Match

A break-free switch (without fallthrough behavior).

Unlike a JavaScript switch, it can be used in expression position. It supports alternatives explicitly (hence the List[MatchableLiteral] in cases), whereas in a switch one would use the fallthrough behavior to implement alternatives. (This is not a pattern matching construct like in Scala.)

The selector must be either an int (IntType) or a java.lang.String. The cases can be any MatchableLiteral, even if they do not make sense for the type of the selecter (they simply will never match).

Because +0.0 === -0.0 in JavaScript, and because those semantics are used in a JS switch, we have to prevent the selector from ever being -0.0. Otherwise, it would be matched by a case IntLiteral(0). At the same time, we must allow at least int and java.lang.String to support all switchable matches from Scala. Since the latter two have no common super type that does not allow -0.0, we really have to special-case those two types.

This is also why we restrict MatchableLiterals to IntLiteral, StringLiteral and Null. Allowing more cases would only make IR checking more complicated, without bringing any added value.