[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/fir/resolve/tests/org/jetbrains/kotlin/fir/FirOldFrontendDiagnosticsTestGenerated.java

@@ -10604,6 +10604,16 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirOldFronte
             public void testNontrivialCallExpression() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/nontrivialCallExpression.kt");
             }
+
+            @TestMetadata("preferArgumentToNullability.kt")
+            public void testPreferArgumentToNullability() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/preferArgumentToNullability.kt");
+            }
+
+            @TestMetadata("preferNothingToBound.kt")
+            public void testPreferNothingToBound() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/preferNothingToBound.kt");
+            }
         }
 
         @TestMetadata("compiler/testData/diagnostics/tests/inference/nothingType")

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

compiler/testData/diagnostics/tests/inference/nestedCalls/preferArgumentToNullability.fir.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +NewInference
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+open class Cls
+
+abstract class In<in I>
+class SubIn : In<Cls?>()
+
+inline fun <reified T : Cls> materialize(): T? = TODO()
+
+fun <D> transform(transformer: In<D>, data: D): Unit = TODO()
+
+fun test(subIn: SubIn) {
+    transform(subIn, materialize()) // D should be inferred to Cls?, not Nothing?
+}

compiler/testData/diagnostics/tests/inference/nestedCalls/preferArgumentToNullability.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +NewInference
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+open class Cls
+
+abstract class In<in I>
+class SubIn : In<Cls?>()
+
+inline fun <reified T : Cls> materialize(): T? = TODO()
+
+fun <D> transform(transformer: In<D>, data: D): Unit = TODO()
+
+fun test(subIn: SubIn) {
+    transform(subIn, materialize()) // D should be inferred to Cls?, not Nothing?
+}

compiler/testData/diagnostics/tests/inference/nestedCalls/preferArgumentToNullability.txt

@@ -0,0 +1,26 @@
+package
+
+public inline fun </*0*/ reified T : Cls> materialize(): T?
+public fun test(/*0*/ subIn: SubIn): kotlin.Unit
+public fun </*0*/ D> transform(/*0*/ transformer: In<D>, /*1*/ data: D): kotlin.Unit
+
+public open class Cls {
+    public constructor Cls()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public abstract class In</*0*/ in I> {
+    public constructor In</*0*/ in I>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class SubIn : In<Cls?> {
+    public constructor SubIn()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/inference/nestedCalls/preferNothingToBound.fir.kt

@@ -0,0 +1,3 @@
+fun <K> id(arg: K): K = arg
+
+val v = id(null)

compiler/testData/diagnostics/tests/inference/nestedCalls/preferNothingToBound.kt

@@ -0,0 +1,3 @@
+fun <K> id(arg: K): K = arg
+
+val v = <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>id(null)<!>

compiler/testData/diagnostics/tests/inference/nestedCalls/preferNothingToBound.txt

@@ -0,0 +1,4 @@
+package
+
+public val v: kotlin.Nothing?
+public fun </*0*/ K> id(/*0*/ arg: K): K

compiler/testData/diagnostics/tests/inference/nothingType/inferArgumentToNothingFromNullConstant.fir.kt

@@ -0,0 +1,12 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION -ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE -UNUSED_VALUE
+
+class Out<out T : Any>(result: T?)
+
+fun main() {
+    val a = Out(null)
+
+    a
+
+    var b: Out<Int>? = null
+    b = a
+}

compiler/testData/diagnostics/tests/inference/regressions/kt32106.fir.kt

@@ -1,5 +1,3 @@
-// !LANGUAGE: +NewInference
-
 class Query<out T : Any> private constructor(
     private val result: T?,
     private val error: Throwable?,

compiler/testData/diagnostics/tests/inference/regressions/kt32106.kt

@@ -1,5 +1,3 @@
-// !LANGUAGE: +NewInference
-
 class Query<out T : Any> private constructor(
     private val result: T?,
     private val error: Throwable?,

compiler/testData/diagnostics/testsWithStdLib/inference/annotationsForResolve/onlyInputTypesCommonConstraintSystem.fir.kt

