[swift-evolution] [Pitch] Remove bit shift traps

[Patrick Pijnappel]

Currently, bit shifting with an amount greater than or equal to the size of
the type traps:

func foo(x: Int32) {
  let y = x << 32 // Runtime trap (for any << or >> with amount >= 32)
}

I propose to make this not trap, and just end up with 0 (or ~0 in case of
right-shifting a negative number):

   - Unlike the traps for integer arithmetic and casts, it is obvious what
   a bitshift past the end does as fundamentally the behavior stays the same.
   - If the intention is to make it analogous with multiplication/division
   by 2**n, the checks don't really change anything. Right shift are still
   identical to divisions by 2**n. Left shifts are like multiplication by 2**n
   but with different overflow behavior, which is already the case with the
   current rules (e.g. Int.max << 1 doesn't trap)
   - It could lead to bugs where users expect this to work, e.g. the
   following crashes when the entire buffer is consumed: buffer = buffer <<
   bitsConsumed
   - Bitshift are often used in performance-sensitive code, and with the
   current behavior any non-constant bit shift introduces a branch.
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