This seems similar to recently fixed SR-12443, just tested seems fixed on master
cc @hamishknight

Yup, this has been fixed on master, please verify using the next available development snapshot from https://swift.org/download/#snapshots.

Comment by David L. Edwards (JIRA)

I tested my test case project against the Swift 5.2 Snapshot 2020-04-07 (Swift a543bfb). The crash remains.

overridebug.zip

1. Apple Swift version 5.2.2-dev (Swift a543bfb)

2. While evaluating request TypeCheckSourceFileRequest(source_file "/Users/dlemex/Downloads/project/ThreadSafeTest/StatusSingleton.swift", 0)

3. While evaluating request TypeCheckFunctionBodyUntilRequest(ThreadSafeTest.(file).StatusSingleton._@/Users/dlemex/Downloads/project/ThreadSafeTest/StatusSingleton.swift:24:25, )

4. While type-checking setter for isStatusEnabled (at /Users/dlemex/Downloads/project/ThreadSafeTest/StatusSingleton.swift:24:25)

5. While evaluating request ParseAbstractFunctionBodyRequest(ThreadSafeTest.(file).StatusSingleton._@/Users/dlemex/Downloads/project/ThreadSafeTest/StatusSingleton.swift:24:25)

0 swift 0x0000000104569138 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40

1 swift 0x0000000104568355 llvm::sys::RunSignalHandlers() + 85

2 swift 0x0000000104569710 SignalHandler(int) + 272

3 libsystem_platform.dylib 0x00007fff712115fd _sigtramp + 29

4 libsystem_platform.dylib 0x0000000000004e68 _sigtramp + 2396993672

5 libsystem_c.dylib 0x00007fff710e7808 abort + 120

6 libsystem_c.dylib 0x00007fff710e6ac6 err + 0

7 swift 0x000000010481ba01 swift::buildSelfReference(swift::VarDecl*, swift::SelfAccessorKind, bool, swift::ASTContext&) (.cold.1) + 33

8 swift 0x000000010128c560 swift::buildArgumentForwardingExpr(llvm::ArrayRef<swift::ParamDecl*>, swift::ASTContext&) + 0

9 swift 0x00000001013d06a7 buildStorageReference(swift::AccessorDecl*, swift::AbstractStorageDecl*, (anonymous namespace)::TargetImpl, bool, swift::ASTContext&) + 951

10 swift 0x00000001013d1b33 synthesizeObservedSetterBody(swift::AccessorDecl*, (anonymous namespace)::TargetImpl, swift::ASTContext&) + 291

11 swift 0x00000001013cf71e synthesizeAccessorBody(swift::AbstractFunctionDecl*, void*) + 3118

12 swift 0x000000010158ba66 swift::ParseAbstractFunctionBodyRequest::evaluate(swift::Evaluator&, swift::AbstractFunctionDecl*) const + 294

13 swift 0x000000010158bb31 swift::SimpleRequest<swift::ParseAbstractFunctionBodyRequest, swift::BraceStmt* (swift::AbstractFunctionDecl*), (swift::CacheKind)2>::evaluateRequest(swift::ParseAbstractFunctionBodyRequest const&, swift::Evaluator&) + 17

14 swift 0x000000010172b591 llvm::Expected<swift::ParseAbstractFunctionBodyRequest::OutputType> swift::Evaluator::getResultUncached<swift::ParseAbstractFunctionBodyRequest>(swift::ParseAbstractFunctionBodyRequest const&) + 465

15 swift 0x000000010172b2c8 llvm::Expected<swift::ParseAbstractFunctionBodyRequest::OutputType> swift::Evaluator::getResultCached<swift::ParseAbstractFunctionBodyRequest, (void*)0>(swift::ParseAbstractFunctionBodyRequest const&) + 72

16 swift 0x00000001016d7353 swift::ParseAbstractFunctionBodyRequest::OutputType swift::evaluateOrDefault<swift::ParseAbstractFunctionBodyRequest>(swift::Evaluator&, swift::ParseAbstractFunctionBodyRequest, swift::ParseAbstractFunctionBodyRequest::OutputType) + 35

dlemex (JIRA User) You need to download the master (trunk) snapshot, not the Swift 5.2 snapshot (as the fix wasn't merged into the 5.2 branch).

@swift-ci create

dlemex (JIRA User) As Suyash says, you need to test against a master development snapshot. I'm going to mark the issue as resolved again, but please do feel free to re-open if it's still reproducible on a master snapshot.