KT-10139:

Non-exhaustive 'when' without 'else' used in expression is an error
regardless of expected type: it can't be an expression, even of type Unit.

compiler/testData/diagnostics/tests/when/NoElseExpectedUnit.kt

@@ -1,5 +1,5 @@
 fun foo(x: Int) {
-    val y: Unit = when (x) {
+    val y: Unit = <!NO_ELSE_IN_WHEN!>when<!> (x) {
         2 -> {}
         3 -> {}
     }

compiler/testData/diagnostics/tests/when/NoElseReturnedUnit.kt

@@ -1,5 +1,5 @@
 fun foo(x: Int) {
-    return when (x) {
+    return <!NO_ELSE_IN_WHEN!>when<!> (x) {
         2 -> {}
         3 -> {}
     }