[PATCH] D121177: [modules] Merge equivalent extensions and diagnose ivar redeclarations for extensions loaded from different modules.

[Volodymyr Sapsai via Phabricator via cfe-commits]

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Emitting metadata for the same ivar multiple times can lead to
miscompilations. Objective-C runtime adds offsets to calculate ivar
position in memory and presence of duplicate offsets causes wrong final
position thus overwriting unrelated memory.

Such a situation is impossible with modules disabled as clang diagnoses
ivar redeclarations during sema checks after parsing
(`Sema::ActOnFields`). Fix the case with modules enabled by checking
during deserialization if ivar is already declared. We also support
a use case where the same category ends up in multiple modules. We
don't want to treat this case as ivar redeclaration and instead merge
corresponding ivars.

rdar://83468070


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121177

Files:
  clang/include/clang/AST/DeclObjC.h
  clang/include/clang/Serialization/ASTReader.h
  clang/lib/AST/DeclObjC.cpp
  clang/lib/Serialization/ASTReader.cpp
  clang/lib/Serialization/ASTReaderDecl.cpp
  clang/test/Modules/merge-extension-ivars.m
  clang/test/Modules/redecl-ivars.m

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