Fold lower constraints like (T!!..T) and (T..T?) into the latter one

#KT-41149 Fixed

compiler/fir/fir2ir/tests/org/jetbrains/kotlin/codegen/ir/FirBlackBoxCodegenTestGenerated.java

@@ -12632,6 +12632,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("manyFlexibleTypeParametersFromJavaAndConversions.kt")
+        public void testManyFlexibleTypeParametersFromJavaAndConversions() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/manyFlexibleTypeParametersFromJavaAndConversions.kt");
+        }
+
         @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
         public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");

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

@@ -8,8 +8,7 @@ package org.jetbrains.kotlin.resolve.calls.inference.model
 import org.jetbrains.kotlin.resolve.calls.inference.trimToSize
 import org.jetbrains.kotlin.resolve.calls.model.KotlinCallDiagnostic
 import org.jetbrains.kotlin.resolve.calls.tower.isSuccess
-import org.jetbrains.kotlin.types.KotlinType
-import org.jetbrains.kotlin.types.UnwrappedType
+import org.jetbrains.kotlin.types.*
 import org.jetbrains.kotlin.types.model.KotlinTypeMarker
 import org.jetbrains.kotlin.types.model.TypeConstructorMarker
 import org.jetbrains.kotlin.types.model.TypeVariableMarker
@@ -49,6 +48,7 @@ class MutableVariableWithConstraints private constructor(
 
     // return new actual constraint, if this constraint is new
     fun addConstraint(constraint: Constraint): Constraint? {
+        val isLowerAndFlexibleTypeWithDefNotNullLowerBound = constraint.isLowerAndFlexibleTypeWithDefNotNullLowerBound()
 
         for (previousConstraint in constraints) {
             if (previousConstraint.typeHashCode == constraint.typeHashCode
@@ -75,12 +75,23 @@ class MutableVariableWithConstraints private constructor(
                     return actualConstraint
                 }
             }
+
+            if (isLowerAndFlexibleTypeWithDefNotNullLowerBound &&
+                previousConstraint.isStrongerThanLowerAndFlexibleTypeWithDefNotNullLowerBound(constraint)
+            ) {
+                return null
+            }
         }
 
         mutableConstraints.add(constraint)
         if (simplifiedConstraints != null && simplifiedConstraints !== mutableConstraints) {
             simplifiedConstraints!!.add(constraint)
         }
+
+        if (simplifiedConstraints != null && isLowerAndFlexibleTypeWithDefNotNullLowerBound) {
+            simplifiedConstraints = null
+        }
+
         return constraint
     }
 
@@ -114,10 +125,50 @@ class MutableVariableWithConstraints private constructor(
         }
     }
 
-    private fun SmartList<Constraint>.simplifyConstraints(): SmartList<Constraint> {
-        val equalityConstraints =
-            filter { it.kind == ConstraintKind.EQUALITY }
-                .groupBy { it.typeHashCode }
+    private fun SmartList<Constraint>.simplifyConstraints(): SmartList<Constraint> =
+        simplifyLowerConstraints().simplifyEqualityConstraints()
+
+    private fun SmartList<Constraint>.simplifyLowerConstraints(): SmartList<Constraint> {
+        val usefulConstraints = SmartList<Constraint>()
+        for (constraint in this) {
+            if (!constraint.isLowerAndFlexibleTypeWithDefNotNullLowerBound()) {
+                usefulConstraints.add(constraint)
+                continue
+            }
+
+            // Now we have to check that some constraint T!!.T? <: K is useless or not
+            // If there is constraint T..T? <: K, then the original one (T!!.T?) is useless
+            // This is so because CST(T..T?, T!!..T?) == CST(T..T?)
+
+            val thereIsStrongerConstraint = this.any { it.isStrongerThanLowerAndFlexibleTypeWithDefNotNullLowerBound(constraint) }
+
+            if (!thereIsStrongerConstraint) {
+                usefulConstraints.add(constraint)
+            }
+        }
+
+        return usefulConstraints
+    }
+
+    // Such constraint is applicable for simplification
+    private fun Constraint.isLowerAndFlexibleTypeWithDefNotNullLowerBound(): Boolean =
+        kind == ConstraintKind.LOWER && type is FlexibleType && type.lowerBound.isDefinitelyNotNullType
+
+    private fun Constraint.isStrongerThanLowerAndFlexibleTypeWithDefNotNullLowerBound(other: Constraint): Boolean {
+        if (this === other) return false
+
+        if (typeHashCode != other.typeHashCode || kind == ConstraintKind.UPPER) return false
+        if (type !is FlexibleType) return false
+
+        val otherFlexibleType = other.type as? FlexibleType ?: return false
+        val otherLowerBound = otherFlexibleType.lowerBound as? DefinitelyNotNullType ?: return false
+        val otherUpperBound = otherFlexibleType.upperBound
+
+        return type.lowerBound == otherLowerBound.original && type.upperBound == otherUpperBound
+    }
+
+    private fun SmartList<Constraint>.simplifyEqualityConstraints(): SmartList<Constraint> {
+        val equalityConstraints = filter { it.kind == ConstraintKind.EQUALITY }.groupBy { it.typeHashCode }
         return when {
             equalityConstraints.isEmpty() -> this
             else -> filterTo(SmartList()) { isUsefulConstraint(it, equalityConstraints) }

