Introduce action to add missing when branches on sealed class

Made via diagnostics NON_EXHAUSTIVE_WHEN_FOR_SEALED_CLASS
with INFO severity and quick-fix
So #KT-17580 Fixed

idea/idea-analysis/src/org/jetbrains/kotlin/idea/highlighter/KotlinPsiChecker.kt

@@ -250,7 +250,7 @@ private class ElementAnnotator(private val element: PsiElement,
                         }
                 )
             }
-            Severity.INFO -> return // Do nothing
+            Severity.INFO -> AnnotationPresentationInfo(ranges, highlightType = ProblemHighlightType.INFORMATION)
         }
 
         setUpAnnotations(diagnostics, presentationInfo)
@@ -298,7 +298,7 @@ private class AnnotationPresentationInfo(
                     holder.createWarningAnnotation(range, defaultMessage)
                 }
             }
-            else -> throw IllegalArgumentException("Only ERROR and WARNING diagnostics are supported")
+            Severity.INFO -> holder.createInfoAnnotation(range, defaultMessage)
         }
 
         annotation.tooltip = getMessage(diagnostic)

idea/src/org/jetbrains/kotlin/idea/quickfix/QuickFixRegistrar.kt

@@ -241,6 +241,8 @@ class QuickFixRegistrar : QuickFixContributor {
         REDUNDANT_ELSE_IN_WHEN.registerFactory(RemoveWhenElseBranchFix)
         NON_EXHAUSTIVE_WHEN.registerFactory(AddWhenElseBranchFix)
         NON_EXHAUSTIVE_WHEN.registerFactory(AddWhenRemainingBranchesFix)
+        NON_EXHAUSTIVE_WHEN_ON_SEALED_CLASS.registerFactory(AddWhenElseBranchFix)
+        NON_EXHAUSTIVE_WHEN_ON_SEALED_CLASS.registerFactory(AddWhenRemainingBranchesFix)
         BREAK_OR_CONTINUE_IN_WHEN.registerFactory(AddLoopLabelFix)
 
         NO_TYPE_ARGUMENTS_ON_RHS.registerFactory(AddStarProjectionsFix.IsExpressionFactory)

idea/testData/quickfix/when/addRemainingBranchesSealedStatement.kt

@@ -0,0 +1,16 @@
+// "Add remaining branches" "true"
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+sealed class Variant {
+    object Singleton : Variant()
+
+    class Something(val x: Int) : Variant()
+
+    object Another : Variant()
+}
+fun test(v: Variant?) {
+    wh<caret>en(v) {
+        Variant.Singleton -> "s"
+    }
+}

idea/testData/quickfix/when/addRemainingBranchesSealedStatement.kt.after

@@ -0,0 +1,19 @@
+// "Add remaining branches" "true"
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+sealed class Variant {
+    object Singleton : Variant()
+
+    class Something(val x: Int) : Variant()
+
+    object Another : Variant()
+}
+fun test(v: Variant?) {
+    when(v) {
+        Variant.Singleton -> "s"
+        is Variant.Something -> TODO()
+        Variant.Another -> TODO()
+        null -> TODO()
+    }
+}

idea/tests/org/jetbrains/kotlin/idea/quickfix/QuickFixTestGenerated.java

@@ -10957,6 +10957,12 @@ public class QuickFixTestGenerated extends AbstractQuickFixTest {
             doTest(fileName);
         }
 
+        @TestMetadata("addRemainingBranchesSealedStatement.kt")
+        public void testAddRemainingBranchesSealedStatement() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/quickfix/when/addRemainingBranchesSealedStatement.kt");
+            doTest(fileName);
+        }
+
         public void testAllFilesPresentInWhen() throws Exception {
             KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/testData/quickfix/when"), Pattern.compile("^([\\w\\-_]+)\\.kt$"), TargetBackend.ANY, true);
         }