Mark Lacey d4a5fe58be Attempt to bind the result of unwrapping optionals if we're asked to bind subtypes. ...

When trying to solve for the test case we attempt to simplify a value
member constraint and it fails because we've bound the LHS type
variable to an optional as a result of other constraints involving
other type variables in the equivalence class of this type
variable.

We don't have enough information to directly deduce the non-optional
type directly from other constraints involving this type variable.

This change results in one interesting type checking anomoly. In Swift
3 mode, we now successfully typecheck an expression that we previously
did not. Although the type checking technically makes sense given the
type checking rules we have in place, it can be a bit surprising to
users. Fortunately we emit a warning that calls out the surprising
behavior of considering an expression unused.

Fixes rdar://problem/30271695.

2017-02-19 14:53:01 -07:00

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