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?>) {}

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

@@ -8,7 +8,7 @@ fun test() {
     val map: Map<String, Int> = mapOf("x" to 1)
 
     val r1 = map.getOrDefault_Exact("y", null)
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>r1<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>r1<!>
 
     val r2 = map.getOrDefault_Exact("y", null as Int?)
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>r2<!>

compiler/testData/ir/irText/declarations/kt47527.fir.ir.txt

@@ -28,12 +28,12 @@ FILE fqName:<root> fileName:/kt47527.kt
     VALUE_PARAMETER name:value index:0 type:kotlin.Any?
     BLOCK_BODY
       RETURN type=kotlin.Nothing from='public final fun test_2 (value: kotlin.Any?): kotlin.String? declared in <root>'
-        CALL 'public final fun run <R> (block: kotlin.Function0<R of kotlin.StandardKt.run>): R of kotlin.StandardKt.run [inline] declared in kotlin.StandardKt' 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?
+        CALL 'public final fun run <R> (block: kotlin.Function0<R of kotlin.StandardKt.run>): R of kotlin.StandardKt.run [inline] declared in kotlin.StandardKt' type=kotlin.String? origin=null
+          <R>: kotlin.String?
+          block: FUN_EXPR type=kotlin.Function0<kotlin.String?> origin=LAMBDA
+            FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.String?
               BLOCK_BODY
-                RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.Nothing? declared in <root>.test_2'
+                RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.String? declared in <root>.test_2'
                   BLOCK type=kotlin.Nothing? origin=SAFE_CALL
                     VAR IR_TEMPORARY_VARIABLE name:tmp_1 type:kotlin.Any? [val]
                       GET_VAR 'value: kotlin.Any? declared in <root>.test_2' type=kotlin.Any? origin=null

compiler/testData/ir/irText/declarations/kt47527.fir.kt.txt

@@ -12,7 +12,7 @@ fun test_1(value: Any?): String? {
 }
 
 fun test_2(value: Any?): String? {
-  return run<Nothing?>(block = local fun <anonymous>(): Nothing? {
+  return run<String?>(block = local fun <anonymous>(): String? {
     return { // BLOCK
       val tmp1_safe_receiver: Any? = value
       when {