public interface Executor {
 fun f(): String = "test"
 
 class A {
-    fun schedule1(e: Executor): Future<String> = <!RETURN_TYPE_MISMATCH, TYPE_MISMATCH!>e.submit(::f)<!>
+    fun schedule1(e: Executor): Future<String> = e.submit(::f)
     fun schedule2(e: Executor): Future<String> = <!RETURN_TYPE_MISMATCH, TYPE_MISMATCH!>e.submit { f() }<!>
 }