John McCall 39b65f49a0 Track the actual DC of a member access in the constraint system. ...

Without this, CSGen/CSSimplify and CSApply may have differing
opinions about whether e.g. a let property is settable, which
can lead to invalid ASTs.

Arguably, a better fix would be to remove the dependency on the
exact nested DC.  For example, we could treat lets as settable
in all contexts and then just complain later about invalid
attempts to set them.  Or we could change CSApply to directly
use the information it already has about how an l-value is used,
rather than trying to figure out whether it *might* be getting set.
But somehow, tracking a new piece of information through the
entire constraint system seems to be the more minimal change.

Fixes rdar://29810997.

2017-01-26 00:14:21 -05:00

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