Why does my explicit init crash?

[Nevin]

The default memberwise initializer does not evaluate variable-initialization expressions when explicit values are passed in. User-written initializers do evaluate them in all cases.

This is a known behavior, which was first discussed during the SE–0242 review thread:

Quote

@tkremenek

The proposal review also discussed an interesting behavioral quirk with initial values and side-effects, for example:

struct Foo {
  var i: Int = getDefaultValue()
}

struct Bar {
  var i: Int = getDefaultValue()
  init(i: Int) { self.i = i }
}

let x = Foo(i: 1)  // No side effects
let y = Bar(i: 1)  // Yes side effects

The Core Team felt that this was a bug in the compiler's implementation of initializers. The failure here is that Bar calls the side effect, and it is not the right behavior. Addressing this would likely require an evolution proposal, or at least an assessment of whether or not fixing this would break existing code.

The core team described the behavior as a bug, however I don’t know of a corresponding bug report issue for it.

• • •

A few more recent threads have mentioned this as well:

• Pre-pitch: remove the implicit initialization of Optional variables

• What is a ‘variable initialization expression’?