Propagate nullness flexibility into the result type from type variables in other constraints during result type finding, to prevent variable fixation to less flexible type

^KT-44540 Fixed

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirOldFrontendDiagnosticsTestGenerated.java

@@ -20213,6 +20213,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                 runTest("compiler/testData/diagnostics/tests/platformTypes/override.kt");
             }
 
+            @Test
+            @TestMetadata("propagateFlexibilityFromOtherConstraints.kt")
+            public void testPropagateFlexibilityFromOtherConstraints() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/platformTypes/propagateFlexibilityFromOtherConstraints.kt");
+            }
+
             @Test
             @TestMetadata("rawOverrides.kt")
             public void testRawOverrides() throws Exception {

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

@@ -40,7 +40,9 @@ class ResultTypeResolver(
         findResultIfThereIsEqualsConstraint(c, variableWithConstraints)?.let { return it }
 
         val subType = c.findSubType(variableWithConstraints)
-        val superType = c.findSuperType(variableWithConstraints)
+        // Super type should be the most flexible, sub type should be the least one
+        val superType = c.findSuperType(variableWithConstraints).makeFlexibleIfNecessary(c, variableWithConstraints.constraints)
+
         return if (direction == ResolveDirection.TO_SUBTYPE || direction == ResolveDirection.UNKNOWN) {
             c.resultType(subType, superType, variableWithConstraints)
         } else {
@@ -48,6 +50,30 @@ class ResultTypeResolver(
         }
     }
 
+    /*
+     * We propagate nullness flexibility into the result type from type variables in other constraints
+     * to prevent variable fixation into less flexible type.
+     *  Constraints:
+     *      UPPER(TypeVariable(T)..TypeVariable(T)?)
+     *      UPPER(Foo?)
+     *  Result type = makeFlexibleIfNecessary(Foo?) = Foo!
+     *
+     * We don't propagate nullness flexibility in depth as it's non-determined for now (see KT-35534):
+     *  CST(Bar<Foo>, Bar<Foo!>) = Bar<Foo!>
+     *  CST(Bar<Foo!>, Bar<Foo>) = Bar<Foo>
+     * But: CST(Foo, Foo!) = CST(Foo!, Foo) = Foo!
+     */
+    private fun KotlinTypeMarker?.makeFlexibleIfNecessary(c: Context, constraints: List<Constraint>) = with(c) {
+        when (val type = this@makeFlexibleIfNecessary) {
+            is SimpleTypeMarker -> {
+                if (constraints.any { it.type.typeConstructor().isTypeVariable() && it.type.hasFlexibleNullability() }) {
+                    createFlexibleType(type.makeSimpleTypeDefinitelyNotNullOrNotNull(), type.withNullability(true))
+                } else type
+            }
+            else -> type
+        }
+    }
+
     private fun Context.resultType(
         firstCandidate: KotlinTypeMarker?,
         secondCandidate: KotlinTypeMarker?,

compiler/testData/diagnostics/tests/platformTypes/propagateFlexibilityFromOtherConstraints.fir.kt

@@ -0,0 +1,32 @@
+// FULL_JDK
+// FILE: test.kt
+
+@file:Suppress("UNUSED_PARAMETER")
+
+import java.util.Comparator
+
+abstract class DataView {
+    abstract val presentationName: String
+}
+
+fun <A> comboBox(
+    model: SortedComboBoxModel<A>,
+    graphProperty: GraphProperty<A>,
+) {}
+
+class GraphProperty<B>
+
+fun test() {
+    val presentationName: (DataView) -> String = { it.presentationName }
+    val parentComboBoxModel/*: SortedComboBoxModel<DataView>*/ = SortedComboBoxModel(Comparator.comparing(presentationName))
+    comboBox(parentComboBoxModel, GraphProperty<DataView>())
+}
+
+// FILE: SortedComboBoxModel.java
+
+import java.util.Comparator;
+
+public class SortedComboBoxModel<C> {
+    public SortedComboBoxModel(Comparator<? super C> comparator) {
+    }
+}

compiler/testData/diagnostics/tests/platformTypes/propagateFlexibilityFromOtherConstraints.kt

@@ -0,0 +1,32 @@
+// FULL_JDK
+// FILE: test.kt
+
+@file:Suppress("UNUSED_PARAMETER")
+
+import java.util.Comparator
+
+abstract class DataView {
+    abstract val presentationName: String
+}
+
+fun <A> comboBox(
+    model: SortedComboBoxModel<A>,
+    graphProperty: GraphProperty<A>,
+) {}
+
+class GraphProperty<B>
+
+fun test() {
+    val presentationName: (DataView) -> String = { it.presentationName }
+    val parentComboBoxModel/*: SortedComboBoxModel<DataView>*/ = SortedComboBoxModel(Comparator.<!INTERFACE_STATIC_METHOD_CALL_FROM_JAVA6_TARGET_ERROR!>comparing<!>(presentationName))
+    comboBox(parentComboBoxModel, GraphProperty<DataView>())
+}
+
+// FILE: SortedComboBoxModel.java
+
+import java.util.Comparator;
+
+public class SortedComboBoxModel<C> {
+    public SortedComboBoxModel(Comparator<? super C> comparator) {
+    }
+}

compiler/testData/diagnostics/tests/platformTypes/propagateFlexibilityFromOtherConstraints.txt

@@ -0,0 +1,26 @@
+package
+
+public fun </*0*/ A> comboBox(/*0*/ model: SortedComboBoxModel<A>, /*1*/ graphProperty: GraphProperty<A>): kotlin.Unit
+public fun test(): kotlin.Unit
+
+public abstract class DataView {
+    public constructor DataView()
+    public abstract val presentationName: kotlin.String
+    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 GraphProperty</*0*/ B> {
+    public constructor GraphProperty</*0*/ B>()
+    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 open class SortedComboBoxModel</*0*/ C : kotlin.Any!> {
+    public constructor SortedComboBoxModel</*0*/ C : kotlin.Any!>(/*0*/ comparator: java.util.Comparator<in C!>!)
+    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/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -20219,6 +20219,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/platformTypes/override.kt");
             }
 
+            @Test
+            @TestMetadata("propagateFlexibilityFromOtherConstraints.kt")
+            public void testPropagateFlexibilityFromOtherConstraints() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/platformTypes/propagateFlexibilityFromOtherConstraints.kt");
+            }
+
             @Test
             @TestMetadata("rawOverrides.kt")
             public void testRawOverrides() throws Exception {