CAST_NEVER_SUCCEEDS: do not report when casting nullable to nullable #KT-260 Fixed

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

@@ -37,9 +37,12 @@ object CastDiagnosticsUtil {
             lhsType: KotlinType,
             rhsType: KotlinType,
             platformToKotlinClassMap: PlatformToKotlinClassMap): Boolean {
-        if (KotlinBuiltIns.isNullableNothing(lhsType) && !TypeUtils.isNullableType(rhsType)) return false
+        val rhsNullable = TypeUtils.isNullableType(rhsType)
+        val lhsNullable = TypeUtils.isNullableType(lhsType)
+        if (KotlinBuiltIns.isNullableNothing(lhsType) && !rhsNullable) return false
         if (KotlinBuiltIns.isNothing(rhsType)) return false
-        if (KotlinBuiltIns.isNullableNothing(rhsType)) return TypeUtils.isNullableType(lhsType)
+        if (KotlinBuiltIns.isNullableNothing(rhsType)) return lhsNullable
+        if (lhsNullable && rhsNullable) return true
         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/NullableToNullable.kt

@@ -0,0 +1,7 @@
+// From KT-13324: always succeeds
+val x = null <!USELESS_CAST!>as String?<!>
+// From KT-260: sometimes succeeds
+fun foo(a: String?): Int? {
+    val c = a as? Int?
+    return c
+}

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

@@ -0,0 +1,4 @@
+package
+
+public val x: kotlin.String? = null
+public fun foo(/*0*/ a: kotlin.String?): kotlin.Int?

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

@@ -2673,6 +2673,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("NullableToNullable.kt")
+            public void testNullableToNullable() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/cast/NullableToNullable.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("WhenErasedDisallowFromAny.kt")
             public void testWhenErasedDisallowFromAny() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/cast/WhenErasedDisallowFromAny.kt");