Doug Gregor e7eac0af22 [Type checker] Extend the diagnostics for unwrapping the base of a member access. ...

Introduce a new fix kind into the constraint solver to cover unwrapping the base
of a member access so we can refer to the a member of the unwrapped base.
Wire this fix kind to the just-added diagnostic that suggests either the
chaining ‘?’ or the force-unwrap ‘!’ via separate, descriptive Fix-Its.

Example:

error: value of optional type 'X?' must be unwrapped to refer to member 'f' of wrapped base type 'X'
  let _: Int = x.f()
               ^
note: chain the optional using '?' to access member 'f' only for non-'nil' base values
  let _: Int = x.f()
               ^
                ?
note: force-unwrap using '!' to abort execution if the optional value contains 'nil'
  let _: Int = x.f()
               ^
                !

Before this, we would sometimes get a Fix-It for just ‘?’ and sometimes get a Fix-It for the
coalescing ‘??’, neither of which is likely to be right.

More work on rdar://problem/42081852.

2018-07-13 16:26:03 -07:00

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