[swift-evolution] [Last second] Precedence of nil-coalescing operator seems too low

[Erica Sadun]

Given: `let x = Optional(3)` then
`let y = 5 + x ?? 2` will not compile
but 
`let y = 5 + (x ?? 2)` will.

Should NilCoalescingPrecedence be raised? The current operator precedence chain is:

BitwiseShiftPrecedence > MultiplicationPrecedence > AdditionPrecedence > RangeFormationPrecedence > CastingPrecedence > NilCoalescingPrecedence > ComparisonPrecedence > LogicalConjunctionPrecedence > LogicalDisjunctionPrecedence > TernaryPrecedence > AssignmentPrecedence > FunctionArrowPrecedence > [nothing]

It seems to me that `NilCoalescingPrecedence` should probably be higher than `MultiplicationPrecedence` and possibly higher `BitwiseShiftPrecedence` as its job is to produce an unwrapped value that can then be operated upon.

I think CastingPrecedence should be even higher because
`expression as? T ?? fallback value`
should be parsed as
`(expression as? T) ?? (fallback value)`

I apologize profusely because I know this is beyond last minute,

-- E

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