[FIR] Check if type variable is bounded in upper by incompatible types (resolution stage)

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

@@ -20,7 +20,8 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckDslScopeViolation,
         CheckLowPriorityInOverloadResolution,
         PostponedVariablesInitializerResolutionStage,
-        ConstraintSystemForks
+        ConstraintSystemForks,
+        CheckIncompatibleTypeVariableUpperBounds,
     )
 
     object SyntheticSelect : CallKind(
@@ -31,6 +32,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckArguments,
         EagerResolveOfCallableReferences,
         ConstraintSystemForks,
+        CheckIncompatibleTypeVariableUpperBounds,
     )
 
     object Function : CallKind(
@@ -52,6 +54,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckLowPriorityInOverloadResolution,
         PostponedVariablesInitializerResolutionStage,
         ConstraintSystemForks,
+        CheckIncompatibleTypeVariableUpperBounds,
     )
 
     object DelegatingConstructorCall : CallKind(
@@ -68,6 +71,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckArguments,
         EagerResolveOfCallableReferences,
         ConstraintSystemForks,
+        CheckIncompatibleTypeVariableUpperBounds,
     )
 
     object CallableReference : CallKind(
@@ -82,6 +86,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckDslScopeViolation,
         CheckCallableReferenceExpectedType,
         CheckLowPriorityInOverloadResolution,
+        CheckIncompatibleTypeVariableUpperBounds,
     )
 
     object SyntheticIdForCallableReferencesResolution : CallKind(
@@ -92,6 +97,7 @@ sealed class CallKind(vararg resolutionSequence: ResolutionStage) {
         CheckArguments,
         EagerResolveOfCallableReferences,
         ConstraintSystemForks,
+        CheckIncompatibleTypeVariableUpperBounds,
     )
 
     internal class CustomForIde(vararg resolutionSequence: ResolutionStage) : CallKind(*resolutionSequence)

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

@@ -576,6 +576,27 @@ internal object CheckLowPriorityInOverloadResolution : CheckerStage() {
     }
 }
 
+internal object CheckIncompatibleTypeVariableUpperBounds : ResolutionStage() {
+    override suspend fun check(candidate: Candidate, callInfo: CallInfo, sink: CheckerSink, context: ResolutionContext) =
+        with(candidate.system.asConstraintSystemCompleterContext()) {
+            val typeVariables = candidate.system.notFixedTypeVariables.values
+
+            for (variableWithConstraints in typeVariables) {
+                val upperTypes = variableWithConstraints.constraints.extractUpperTypes()
+
+                if (upperTypes.isEmptyIntersection()) {
+                    sink.yieldDiagnostic(
+                        @Suppress("UNCHECKED_CAST")
+                        InferredEmptyIntersectionDiagnostic(
+                            upperTypes as Collection<ConeKotlinType>,
+                            variableWithConstraints.typeVariable as ConeTypeVariable
+                        )
+                    )
+                }
+            }
+        }
+}
+
 internal object PostponedVariablesInitializerResolutionStage : ResolutionStage() {
 
     override suspend fun check(candidate: Candidate, callInfo: CallInfo, sink: CheckerSink, context: ResolutionContext) {

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -261,7 +261,6 @@ enum class LanguageFeature(
     ForbidUsingExtensionPropertyTypeParameterInDelegate(KOTLIN_1_8, kind = BUG_FIX),
     ModifierNonBuiltinSuspendFunError(KOTLIN_1_8),
     SynchronizedSuspendError(KOTLIN_1_8),
-    ForbidInferringTypeVariablesIntoEmptyIntersection(KOTLIN_1_8, kind = BUG_FIX), // KT-51221
 
     // 1.9
 
@@ -275,6 +274,7 @@ enum class LanguageFeature(
     StopPropagatingDeprecationThroughOverrides(KOTLIN_1_9, kind = BUG_FIX), // KT-47902
     ReportTypeVarianceConflictOnQualifierArguments(KOTLIN_1_9, kind = BUG_FIX), // KT-50947
     ReportErrorsOnRecursiveTypeInsidePlusAssignment(KOTLIN_1_9, kind = BUG_FIX), // KT-48546
+    ForbidInferringTypeVariablesIntoEmptyIntersection(KOTLIN_1_9, kind = BUG_FIX), // KT-51221
 
     // Disabled for indefinite time. See KT-48535 and related discussion
     ApproximateIntegerLiteralTypesInReceiverPosition(sinceVersion = null),