[NI] Discriminate Nothing? result type from nullability constraints

Nullability constraints should not be chosen when proper
argument constraints for variables in contravariant position present.

^KT-32106 Fixed
^KT-33166 Fixed

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

@@ -175,7 +175,10 @@ class ConstraintIncorporator(
         val inputTypePosition = if (baseConstraint.position.from is OnlyInputTypeConstraintPosition)
             baseConstraint.position else null
 
-        addNewIncorporatedConstraint(targetVariable, newConstraint, ConstraintContext(kind, derivedFrom, inputTypePosition))
+        val isNullabilityConstraint = isUsefulForNullabilityConstraint && newConstraint.isNullableNothing()
+        val constraintContext = ConstraintContext(kind, derivedFrom, inputTypePosition, isNullabilityConstraint)
+
+        addNewIncorporatedConstraint(targetVariable, newConstraint, constraintContext)
     }
 
     fun Context.containsConstrainingTypeWithoutProjection(

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

@@ -178,7 +178,7 @@ class ConstraintInjector(
             val typeVariable = c.allTypeVariables[typeVariableConstructor]
                 ?: error("Should by type variableConstructor: $typeVariableConstructor. ${c.allTypeVariables.values}")
 
-            addNewIncorporatedConstraint(typeVariable, type, ConstraintContext(kind, emptySet()))
+            addNewIncorporatedConstraint(typeVariable, type, ConstraintContext(kind, emptySet(), isNullabilityConstraint = false))
         }
 
         // from ConstraintIncorporator.Context
@@ -194,7 +194,7 @@ class ConstraintInjector(
             type: KotlinTypeMarker,
             constraintContext: ConstraintContext
         ) {
-            val (kind, derivedFrom, inputTypePosition) = constraintContext
+            val (kind, derivedFrom, inputTypePosition, isNullabilityConstraint) = constraintContext
 
             var targetType = type
             if (targetType.isUninferredParameter()) {
@@ -231,7 +231,12 @@ class ConstraintInjector(
                 }
             }
 
-            possibleNewConstraints.add(typeVariable to Constraint(kind, targetType, position, derivedFrom = derivedFrom))
+            val newConstraint = Constraint(
+                kind, targetType, position,
+                derivedFrom = derivedFrom,
+                isNullabilityConstraint = isNullabilityConstraint
+            )
+            possibleNewConstraints.add(typeVariable to newConstraint)
             inputTypePosition?.let { preservedInputTypePositions.add(it) }
         }
 
@@ -264,5 +269,6 @@ class ConstraintInjector(
 data class ConstraintContext(
     val kind: ConstraintKind,
     val derivedFrom: Set<TypeVariableMarker>,
-    val inputTypePosition: ConstraintPosition? = null
+    val inputTypePosition: ConstraintPosition? = null,
+    val isNullabilityConstraint: Boolean
 )

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

@@ -80,23 +80,27 @@ class ResultTypeResolver(
     }
 
     private fun Context.isSuitableType(resultType: KotlinTypeMarker, variableWithConstraints: VariableWithConstraints): Boolean {
-        for (constraint in variableWithConstraints.constraints) {
-            if (!isProperType(constraint.type)) continue
+        val filteredConstraints = variableWithConstraints.constraints.filter { isProperType(it.type) }
+        for (constraint in filteredConstraints) {
             if (!checkConstraint(this, constraint.type, constraint.kind, resultType)) return false
         }
-        if (
-            !trivialConstraintTypeInferenceOracle.isSuitableResultedType(resultType)
-            && isNothingNotSuitableFor(variableWithConstraints.typeVariable)
-        ) return false
+        if (!trivialConstraintTypeInferenceOracle.isSuitableResultedType(resultType)) {
+            if (nothingIsForbiddenFor(variableWithConstraints.typeVariable)) return false
+            if (resultType.isNullableType() && checkSingleLowerNullabilityConstraint(filteredConstraints)) return false
+        }
 
         return true
     }
 
-    private fun isNothingNotSuitableFor(variable: TypeVariableMarker): Boolean {
+    private fun nothingIsForbiddenFor(variable: TypeVariableMarker): Boolean {
         val parameterName = variable.safeAs<TypeVariableFromCallableDescriptor>()?.originalTypeParameter?.name ?: return false
         val isSpecialFunctionParameter = statelessCallbacks?.isSpecialFunctionTypeParameterName(parameterName) ?: false
-        val isBangBangParameter = isSpecialFunctionParameter && statelessCallbacks?.isExclExclTypeParameterName(parameterName) ?: false
-        return isSpecialFunctionParameter && !isBangBangParameter
+        val isFromExclExcl = isSpecialFunctionParameter && statelessCallbacks?.isExclExclTypeParameterName(parameterName) ?: false
+        return isSpecialFunctionParameter && !isFromExclExcl
+    }
+
+    private fun checkSingleLowerNullabilityConstraint(constraints: List<Constraint>): Boolean {
+        return constraints.singleOrNull { it.kind.isLower() }?.isNullabilityConstraint ?: false
     }
 
     private fun Context.findSubType(variableWithConstraints: VariableWithConstraints): KotlinTypeMarker? {

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

@@ -68,7 +68,8 @@ class Constraint(
     val type: KotlinTypeMarker, // flexible types here is allowed
     val position: IncorporationConstraintPosition,
     val typeHashCode: Int = type.hashCode(),
-    val derivedFrom: Set<TypeVariableMarker>
+    val derivedFrom: Set<TypeVariableMarker>,
+    val isNullabilityConstraint: Boolean
 ) {
     override fun equals(other: Any?): Boolean {
         if (this === other) return true

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

@@ -68,7 +68,8 @@ class MutableVariableWithConstraints(
                 constraint.type,
                 constraint.position,
                 constraint.typeHashCode,
-                derivedFrom = constraint.derivedFrom
+                derivedFrom = constraint.derivedFrom,
+                isNullabilityConstraint = false
             )
         else
             constraint