Build type enhancement properly, by taking into account both bounds of the original flexible type

^KT-44420 Fixed

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

@@ -20852,6 +20852,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
                 }
 
+                @Test
+                @TestMetadata("buildFlexibleEnhancement.kt")
+                public void testBuildFlexibleEnhancement() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/buildFlexibleEnhancement.kt");
+                }
+
                 @Test
                 @TestMetadata("overriddenExtensions.kt")
                 public void testOverriddenExtensions() throws Exception {

compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/buildFlexibleEnhancement.kt

@@ -0,0 +1,35 @@
+// FIR_IDENTICAL
+// FULL_JDK
+// WITH_RUNTIME
+// WITH_REFLECT
+
+// FILE: NonNullApi.java
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target({ElementType.TYPE, ElementType.PACKAGE})
+@javax.annotation.Nonnull
+@javax.annotation.meta.TypeQualifierDefault({ElementType.METHOD, ElementType.PARAMETER})
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+public @interface NonNullApi { }
+
+// FILE: Foo.java
+
+import java.util.Collection;
+
+@NonNullApi
+public class Foo<E> {
+    public Foo(Collection<? extends E> c) {}
+}
+
+// FILE: main.kt
+
+fun test() {
+    val collection: Collection<Int> = listOf(1, 2, 3)
+    Foo(collection)
+}

compiler/testData/foreignAnnotations/tests/jsr305/nullabilityWarnings/typeQualifierDefault/springNullable.kt

@@ -77,7 +77,7 @@ fun main(a: A) {
     a.field?.length
     a.field.length
 
-    <!RECEIVER_NULLABILITY_MISMATCH_BASED_ON_JAVA_ANNOTATIONS("MutableList<String!>?")!>a.baz()<!>.get(0)
+    <!RECEIVER_NULLABILITY_MISMATCH_BASED_ON_JAVA_ANNOTATIONS("(Mutable)List<String!>?")!>a.baz()<!>.get(0)
     a.baz()!!.get(0).get(0)
     a.baz()!!.get(0)?.get(0)
 }

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -20858,6 +20858,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement"), Pattern.compile("^(.*)\\.kts?$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
                 }
 
+                @Test
+                @TestMetadata("buildFlexibleEnhancement.kt")
+                public void testBuildFlexibleEnhancement() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/buildFlexibleEnhancement.kt");
+                }
+
                 @Test
                 @TestMetadata("overriddenExtensions.kt")
                 public void testOverriddenExtensions() throws Exception {

core/descriptors.jvm/src/org/jetbrains/kotlin/load/java/typeEnhancement/typeEnhancement.kt

@@ -58,6 +58,15 @@ class JavaTypeEnhancement(private val javaResolverSettings: JavaResolverSettings
     // For flexible types, both bounds are indexed in the same way: `(A<B>..C<D>)` gives `0 - (A<B>..C<D>), 1 - B and D`.
     fun KotlinType.enhance(qualifiers: (Int) -> JavaTypeQualifiers) = unwrap().enhancePossiblyFlexible(qualifiers, 0).typeIfChanged
 
+    private fun buildEnhancementByFlexibleTypeBounds(lowerBound: KotlinType, upperBound: KotlinType): KotlinType? {
+        val upperEnhancement = upperBound.getEnhancement()
+        val lowerEnhancement = lowerBound.getEnhancement() ?: upperEnhancement ?: return null
+
+        if (upperEnhancement == null) return lowerEnhancement
+
+        return KotlinTypeFactory.flexibleType(lowerEnhancement.lowerIfFlexible(), upperEnhancement.upperIfFlexible())
+    }
+
     private fun UnwrappedType.enhancePossiblyFlexible(qualifiers: (Int) -> JavaTypeQualifiers, index: Int): Result {
         if (isError) return Result(this, 1, false)
         return when (this) {
@@ -72,12 +81,13 @@ class JavaTypeEnhancement(private val javaResolverSettings: JavaResolverSettings
                 }
 
                 val wereChanges = lowerResult.wereChanges || upperResult.wereChanges
-                val enhancement = lowerResult.type.getEnhancement() ?: upperResult.type.getEnhancement()
-                val type = if (!wereChanges) this@enhancePossiblyFlexible
-                else when {
-                    this is RawTypeImpl -> RawTypeImpl(lowerResult.type, upperResult.type)
-                    else -> KotlinTypeFactory.flexibleType(lowerResult.type, upperResult.type)
-                }.wrapEnhancement(enhancement)
+                val enhancement = buildEnhancementByFlexibleTypeBounds(lowerResult.type, upperResult.type)
+                val type = if (wereChanges) {
+                    when (this) {
+                        is RawTypeImpl -> RawTypeImpl(lowerResult.type, upperResult.type)
+                        else -> KotlinTypeFactory.flexibleType(lowerResult.type, upperResult.type)
+                    }.wrapEnhancement(enhancement)
+                } else this@enhancePossiblyFlexible
 
                 Result(
                     type,