Put not-null original type as last possible smart cast target

This fixes muted tests testSmartCast (IDE highlighting) and
testNestedSealed (Diagnostics).
Together with commit 555b3f12 this makes #KT-15901 Fixed

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/DataFlowInfo.kt

@@ -46,6 +46,7 @@ interface DataFlowInfo {
      *
      * IMPORTANT: by default, the original (native) type for this value
      * are NOT included. So it's quite possible to get an empty set here.
+     * Also, type order in the result set MAKES SENSE so keep it stable and do not change without reason
      */
     fun getCollectedTypes(key: DataFlowValue): Set<KotlinType>
 
@@ -55,6 +56,7 @@ interface DataFlowInfo {
      *
      * IMPORTANT: by default, the original (native) type for this value
      * are NOT included. So it's quite possible to get an empty set here.
+     * Also, type order in the result set MAKES SENSE so keep it stable and do not change without reason
      */
     fun getStableTypes(key: DataFlowValue): Set<KotlinType>
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/DelegatingDataFlowInfo.kt

@@ -122,12 +122,12 @@ internal class DelegatingDataFlowInfo private constructor(
 
         val enrichedTypes = newLinkedHashSetWithExpectedSize<KotlinType>(types.size + 1)
         val originalType = key.type
-        if (originalType.isMarkedNullable) {
-            enrichedTypes.add(TypeUtils.makeNotNullable(originalType))
-        }
         for (type in types) {
             enrichedTypes.add(TypeUtils.makeNotNullable(type))
         }
+        if (originalType.isMarkedNullable) {
+            enrichedTypes.add(TypeUtils.makeNotNullable(originalType))
+        }
 
         return enrichedTypes
     }

idea/testData/checker/infos/SmartCastTarget.kt

@@ -0,0 +1,14 @@
+// See KT-15901
+
+<info>enum</info> class En { A }
+
+fun foo(): Any {
+    val en2: Any? = En.A
+    if (en2 is En) {
+        // Here we had smart casts to En / Any
+        // should be always En
+        val a: Any = <info descr="Smart cast to En">en2</info>
+        return a
+    }
+    return ""
+}

idea/testData/checker/infos/SmartCasts.kt

@@ -15,7 +15,7 @@ fun f9(a : A?) {
   a<info>?.</info><error descr="[UNRESOLVED_REFERENCE] Unresolved reference: bar">bar</error>()
   if (a is B) {
     <info descr="Smart cast to B">a</info>.bar()
-    <info descr="Smart cast to A">a</info>.foo()
+    <info descr="Smart cast to B">a</info>.foo()
   }
   a<info>?.</info>foo()
   a<info>?.</info><error descr="[UNRESOLVED_REFERENCE] Unresolved reference: bar">bar</error>()
@@ -30,7 +30,7 @@ fun f9(a : A?) {
     return;
   }
   <info descr="Smart cast to B">a</info>.bar()
-  <info descr="Smart cast to A">a</info>.foo()
+  <info descr="Smart cast to B">a</info>.foo()
 }
 
 fun fAny(a : Any?) {
@@ -96,7 +96,7 @@ fun f12(a : A?) {
 
 fun f13(a : A?) {
   if (a is B) {
-    <info descr="Smart cast to A">a</info>.foo()
+    <info descr="Smart cast to B">a</info>.foo()
     <info descr="Smart cast to B">a</info>.bar()
   }
   else {
@@ -109,12 +109,12 @@ fun f13(a : A?) {
     a<info>?.</info>foo()
   }
   else {
-    <info descr="Smart cast to A">a</info>.foo()
+    <info descr="Smart cast to B">a</info>.foo()
   }
 
   a<info>?.</info>foo()
-  if (a is B && <info descr="Smart cast to A">a</info>.foo() == Unit) {
-    <info descr="Smart cast to A">a</info>.foo()
+  if (a is B && <info descr="Smart cast to B">a</info>.foo() == Unit) {
+    <info descr="Smart cast to B">a</info>.foo()
     <info descr="Smart cast to B">a</info>.bar()
   }
   else {

idea/testData/checker/infos/SmartCastsWithSafeAccess.kt

@@ -16,7 +16,7 @@ fun test(a: A?) {
     }
 
     if (a is B && a is C) {
-        <info descr="Smart cast to A">a</info>.foo()
+        <info descr="Smart cast to B">a</info>.foo()
     }
 
     if (a is B? && a is C?) {

idea/tests/org/jetbrains/kotlin/checkers/PsiCheckerTestGenerated.java

@@ -917,6 +917,12 @@ public class PsiCheckerTestGenerated extends AbstractPsiCheckerTest {
             doTestWithInfos(fileName);
         }
 
+        @TestMetadata("SmartCastTarget.kt")
+        public void testSmartCastTarget() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/checker/infos/SmartCastTarget.kt");
+            doTestWithInfos(fileName);
+        }
+
         @TestMetadata("SmartCastToEnum.kt")
         public void testSmartCastToEnum() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/checker/infos/SmartCastToEnum.kt");