Cast never succeeds: special 'Nothing' / 'Nothing?' case

(cherry picked from commit 63435b4)

compiler/frontend/src/org/jetbrains/kotlin/types/CastDiagnosticsUtil.kt

@@ -38,6 +38,8 @@ object CastDiagnosticsUtil {
             rhsType: KotlinType,
             platformToKotlinClassMap: PlatformToKotlinClassMap): Boolean {
         if (KotlinBuiltIns.isNullableNothing(lhsType) && !TypeUtils.isNullableType(rhsType)) return false
+        if (KotlinBuiltIns.isNothing(rhsType)) return false
+        if (KotlinBuiltIns.isNullableNothing(rhsType)) return TypeUtils.isNullableType(lhsType)
         if (lhsType.isError) return true
         if (isRelated(lhsType, rhsType, platformToKotlinClassMap)) return true
         // This is an oversimplification (which does not render the method incomplete):

compiler/testData/diagnostics/tests/cast/AsNothing.kt

@@ -0,0 +1,14 @@
+// Nothing can be cast to Nothing
+fun foo(x: String) {
+    x <!CAST_NEVER_SUCCEEDS!>as<!> Nothing
+}
+
+fun gav(y: String?) {
+    y <!CAST_NEVER_SUCCEEDS!>as<!> Nothing
+}
+
+// Only nullable can be cast to Nothing?
+fun bar(x: String, y: String?) {
+    x <!CAST_NEVER_SUCCEEDS!>as<!> Nothing?
+    y as Nothing?
+}

compiler/testData/diagnostics/tests/cast/AsNothing.txt

@@ -0,0 +1,5 @@
+package
+
+public fun bar(/*0*/ x: kotlin.String, /*1*/ y: kotlin.String?): kotlin.Unit
+public fun foo(/*0*/ x: kotlin.String): kotlin.Unit
+public fun gav(/*0*/ y: kotlin.String?): kotlin.Unit

compiler/testData/diagnostics/testsWithStdLib/reified/reifiedNothingSubstitution.kt

@@ -9,8 +9,8 @@ fun box() {
     val b = <!UNSUPPORTED!>Array<!><<!REIFIED_TYPE_FORBIDDEN_SUBSTITUTION!>Nothing?<!>>(5) { null!! }
     val c = <!REIFIED_TYPE_FORBIDDEN_SUBSTITUTION!>foo<!>() { null!! }
     val d = foo<Any> { null!! }
-    val e = <!REIFIED_TYPE_FORBIDDEN_SUBSTITUTION!>foo<!> { "1" as Nothing }
+    val e = <!REIFIED_TYPE_FORBIDDEN_SUBSTITUTION!>foo<!> { "1" <!CAST_NEVER_SUCCEEDS!>as<!> Nothing }
-    val e1 = <!REIFIED_TYPE_FORBIDDEN_SUBSTITUTION!>foo<!> { "1" as Nothing? }
+    val e1 = <!REIFIED_TYPE_FORBIDDEN_SUBSTITUTION!>foo<!> { "1" <!CAST_NEVER_SUCCEEDS!>as<!> Nothing? }
 
     val f = javaClass<<!REIFIED_TYPE_FORBIDDEN_SUBSTITUTION!>Nothing<!>>()
 }

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

@@ -2421,6 +2421,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("AsNothing.kt")
+            public void testAsNothing() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/cast/AsNothing.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("AsWithOtherParameter.kt")
             public void testAsWithOtherParameter() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/cast/AsWithOtherParameter.kt");