[4.0] IRGen: Fix linkage for shared declarations #12164
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They need external linkage not linkonce_odr which can only be used by defintions not declarations. This got exposed by clang imported protocol witness table declarations. They get assigned shared linkage since they are potentially not unique. rdar://26563441 (cherry picked from commit 3d34fb3)
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@swift-ci Please test |
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@swift-ci Please test source compatibility |
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They need external linkage not linkonce_odr which can only be used by defintions
not declarations. This got exposed by clang imported protocol witness table declarations. They get assigned shared linkage since they are potentially not unique.
Explanation: There are situations where we end up with a SIL declaration of a protocol witness table that has shared linkage. We will unconditionally assign llvm’s linkonce_odr linkage to shared definitions and declarations. However, only definitions are allowed with this linkage. Instead we should use external linkage for such declarations.
This situation can arise under optimized incremental compilation when there are two files. One contains a protocol defintion that extends a clang importer synthesized protocol and another file that uses that protocol.
Scope of Issue: Compiler error when one uses incremental compilation with a definition of a hashable protocol conformance to a NS_OPTION type in one file and the use of the NS_OPTION type hashable conformance in another file. This is an older bug that has been around since at least last year.
Risk: Low. We are changing the linkage of a declaration from linkonce (which crashes) to a declaration with external linkage. What can at most happen is that we change an compiler error into a linker error (because the declaration is not satisfied by a definition somewhere else).
Testing: Added swift regression test
rdar://26563441