I tried writing a generic “FirstNonNil” function, but got stuck at the "==
nil" part.
First I thought to maybe add a type list enumerating all of the nillable
types, but I don't have a way to enumerate all of the possible interface
types as part of that list, and there is no constraint for “any interface
type”.
Then I thought I could use a `var zero T` variable and compare `== zero`
instead of `== nil`. But that doesn't work either: I need the `comparable`
constraint to get `==`, but not every nillable type is comparable with an
arbitrary variable (slices, maps, and functions are not).
So it seems that the best I can do is to write a generic `First` function
and make the caller write out `func (x T) bool { return x != nil }`
explicitly at every call site. Did I miss a better option?
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