[5.3] Compatibility51: Backport the 5.2 implementation of the conformance cache. #31281
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
The runtime that shipped with Swift 5.1 and earlier had a bug that interfered with backward
deployment of binaries that dynamically check for protocol conformances on conditionally-available tests. This was fixed in the top-of-tree Swift runtime by #29887; however, that doesn't do much good for running binaries on older OSes that don't have that fix. In order for binaries built with a newer Swift compiler to run successfully on older OSes, introduce a compatibility hook that replaces the conformance cache implementation in the original OS runtime with a version based on the current implementation that has the fix for the protocol conformance bug. Fixes rdar://problem/59460603