compiler/testData/codegen/box/inference/manyFlexibleTypeParametersFromJavaAndConversions.kt

@@ -0,0 +1,64 @@
+// TARGET_BACKEND: JVM
+
+// FILE: Combiner.java
+
+public class Combiner {
+    public static <T1, T2, T3, T4, T5, R> void combine(
+        Inv<? extends T1> source1, Inv<? extends T2> source2,
+        Inv<? extends T3> source3, Inv<? extends T4> source4,
+        Inv<? extends T5> source5,
+        Function5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? extends R> combiner
+    ) {
+        return;
+    }
+}
+
+// FILE: Function5.java
+
+public interface Function5<T1, T2, T3, T4, T5, R> {
+    R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5);
+}
+
+// FILE: Test.kt
+
+fun <T1, T2, T3, T4, T5, R> kCombine(
+    s1: Inv<out T1>, s2: Inv<out T2>, s3: Inv<out T3>, s4: Inv<out T4>, s5: Inv<out T5>,
+    f: Function5<in T1, in T2, in T3, in T4, in T5, out R>
+) {
+}
+
+data class Quantiple<out T1, out T2, out T3, out T4, out T5>(
+    val first: T1,
+    val second: T2,
+    val third: T3,
+    val fourth: T4,
+    val fifth: T5
+)
+
+fun <K1, K2, K3, K4, K5> materialize(): (K1, K2, K3, K4, K5) -> Quantiple<K1, K2, K3, K4, K5> = { _, _, _, _, _ -> TODO() }
+
+class Inv<T>
+
+fun <P1, P2, P3, P4, P5> test(
+    p1: Inv<out P1>, p2: Inv<out P2>, p3: Inv<out P3>, p4: Inv<out P4>, p5: Inv<out P5>
+) {
+    Combiner.combine(
+        p1, p2, p3, p4, p5,
+        materialize()
+    )
+
+    Combiner.combine(
+        p1, p2, p3, p4, p5,
+        ::Quantiple
+    )
+
+    kCombine(
+        p1, p2, p3, p4, p5,
+        materialize()
+    )
+}
+
+fun box(): String {
+    test(Inv<Int>(), Inv<Byte>(), Inv<Short>(), Inv<Long>(), Inv<String>())
+    return "OK"
+}

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -13857,6 +13857,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("manyFlexibleTypeParametersFromJavaAndConversions.kt")
+        public void testManyFlexibleTypeParametersFromJavaAndConversions() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/manyFlexibleTypeParametersFromJavaAndConversions.kt");
+        }
+
         @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
         public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");

compiler/tests/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -13857,6 +13857,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("manyFlexibleTypeParametersFromJavaAndConversions.kt")
+        public void testManyFlexibleTypeParametersFromJavaAndConversions() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/manyFlexibleTypeParametersFromJavaAndConversions.kt");
+        }
+
         @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
         public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/IrBlackBoxCodegenTestGenerated.java

@@ -12632,6 +12632,11 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("manyFlexibleTypeParametersFromJavaAndConversions.kt")
+        public void testManyFlexibleTypeParametersFromJavaAndConversions() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/manyFlexibleTypeParametersFromJavaAndConversions.kt");
+        }
+
         @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
         public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");