Fix exhaustiveness check for when with subject variable

compiler/testData/diagnostics/tests/when/withSubjectVariable/smartcastToSealed.kt

@@ -0,0 +1,20 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+sealed class Either
+class Left : Either()
+class Right : Either()
+
+fun testSmartcastToSealedInSubjectInitializer1(x: Any?) {
+    val y1 = when (val either = x as Either) {
+        is Left -> "L"
+        is Right -> "R"
+    }
+}
+
+fun testSmartcastToSealedInSubjectInitializer2(x: Any?) {
+    val y2 = <!NO_ELSE_IN_WHEN!>when<!> (val either: Any = x as Either) { // NB explicit type annotation
+        is Left -> "L"
+        is Right -> "R"
+    }
+}