@@ -36,7 +36,6 @@ fun testOK(first: First, bound: Bound, second: Second) {
     strictSelect(OutB(first), OutB(bound))
     strictSelect(In(first), In(bound))
     strictSelect(InB(first), InB(bound))
-    strictSelect(InB(first), InB(second)) // different behaviour in contravariant position
 
     val out: Out<Bound> = strictSelect(Out(first), Out(second))
     val outb: OutB<Bound> = strictSelect(OutB(first), OutB(second))
@@ -49,6 +48,7 @@ fun testFail(first: First, bound: Bound, second: Second) {
     strictSelect(Inv(first), Inv(bound))
     strictSelect(Out(first), Out(second))
     strictSelect(In(first), In(second))
+    strictSelect(InB(first), InB(second))
     strictSelect(Out(Inv(first)), Out(Inv(second)))
     strictSelect(In(Inv(first)), In(Inv(second)))
 }

compiler/testData/diagnostics/testsWithStdLib/inference/annotationsForResolve/onlyInputTypesCommonConstraintSystem.kt

@@ -36,7 +36,6 @@ fun testOK(first: First, bound: Bound, second: Second) {
     strictSelect(OutB(first), OutB(bound))
     strictSelect(In(first), In(bound))
     strictSelect(InB(first), InB(bound))
-    <!OI;TYPE_INFERENCE_ONLY_INPUT_TYPES!>strictSelect<!>(InB(first), InB(second)) // different behaviour in contravariant position
 
     val out: Out<Bound> = strictSelect(Out(first), Out(second))
     val outb: OutB<Bound> = strictSelect(OutB(first), OutB(second))
@@ -49,6 +48,7 @@ fun testFail(first: First, bound: Bound, second: Second) {
     <!TYPE_INFERENCE_ONLY_INPUT_TYPES!>strictSelect<!>(Inv(first), Inv(bound))
     <!TYPE_INFERENCE_ONLY_INPUT_TYPES!>strictSelect<!>(Out(first), Out(second))
     <!TYPE_INFERENCE_ONLY_INPUT_TYPES!>strictSelect<!>(In(first), In(second))
+    <!TYPE_INFERENCE_ONLY_INPUT_TYPES!>strictSelect<!>(InB(first), InB(second))
     <!TYPE_INFERENCE_ONLY_INPUT_TYPES!>strictSelect<!>(Out(Inv(first)), Out(Inv(second)))
     <!TYPE_INFERENCE_ONLY_INPUT_TYPES!>strictSelect<!>(In(Inv(first)), In(Inv(second)))
 }

compiler/testData/diagnostics/testsWithStdLib/tryCatch/tryExpression.kt

@@ -27,7 +27,7 @@ fun test1(): Map<Int, Int> = run {
 }
 
 fun test2(): Map<Int, Int> = run {
-    <!NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH!>try {
+    <!NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH!>try {
         emptyMap()
     } catch (e: ExcA) {
         <!NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, OI;TYPE_INFERENCE_EXPECTED_TYPE_MISMATCH!>mapOf(<!NI;TYPE_MISMATCH, NI;TYPE_MISMATCH!>"" to ""<!>)<!>

compiler/testData/ir/irText/expressions/whenReturn.fir.txt

@@ -2,10 +2,10 @@ FILE fqName:<root> fileName:/whenReturn.kt
   FUN name:toString visibility:public modality:FINAL <> (grade:kotlin.String) returnType:kotlin.String
     VALUE_PARAMETER name:grade index:0 type:kotlin.String
     BLOCK_BODY
-      BLOCK type=kotlin.String origin=WHEN
+      BLOCK type=kotlin.Nothing origin=WHEN
         VAR IR_TEMPORARY_VARIABLE name:tmp_0 type:kotlin.String [val]
           GET_VAR 'grade: kotlin.String declared in <root>.toString' type=kotlin.String origin=null
-        WHEN type=kotlin.String origin=WHEN
+        WHEN type=kotlin.Nothing origin=WHEN
           BRANCH
             if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
               arg0: GET_VAR 'val tmp_0: kotlin.String [val] declared in <root>.toString' type=kotlin.String origin=null

