class Class {}
public struct Test<T: AnyObject> {
    internal unowned let t: Class!

    public init() {
        self.t = nil
    }
}

public struct Test<T: AnyObject> {
    internal unowned(unsafe) let t: T!

    public init() {
        self.t = nil
    }
}

public struct Test<T: AnyObject> {
    internal unowned let t: T!

    public init() {
        self.t = nil
    }
}

Stack dump:
0.  Program arguments: /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c -primary-file ../../Libraries/TestFail/Sources/TestFail/Test.swift  -emit-module-path Libraries/TestFail/CMakeFiles/TestFail.dir/Sources/TestFail/Test.swift.swiftmodule -emit-module-doc-path Libraries/TestFail/CMakeFiles/TestFail.dir/Sources/TestFail/Test.swift.swiftdoc -emit-module-source-info-path Libraries/TestFail/CMakeFiles/TestFail.dir/Sources/TestFail/Test.swift.swiftsourceinfo -emit-dependencies-path Libraries/TestFail/CMakeFiles/TestFail.dir/Sources/TestFail/Test.swift.o.d -emit-reference-dependencies-path Libraries/TestFail/CMakeFiles/TestFail.dir/Sources/TestFail/Test.swift.o.swiftdeps -target x86_64-apple-darwin19.5.0 -enable-objc-interop -stack-check -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -I .build/swift -g -module-link-name TestFail -D TestFail_EXPORTS -enable-anonymous-context-mangled-names -target-sdk-version 10.15.4 -parse-as-library -module-name TestFail -o Libraries/TestFail/CMakeFiles/TestFail.dir/Sources/TestFail/Test.swift.o 
1.  Apple Swift version 5.3 (swiftlang-1200.0.16.9 clang-1200.0.22.5)
2.  While evaluating request SILGenSourceFileRequest(SIL Generation for file "../../Libraries/TestFail/Sources/TestFail/Test.swift")
3.  While emitting SIL for getter for t (at ../../Libraries/TestFail/Sources/TestFail/Test.swift:45:26)
0  swift                    0x00000001054bdf65 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x00000001054bcf65 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x00000001054be51f SignalHandler(int) + 111
3  libsystem_platform.dylib 0x00007fff6f1d75fd _sigtramp + 29
4  libsystem_platform.dylib 0x00007fdb1d861b10 _sigtramp + 18446743917721855280
5  swift                    0x000000010238d735 swift::UnownedStorageType::isLoadable(swift::ResilienceExpansion) const + 181
6  swift                    0x0000000101c37c0f swift::CanTypeVisitor<(anonymous namespace)::TypeClassifier, swift::Lowering::TypeLowering::RecursiveProperties, swift::Lowering::AbstractionPattern>::visit(swift::CanType, swift::Lowering::AbstractionPattern) + 2079
7  swift                    0x0000000101c3b3ea (anonymous namespace)::LowerType::visitAnyStructType(swift::CanType, swift::Lowering::AbstractionPattern, swift::StructDecl*) + 426
8  swift                    0x0000000101c30c87 swift::Lowering::TypeConverter::getTypeLowering(swift::Lowering::AbstractionPattern, swift::Type, swift::TypeExpansionContext) + 1015
9  swift                    0x0000000101bbf6d4 (anonymous namespace)::DestructureInputs::visit(swift::ValueOwnership, bool, swift::Lowering::AbstractionPattern, swift::CanType, swift::SILFunctionTypeRepresentation, bool) + 452
10 swift                    0x0000000101bbd82d getSILFunctionType(swift::Lowering::TypeConverter&, swift::TypeExpansionContext, swift::Lowering::AbstractionPattern, swift::CanTypeWrapper<swift::AnyFunctionType>, swift::AnyFunctionType::ExtInfo, (anonymous namespace)::Conventions const&, swift::ForeignInfo const&, llvm::Optional<swift::SILDeclRef>, llvm::Optional<swift::SILDeclRef>, llvm::Optional<swift::SubstitutionMap>, swift::ProtocolConformanceRef) + 2109
