[NI] Resolve type variable to Nothing for TO_SUBTYPE direction.

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

@@ -38,6 +38,8 @@ class ResultTypeResolver(
     fun findResultType(c: Context, variableWithConstraints: VariableWithConstraints, direction: ResolveDirection): UnwrappedType? {
         findResultIfThereIsEqualsConstraint(c, variableWithConstraints, allowedFixToNotProperType = false)?.let { return it }
 
+        val builtIns = variableWithConstraints.typeVariable.freshTypeConstructor.builtIns
+
         if (direction == ResolveDirection.TO_SUBTYPE || direction == ResolveDirection.UNKNOWN) {
             val lowerConstraints = variableWithConstraints.constraints.filter { it.kind == ConstraintKind.LOWER && c.isProperType(it.type) }
             if (lowerConstraints.isNotEmpty()) {
@@ -64,6 +66,8 @@ class ResultTypeResolver(
             }
         }
 
+        if (direction == ResolveDirection.TO_SUBTYPE) return builtIns.nothingType
+
         // direction == TO_SUPER or there is no LOWER bounds
         val upperConstraints = variableWithConstraints.constraints.filter { it.kind == ConstraintKind.UPPER && c.isProperType(it.type) }
         if (upperConstraints.isNotEmpty()) {
@@ -72,7 +76,7 @@ class ResultTypeResolver(
             return typeApproximator.approximateToSubType(upperType, TypeApproximatorConfiguration.CapturedTypesApproximation) ?: upperType
         }
 
-        return null
+        return return builtIns.anyType
     }
 
     fun findResultIfThereIsEqualsConstraint(

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

@@ -0,0 +1,15 @@
+
+// completion order here: X, Y, WHEN_VARIABLE
+fun <T> List<T>.optimizeReadOnlyList() = when (size) {
+    0 -> emptyList() // here type variable Y will be fixed to Nothing
+    1 -> listOf(this[0])
+    else -> this
+}
+fun <X> listOf(<!UNUSED_PARAMETER!>element<!>: X): List<X> = TODO()
+fun <Y> emptyList(): List<Y> = TODO()
+
+
+fun test(l: List<String>): List<String> {
+    val foo = l.optimizeReadOnlyList()
+    return foo
+}

compiler/testData/diagnostics/tests/inference/fixVariableToNothing.txt

@@ -0,0 +1,6 @@
+package
+
+public fun </*0*/ Y> emptyList(): kotlin.collections.List<Y>
+public fun </*0*/ X> listOf(/*0*/ element: X): kotlin.collections.List<X>
+public fun test(/*0*/ l: kotlin.collections.List<kotlin.String>): kotlin.collections.List<kotlin.String>
+public fun </*0*/ T> kotlin.collections.List<T>.optimizeReadOnlyList(): kotlin.collections.List<T>

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -10048,6 +10048,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("fixVariableToNothing.kt")
+            public void testFixVariableToNothing() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/inference/fixVariableToNothing.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("flexibleTypesAsUpperBound.kt")
             public void testFlexibleTypesAsUpperBound() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/inference/flexibleTypesAsUpperBound.kt");