[FIR] Resolve infix calls to infix functions only

Implements INFIX_MODIFIER_REQUIRED diagnostics

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

@@ -153,15 +153,28 @@ class FirCallResolver(
         towerResolver.reset()
         val result = towerResolver.runResolver(info, transformer.resolutionContext)
         val bestCandidates = result.bestCandidates()
-        var reducedCandidates = if (!result.currentApplicability.isSuccess) {
-            bestCandidates.toSet()
-        } else {
+
+        fun chooseMostSpecific(): Set<Candidate> {
             val onSuperReference = (explicitReceiver as? FirQualifiedAccessExpression)?.calleeReference is FirSuperReference
-            conflictResolver.chooseMaximallySpecificCandidates(
+            return conflictResolver.chooseMaximallySpecificCandidates(
                 bestCandidates, discriminateGenerics = true, discriminateAbstracts = onSuperReference
             )
         }
 
+        var reducedCandidates = if (!result.currentApplicability.isSuccess) {
+            val distinctApplicabilities = bestCandidates.flatMapTo(mutableSetOf()) { candidate ->
+                candidate.diagnostics.map { it.applicability }
+            }
+            //if all candidates have the same kind on inApplicability - try to choose the most specific one
+            if (distinctApplicabilities.size == 1 && distinctApplicabilities.single() > CandidateApplicability.INAPPLICABLE) {
+                chooseMostSpecific()
+            } else {
+                bestCandidates.toSet()
+            }
+        } else {
+            chooseMostSpecific()
+        }
+
         reducedCandidates = overloadByLambdaReturnTypeResolver.reduceCandidates(qualifiedAccess, bestCandidates, reducedCandidates)
 
         return ResolutionResult(info, result.currentApplicability, reducedCandidates)

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

@@ -39,6 +39,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckDispatchReceiver,
         CheckExtensionReceiver,
         CheckArguments,
+        CheckInfixCall,
         EagerResolveOfCallableReferences,
         CheckLowPriorityInOverloadResolution,
         PostponedVariablesInitializerResolutionStage

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

@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.fir.declarations.FirFunction
 import org.jetbrains.kotlin.fir.declarations.FirValueParameter
 import org.jetbrains.kotlin.fir.expressions.FirExpression
 import org.jetbrains.kotlin.fir.expressions.FirNamedArgumentExpression
+import org.jetbrains.kotlin.fir.symbols.impl.FirNamedFunctionSymbol
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.resolve.ForbiddenNamedArgumentsTarget
 import org.jetbrains.kotlin.resolve.calls.tower.CandidateApplicability
@@ -101,4 +102,6 @@ class NullForNotNullType(
 
 class ManyLambdaExpressionArguments(
     val argument: FirExpression
-) : ResolutionDiagnostic(INAPPLICABLE_ARGUMENTS_MAPPING_ERROR)
+) : ResolutionDiagnostic(INAPPLICABLE_ARGUMENTS_MAPPING_ERROR)
+
+class InfixCallOfNonInfixFunction(val function: FirNamedFunctionSymbol) : ResolutionDiagnostic(INAPPLICABLE_MODIFIER)

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

@@ -9,15 +9,14 @@ import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.fir.FirSymbolOwner
 import org.jetbrains.kotlin.fir.FirVisibilityChecker
 import org.jetbrains.kotlin.fir.declarations.*
-import org.jetbrains.kotlin.fir.expressions.FirExpression
-import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccessExpression
-import org.jetbrains.kotlin.fir.expressions.FirResolvedQualifier
+import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.references.FirSuperReference
 import org.jetbrains.kotlin.fir.resolve.inference.*
 import org.jetbrains.kotlin.fir.resolve.toSymbol
 import org.jetbrains.kotlin.fir.symbols.SyntheticSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirCallableSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirFunctionSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirNamedFunctionSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
 import org.jetbrains.kotlin.fir.typeContext
 import org.jetbrains.kotlin.fir.types.*
@@ -267,3 +266,16 @@ internal object PostponedVariablesInitializerResolutionStage : ResolutionStage()
         }
     }
 }
+
+internal object CheckInfixCall : CheckerStage() {
+    override suspend fun check(candidate: Candidate, callInfo: CallInfo, sink: CheckerSink, context: ResolutionContext) {
+        if (callInfo.callSite is FirFunctionCall) {
+            if (callInfo.callSite.origin == FirFunctionCallOrigin.INFIX) {
+                val functionSymbol = candidate.symbol as? FirNamedFunctionSymbol ?: return
+                if (!functionSymbol.fir.isInfix) {
+                    sink.reportDiagnostic(InfixCallOfNonInfixFunction(functionSymbol))
+                }
+            }
+        }
+    }
+}

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirIntegerOperatorCall.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.fir.builder.FirBuilderDsl
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirArgumentList
 import org.jetbrains.kotlin.fir.expressions.FirExpression
+import org.jetbrains.kotlin.fir.expressions.FirFunctionCallOrigin
 import org.jetbrains.kotlin.fir.expressions.builder.FirCallBuilder
 import org.jetbrains.kotlin.fir.expressions.builder.FirExpressionBuilder
 import org.jetbrains.kotlin.fir.expressions.builder.FirQualifiedAccessBuilder
@@ -43,6 +44,7 @@ class FirIntegerOperatorCall @FirImplementationDetail constructor(
     extensionReceiver,
     argumentList,
     calleeReference,
+    FirFunctionCallOrigin.OPERATOR
 )
 
 @FirBuilderDsl