If the input has labels, including them in the pattern has clear value: the
compiler can check that the labels you expected are there, preventing value
swapping bugs. Being able to omit the labels in the pattern is a reasonable
convenience to avoid repeating yourself over and over. But being able to insert
arbitrary labels that don’t match anything from the input is very weird,
because it doesn’t really make anything more convenient or give the compiler
fuel to catch mistakes.
The proposal is essentially asking for slightly nicer syntax for comments in
patterns. That is,
switch (1, 2) {
case (width: 0, height: 0):
…
}
is entirely equivalent to:
switch (1, 2) {
case (/*width:*/ 0, /*height:*/ 0):
…
}
That said, there’s a very clear inconsistency here between if-case-let and
switch-case-let, which you would expect to be semantically equivalent:
let t1: (Int, Int) = (0, 0)
// This works
if case let (a: 0, b: y5) = t1 {
print("hello")
}
// This doesn't
switch t1 {
case let (a: 0, b: y6):
print("hallo")
}
So unless someone has a compelling argument that if-case-let and
switch-case-let should be different, one of these should probably be changed.
Unless the behaviour of if-case-let is *clearly* a random compiler bug that no
one is relying on, then source stability dictates that switch should become
more permissive and allow these “comment" labels. I don’t have enough
background in this feature’s design to say either way.
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