Doug Gregor e97ab635ea [Constraint solver] Separate bridging conversions from other conversions. ...

Previously, bridging conversions were handled as a form of "explicit
conversion" that was treated along the same path as normal
conversions in matchTypes(). Historically, this made some
sense---bridging was just another form of conversion---however, Swift
now separates out bridging into a different kind of conversion that is
available only via an explicit "as". This change accomplishes a few
things:

* Improves type inference around "as" coercions. We were incorrectly
  inferring type variables of the "x" in "x as T" in cases where a
  bridging conversion was expected, which cause some type inference
  failures (e.g., the SR-3319 regression).

* Detangles checking for bridging conversions from other conversions,
  so it's easier to isolate when we're applying a bridging
  conversion.

* Explicitly handle optionals when dealing with bridging conversions,
  addressing a number of problems with incorrect diagnostics, e.g.,
  complains about "unrelated type" cast failures that would succeed at
  runtime.

Addresses rdar://problem/29496775 / SR-3319 / SR-2365.

2016-12-21 13:46:14 -08:00

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