Sema: Fix source compatibility break from relaxed witness matching rules #17737
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This is fix for a source compat regression from: commit 790625a Author: Doug Gregor <dgregor@apple.com> Date: Mon Mar 19 15:29:32 2018 -0700 Allow a witness's noescape parameter to match a requirement's escaping parameter The regression is not severe but its easy enough to fix. With the above change, it was possible for an optional requirement that did not have a witness in Swift 4.1 to pick up a witness in Swift 4.2, because the escaping/noescape mismatch prevented it from being considered in Swift 4.1. If the new witness was not sufficiently visible, this caused a source compatibility regression. Work around this by discarding the witness if its not sufficiently visible. In -swift-version 5, the hack expires, and we revert to the stricter, more consistent behavior. Fixes <rdar://problem/39614880>.
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This is fix for a source compat regression from:
The regression is not severe but its easy enough to fix.
With the above change, it was possible for an optional requirement that did
not have a witness in Swift 4.1 to pick up a witness in Swift 4.2, because
the escaping/noescape mismatch prevented it from being considered in Swift 4.1.
If the new witness was not sufficiently visible, this caused a source
compatibility regression.
Work around this by discarding the witness if its not sufficiently
visible. In -swift-version 5, the hack expires, and we revert to the
stricter, more consistent behavior.
Fixes rdar://problem/39614880.