[swift-evolution] Different types for getter and setter

[Nevin Brackett-Rozinsky]

This may sound rather strange in the abstract, but recently I have
encountered two situations where I would like to have a setter that accepts
a different type than the getter returns.

In the first, the getter returns Foo and the setter should accept
“@escaping @autoclosure () -> Foo”, so that the expression assigned to the
property is not evaluated until it is needed. (The closure is stored in a
private property, which the getter evaluates then caches the result.)

In the second, I want a subscript whose getter returns a concrete type (in
my case, subscripting a matrix by row returns an ArraySlice<Element>) while
the setter can accept something more generic (any kind of Collection with
the correct Element type).

Thoughts?

Nevin
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