K2: Avoid using Nothing? as inference result in the majority of cases

Namely, do not choose `Nothing?` result type when fixing a variable
that has other constraints besides the ones that came from
the relevant type parameter's upper bounds.

See more details in KT-55691.

In K1, the case from specialCallWithMaterializeAndExpectedType.kt
was working (inferred to String?) just because the branches
were analyzed independently with `String?` expected type.

This change became necessary after the previous commit when we united
inference subsystems for if/when branches (see motivation there).

NB: For K1, the behavior is left the same, but the code
was refactored a bit.

^KT-55691 Fixed
^KT-56448 Fixed

compiler/testData/diagnostics/tests/inference/nothingType/kt56448.fir.kt

@@ -0,0 +1,43 @@
+// SKIP_TXT
+// WITH_STDLIB
+// ISSUE: KT-56448
+// FILE: SomeJavaClass.java
+
+public class SomeJavaClass {
+    public static String getString() {
+        return "";
+    }
+}
+
+// FILE: main.kt
+import kotlin.reflect.KProperty1
+
+interface XdEntity
+class XdIssue : XdEntity {
+    val isRemoved: Boolean = true
+    var votes: Int = 0
+}
+
+fun getNullableIssue(): XdIssue? = null
+
+fun <T : XdEntity> CharSequence.toXd(): T = null!!
+
+fun XdIssue.duplicatesRootSearch(): XdIssue = this
+fun <T : XdEntity, R : Comparable<*>?> T.getOldValue(property: KProperty1<T, R>): R? = null
+
+internal fun updateVotesForDuplicates(issue: XdIssue) {
+    val toRecount = HashSet<XdIssue>()
+    var oldDup = getNullableIssue()
+    if (oldDup != null) {
+        oldDup = try {
+            <!DEBUG_INFO_EXPRESSION_TYPE("XdIssue")!>SomeJavaClass.getString().toXd()<!>
+        } catch (_: Exception) {
+            null
+        }
+    }
+    val newDup = getNullableIssue()
+
+    if (oldDup != null && !oldDup.isRemoved) {
+        toRecount.add(oldDup.duplicatesRootSearch())
+    }
+}

compiler/testData/diagnostics/tests/inference/nothingType/kt56448.kt

@@ -0,0 +1,43 @@
+// SKIP_TXT
+// WITH_STDLIB
+// ISSUE: KT-56448
+// FILE: SomeJavaClass.java
+
+public class SomeJavaClass {
+    public static String getString() {
+        return "";
+    }
+}
+
+// FILE: main.kt
+import kotlin.reflect.KProperty1
+
+interface XdEntity
+class XdIssue : XdEntity {
+    val isRemoved: Boolean = true
+    var votes: Int = 0
+}
+
+fun getNullableIssue(): XdIssue? = null
+
+fun <T : XdEntity> CharSequence.toXd(): T = null!!
+
+fun XdIssue.duplicatesRootSearch(): XdIssue = this
+fun <T : XdEntity, R : Comparable<*>?> T.getOldValue(property: KProperty1<T, R>): R? = null
+
+internal fun updateVotesForDuplicates(issue: XdIssue) {
+    val toRecount = HashSet<XdIssue>()
+    var oldDup = getNullableIssue()
+    if (oldDup != null) {
+        oldDup = try {
+            <!DEBUG_INFO_EXPRESSION_TYPE("XdIssue")!>SomeJavaClass.getString().toXd()<!>
+        } catch (_: Exception) {
+            null
+        }
+    }
+    val newDup = getNullableIssue()
+
+    if (oldDup != null && !<!DEBUG_INFO_SMARTCAST!>oldDup<!>.isRemoved) {
+        toRecount.add(<!DEBUG_INFO_SMARTCAST!>oldDup<!>.duplicatesRootSearch())
+    }
+}

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

@@ -0,0 +1,13 @@
+// SKIP_TXT
+// ISSUE: KT-55691
+
+fun <T> materialize(): T = TODO()
+fun <S> select(x: S, y: S): S = x
+
+fun main() {
+    // materialize's type argument is inferred to `Nothing?` in K1 and `String?` in K2
+    val x: String? = select(<!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String?")!>materialize()<!>, null)
+
+    // materialize's type argument is inferred to `Nothing?` both in K1 and K2
+    select(<!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>materialize()<!>, null)
+}

compiler/testData/diagnostics/tests/inference/nothingType/selectWithNull.kt

@@ -0,0 +1,13 @@
+// SKIP_TXT
+// ISSUE: KT-55691
+
+fun <T> materialize(): T = TODO()
+fun <S> select(x: S, y: S): S = x
+
+fun main() {
+    // materialize's type argument is inferred to `Nothing?` in K1 and `String?` in K2
+    val x: String? = select(<!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!><!IMPLICIT_NOTHING_TYPE_ARGUMENT_AGAINST_NOT_NOTHING_EXPECTED_TYPE!>materialize<!>()<!>, null)
+
+    // materialize's type argument is inferred to `Nothing?` both in K1 and K2
+    select(<!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>materialize()<!>, null)
+}

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

@@ -1,7 +0,0 @@
-// !DIAGNOSTICS: -UNUSED_VARIABLE
-
-fun foo() {
-    val s: String? = if (true) <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>materialize()<!> else null
-}
-
-fun <K> materialize(): K = TODO()

compiler/testData/diagnostics/tests/inference/nothingType/specialCallWithMaterializeAndExpectedType.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_VARIABLE
 
 fun foo() {

compiler/testData/diagnostics/tests/inference/nothingVsParameterBound.kt

@@ -0,0 +1,15 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+
+class Out<out T : CharSequence?>(val t: T)
+
+fun foo() {
+    // We have two constraints here:
+    // Nothing? <: T (from argument `null` type)
+    // T <: CharSequence?
+    // And we fix T to `Nothing?`, because it's still more preferrable than constraint from the upper bound
+    val x1 = Out(null)
+    bar(<!DEBUG_INFO_EXPRESSION_TYPE("Out<kotlin.Nothing?>")!>x1<!>)
+}
+
+fun bar(w: Out<String?>) {}