swift234

Sema: Implement type-checking for inout writeback conversion.

Add a third branch to the constraint system for '&x' expressions that allows conversion from an lvalue to a type via an additional writeback step:

- Add an LValueConversionExpr node that converts from @lvalue T to @lvalue U, given a pair of functions that convert T -> U and U -> T, to represent the writeback temporary.
- Allow conversion in an inout expression from @lvalue T to a type U that has the following members:

  static func __writeback_conversion(Builtin.RawPointer, T.Type) -> U
  static func __writeback_conversion_get(T) -> V
  static func __writeback_conversion_set(V) -> T

which builds a solution that produces an LValueConversion from the get/set pair before passing the pointer to the writeback temporary off to the conversion function.

Swift SVN r15764