That is, if there's a problem with a witness, and the witness comes
from a different extension from the conformance (or the original type,
when the conformance is on an extension), put the main diagnostic on
the conformance, with a note on the witness. This involves some
shuffling and rephrasing of existing diagnostics too.
There's a few reasons for this change:
- More context. It may not be obvious why a declaration in file
A.swift needs to be marked 'public' if you can't see the conformance
in B.swift.
- Better locations for imported declarations. If you're checking a
conformance in a source file but the witness came from an imported
module, it's better to put the diagnostic on the part you have
control over. (This is especially true in Xcode, which can't display
diagnostics on imported declarations in the source editor.)
- Plays better with batch mode. Without this change, you can have
diagnostics being reported in file A.swift that are tied to a
conformance declared in file B.swift. Of course the contents of
A.swift also affect the diagnostic, but compiling A.swift on its
own wouldn't produce the diagnostic, and so putting it there is
problematic.
The change does in some cases make for a worse user experience,
though; if you just want to apply the changes and move on, the main
diagnostic isn't in the "right place". It's the note that has the info
and possible fix-it. It's also a slightly more complicated
implementation.
Currently, when we reference a (non-generic) typealias within a
generic context, we would completely lose type sugar for the
typealias, replacing it with the underlying type. Instead, use
BoundNameAliasType for this purpose, which allows us to maintain all
of the type sugar as well as storing complete substitutions for later
use.
Objective-C type parameters are not recoverable from an instance of
a generic Objective-C class, so we cannot refer to them anywhere in
type metadata. However, they could slip in via associated type
inference, which bypassed the existing checks. This would always crash
later in IRGen; ban it in the type checker so that doesn't happen.
Fixes rdar://problem/34979938.