11 swift                    0x0000000101bb850c getNativeSILFunctionType(swift::Lowering::TypeConverter&, swift::TypeExpansionContext, swift::Lowering::AbstractionPattern, swift::CanTypeWrapper<swift::AnyFunctionType>, swift::AnyFunctionType::ExtInfo, llvm::Optional<swift::SILDeclRef>, llvm::Optional<swift::SILDeclRef>, llvm::Optional<swift::SubstitutionMap>, swift::ProtocolConformanceRef) + 1724
12 swift                    0x0000000101bb97b9 getUncachedSILFunctionTypeForConstant(swift::Lowering::TypeConverter&, swift::TypeExpansionContext, swift::SILDeclRef, swift::CanTypeWrapper<swift::AnyFunctionType>) + 1625
13 swift                    0x0000000101bba0fc swift::Lowering::TypeConverter::getConstantInfo(swift::TypeExpansionContext, swift::SILDeclRef) + 1148
14 swift                    0x0000000101bb21e1 swift::SILFunctionBuilder::getOrCreateFunction(swift::SILLocation, swift::SILDeclRef, swift::ForDefinition_t, llvm::function_ref<swift::SILFunction* (swift::SILLocation, swift::SILDeclRef)>, swift::ProfileCounter) + 145
15 swift                    0x00000001014c3123 swift::Lowering::SILGenModule::getFunction(swift::SILDeclRef, swift::ForDefinition_t) + 467
16 swift                    0x00000001014c468e swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 846
17 swift                    0x00000001015db238 void llvm::function_ref<void (swift::AccessorDecl*)>::callback_fn<(anonymous namespace)::SILGenType::visitAccessors(swift::AbstractStorageDecl*)::'lambda'(swift::AccessorDecl*)>(long, swift::AccessorDecl*) + 24
18 swift                    0x00000001022505ab swift::AbstractStorageDecl::visitExpectedOpaqueAccessors(llvm::function_ref<void (swift::AccessorKind)>) const + 219
19 swift                    0x00000001015db211 (anonymous namespace)::SILGenType::visitVarDecl(swift::VarDecl*) + 977
20 swift                    0x00000001015d5b0b (anonymous namespace)::SILGenType::emitType() + 427
21 swift                    0x00000001014cf2a2 swift::ASTVisitor<swift::Lowering::SILGenModule, void, void, void, void, void, void>::visit(swift::Decl*) + 82
22 swift                    0x00000001014cd5c9 (anonymous namespace)::SILGenModuleRAII::emitSourceFile(swift::SourceFile*) + 1417
23 swift                    0x00000001014ccf89 swift::SILGenSourceFileRequest::evaluate(swift::Evaluator&, swift::SILGenDescriptor) const + 137
24 swift                    0x00000001015c7c41 swift::SimpleRequest<swift::SILGenSourceFileRequest, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> > (swift::SILGenDescriptor), (swift::RequestFlags)9>::evaluateRequest(swift::SILGenSourceFileRequest const&, swift::Evaluator&) + 33
25 swift                    0x00000001014d3dbc llvm::Expected<swift::SILGenSourceFileRequest::OutputType> swift::Evaluator::getResultUncached<swift::SILGenSourceFileRequest>(swift::SILGenSourceFileRequest const&) + 972
26 swift                    0x00000001014cee1d swift::performSILGeneration(swift::FileUnit&, swift::Lowering::TypeConverter&, swift::SILOptions const&) + 173
27 swift                    0x00000001010414bf swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 20639
28 swift                    0x0000000100fc2a97 main + 1255
29 libdyld.dylib            0x00007fff6efdecc9 start + 1
30 libdyld.dylib            0x0000000000000025 start + 18446603338654028637
<unknown>:0: error: unable to execute command: Illegal instruction: 4
<unknown>:0: error: compile command failed due to signal 4 (use -v to see invocation)
ninja: build stopped: subcommand failed.