Improve diagnostic when attempting to use `+=` to append a single element

[hamishknight]

For e.g:

struct S {}
var arr: [S] = []
arr += S() // error: Operator function '+=' requires that 'S' conform to 'Sequence'

The diagnostic isn't too helpful. More helpful would be suggesting using [S()] (which could be a general fix where we attempt to match types using an array instead), though I think the ideal suggestion in this case would be to use append instead.

[grynspan]

Since the user's intent is generally clear, we could instead add an overload of += that lets you append an element to a range-replaceable collection like Array:

extension RangeReplaceableCollection {
  @inlinable public static func +=(lhs: inout Self, rhs: Element) {
    lhs.append(rhs)
  }

  @inlinable public static func +(lhs: Self, rhs: Element) -> Self {
    var lhs = lhs
    lhs += rhs
    return lhs
  }
}

[hamishknight]

@grynspan I think that has been suggested before, though it has the potential to cause confusion for both the user and the type-checker in cases where the argument could be used with either overload, e.g:

var arr: [Any] = [1]
arr += [2, 3]

Should arr be [1, 2, 3] or [1, [2, 3]]?

[grynspan]

I would personally expect that to take the append(contentsOf:)-based operator overload. Which we could ensure with judicious use of @_disfavoredOverload.

I know it's been suggested before, I just want to add my two cents. 😁