We encountered a very surprising behaviour in one of our apps, and believe it’s
a bug/missing warning in the Swift compilter, but it would be great if someone
could confirm we’re not missing a piece before I file a bug report.
This snippet reproduces the issue:
protocol MyProto {
var title: String? { get }
}
let tuple = (x: 100, y: 100)
// Results in a warning as expected: Cast always fails
let castedTupleToInt = tuple as? Int
// No warning, although cast will always fail
let castedTuple = tuple as? MyProto
The way I see it, it should be pretty easy for the compiler to detect that
casting a tuple to a protocol always fails, since there’s no way for a tuple to
conform to a protocol.
We came across the bug when we refactored a method to return a tuple instead of
a protocol type, and thought the compiler would make us aware of all the call
sites that need to adopt the the refactored return type. However, wherever we
casted the return type, no warning was raised and the code just silently
returned early because the cast always fails at runtime (as it should, of
course).
Thanks for your attention,
Sebastian
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