compiler/testData/ir/irText/lambdas/nonLocalReturn.fir.txt

@@ -1,10 +1,10 @@
 FILE fqName:<root> fileName:/nonLocalReturn.kt
   FUN name:test0 visibility:public modality:FINAL <> () returnType:kotlin.Unit
     BLOCK_BODY
-      CALL 'public final fun run <R> (block: kotlin.Function0<R of kotlin.run>): R of kotlin.run [inline] declared in kotlin' type=kotlin.Unit origin=null
-        <R>: kotlin.Unit
-        block: FUN_EXPR type=kotlin.Function0<kotlin.Unit> origin=LAMBDA
-          FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Unit
+      CALL 'public final fun run <R> (block: kotlin.Function0<R of kotlin.run>): R of kotlin.run [inline] declared in kotlin' type=kotlin.Nothing origin=null
+        <R>: kotlin.Nothing
+        block: FUN_EXPR type=kotlin.Function0<kotlin.Nothing> origin=LAMBDA
+          FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Nothing
             BLOCK_BODY
               RETURN type=kotlin.Nothing from='public final fun test0 (): kotlin.Unit declared in <root>'
                 GET_OBJECT 'CLASS IR_EXTERNAL_DECLARATION_STUB OBJECT name:Unit modality:FINAL visibility:public superTypes:[kotlin.Any]' type=kotlin.Unit
@@ -33,12 +33,12 @@ FILE fqName:<root> fileName:/nonLocalReturn.kt
         block: FUN_EXPR type=kotlin.Function0<kotlin.Unit> origin=LAMBDA
           FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Unit
             BLOCK_BODY
-              CALL 'public final fun run <R> (block: kotlin.Function0<R of kotlin.run>): R of kotlin.run [inline] declared in kotlin' type=kotlin.Unit origin=null
-                <R>: kotlin.Unit
-                block: FUN_EXPR type=kotlin.Function0<kotlin.Unit> origin=LAMBDA
-                  FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Unit
+              CALL 'public final fun run <R> (block: kotlin.Function0<R of kotlin.run>): R of kotlin.run [inline] declared in kotlin' type=kotlin.Nothing origin=null
+                <R>: kotlin.Nothing
+                block: FUN_EXPR type=kotlin.Function0<kotlin.Nothing> origin=LAMBDA
+                  FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Nothing
                     BLOCK_BODY
-                      RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.Unit declared in <root>.test3.<anonymous>'
+                      RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.Nothing declared in <root>.test3.<anonymous>'
                         GET_OBJECT 'CLASS IR_EXTERNAL_DECLARATION_STUB OBJECT name:Unit modality:FINAL visibility:public superTypes:[kotlin.Any]' type=kotlin.Unit
   FUN name:testLrmFoo1 visibility:public modality:FINAL <> (ints:kotlin.collections.List<kotlin.Int>) returnType:kotlin.Unit
     VALUE_PARAMETER name:ints index:0 type:kotlin.collections.List<kotlin.Int>

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/4.kt

@@ -285,7 +285,7 @@ fun case_24(a: ((() -> Unit) -> Unit), b: (() -> Unit)) {
 }
 
 // TESTCASE NUMBER: 25
-fun case_25(a: (() -> Unit) -> Unit, b: (() -> Unit) -> Unit = if (<!SENSELESS_COMPARISON!>a == null<!>) <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit & kotlin.Nothing"), DEBUG_INFO_SMARTCAST!>a<!> else <!TYPE_MISMATCH!>{{}}<!>) {
+fun case_25(a: (() -> Unit) -> Unit, b: (() -> Unit) -> Unit = if (<!SENSELESS_COMPARISON!>a == null<!>) <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit & kotlin.Nothing")!>a<!> else {{}}) {
     <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit")!>a<!>
     <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit")!>b<!>
 }

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -10611,6 +10611,16 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 public void testNontrivialCallExpression() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/nontrivialCallExpression.kt");
                 }
+
+                @TestMetadata("preferArgumentToNullability.kt")
+                public void testPreferArgumentToNullability() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/preferArgumentToNullability.kt");
+                }
+
+                @TestMetadata("preferNothingToBound.kt")
+                public void testPreferNothingToBound() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/preferNothingToBound.kt");
+                }
             }
 
             @TestMetadata("compiler/testData/diagnostics/tests/inference/nothingType")

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -10606,6 +10606,16 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                 public void testNontrivialCallExpression() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/nontrivialCallExpression.kt");
                 }
+
+                @TestMetadata("preferArgumentToNullability.kt")
+                public void testPreferArgumentToNullability() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/preferArgumentToNullability.kt");
+                }
+
+                @TestMetadata("preferNothingToBound.kt")
+                public void testPreferNothingToBound() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/nestedCalls/preferNothingToBound.kt");
+                }
             }
 
             @TestMetadata("compiler/testData/diagnostics/tests/inference/nothingType")