>>> val a = when(null) { }
error: 'when' expression must be exhaustive, add necessary 'else' branch
val a = when(null) { }
        ^
>>> "" + when (true) { true -> 42 }
error: 'when' expression must be exhaustive, add necessary 'false' branch or 'else' branch instead
"" + when (true) { true -> 42 }
     ^