Robert Widmann 9bd02e1d17 Allow eliding ownership conventions in a copyable context ...

Allow witnesses to protocols in a copyable context to elide explicit
ownership conventions.  This allows clients like the standard library to
standardize on one ownership convention without an ABI or API breaking
change in 3rd party code.

In the future, moveonly contexts must disallow this default behavior
else the witness thunks could silently transfer ownership.

rdar://40774922

protocol P {
  __consuming func implicit(x: __shared String)
  __consuming func explicit(x: __owned String)
  __consuming func mismatch(x: __shared String)
}

class C : P {
  // C.implicit(x:) takes self and x '@guaranteed' thru the witness thunk
  func implicit(x: String) {}
  // C.explicit(x:) takes self and x @owned with no convention changes
  __consuming func explicit(x: __owned String) {}
  // Would inherit __consuming, but x has been spelled __owned so the requirement match fails.
  func mismatch(x: __owned String) {}
}

2018-06-19 12:25:50 -07:00

7.9 KiB

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