Crash when accessing generic property wrapper's `some` type-using wrappedValue

[lukaskollmer]

Description

The program below crashes the compiler.

Changing the type of wrappedValue to a non-some type, or removing the where T: P constraint from the property wrapper type makes it work fine.

Additionally, it seems like the crash is caused by the fact that the program is accessing a property on the wrappedValue. If we change hmmm to return elements instead of elements.count (and adjust its type accordingly), the program compiles without issues.

Alternatively, removing the where Self: Base constraint from the P protocol also makes it compile.

Reproduction

class Base {}
class Derived: Base {}

protocol P: Base {}
extension Derived: P {}


@propertyWrapper
struct Query<T: P> {
    var wrappedValue: some RandomAccessCollection<T> {
        Array<T>()
    }
}


struct S {
    @Query<Derived>
    private var elements
    
    var hmmm: Int {
        elements.count
    }
}

Stack dump

1.	Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
2.	Compiling with the current language version
3.	While evaluating request ASTLoweringRequest(Lowering AST to SIL for module test_2025_01_02)
4.	While silgen emitFunction SIL function "@$s15test_2025_01_021SV5countSivg".
 for getter for count (at /Users/lukas/Developer/test/test_2025-01-02/test_2025-01-02/main.swift:30:9)
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  swift-frontend           0x0000000105e26a9c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x0000000105e24cf0 llvm::sys::RunSignalHandlers() + 112
2  swift-frontend           0x0000000105e27068 SignalHandler(int) + 292
3  libsystem_platform.dylib 0x000000019179a584 _sigtramp + 56
4  libsystem_pthread.dylib  0x0000000191769c20 pthread_kill + 288
5  libsystem_c.dylib        0x0000000191676a30 abort + 180
6  swift-frontend           0x0000000100b87ce8 createDispatchingDiagnosticConsumerIfNeeded(swift::FrontendInputsAndOutputs const&, llvm::function_ref<std::__1::unique_ptr<swift::DiagnosticConsumer, std::__1::default_delete<swift::DiagnosticConsumer>> (swift::InputFile const&)>) + 0
7  swift-frontend           0x0000000105d96820 llvm::report_fatal_error(llvm::Twine const&, bool) + 280
8  swift-frontend           0x0000000105d96708 llvm::report_fatal_error(llvm::Twine const&, bool) + 0
9  swift-frontend           0x00000001012a8094 std::__1::deque<llvm::Function*, std::__1::allocator<llvm::Function*>>::push_back(llvm::Function* const&) + 0
10 swift-frontend           0x00000001012a7ef0 swift::Lowering::SILGenModule::useConformance(swift::ProtocolConformanceRef) + 244
11 swift-frontend           0x00000001011eb1d8 swift::Lowering::SILGenModule::postEmitFunction(swift::SILDeclRef, swift::SILFunction*) + 96
12 swift-frontend           0x00000001011eab44 swift::Lowering::SILGenModule::emitFunctionDefinition(swift::SILDeclRef, swift::SILFunction*) + 8460
13 swift-frontend           0x00000001011eb3bc swift::Lowering::SILGenModule::emitOrDelayFunction(swift::SILDeclRef) + 216
14 swift-frontend           0x00000001011e89ec swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 364
15 swift-frontend           0x000000010133ec8c (anonymous namespace)::SILGenType::visitFuncDecl(swift::FuncDecl*) + 32
16 swift-frontend           0x000000010133ee78 (anonymous namespace)::SILGenType::visitAbstractStorageDecl(swift::AbstractStorageDecl*) + 248
17 swift-frontend           0x000000010133ec08 (anonymous namespace)::SILGenType::visitVarDecl(swift::VarDecl*) + 456
18 swift-frontend           0x000000010133b290 (anonymous namespace)::SILGenType::emitType() + 516
19 swift-frontend           0x00000001011e8624 swift::ASTVisitor<swift::Lowering::SILGenModule, void, void, void, void, void, void>::visit(swift::Decl*) + 104
20 swift-frontend           0x00000001011ef038 swift::ASTLoweringRequest::evaluate(swift::Evaluator&, swift::ASTLoweringDescriptor) const + 1804
21 swift-frontend           0x000000010132592c swift::SimpleRequest<swift::ASTLoweringRequest, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>> (swift::ASTLoweringDescriptor), (swift::RequestFlags)9>::evaluateRequest(swift::ASTLoweringRequest const&, swift::Evaluator&) + 196
22 swift-frontend           0x00000001011f3f14 swift::ASTLoweringRequest::OutputType swift::Evaluator::getResultUncached<swift::ASTLoweringRequest, swift::ASTLoweringRequest::OutputType swift::evaluateOrFatal<swift::ASTLoweringRequest>(swift::Evaluator&, swift::ASTLoweringRequest)::'lambda'()>(swift::ASTLoweringRequest const&, swift::ASTLoweringRequest::OutputType swift::evaluateOrFatal<swift::ASTLoweringRequest>(swift::Evaluator&, swift::ASTLoweringRequest)::'lambda'()) + 528
23 swift-frontend           0x00000001007ce304 swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 2828
24 swift-frontend           0x00000001007d0e88 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 1684
25 swift-frontend           0x00000001007cfbb4 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3572
26 swift-frontend           0x0000000100756a5c swift::mainEntry(int, char const**) + 3680
27 dyld                     0x00000001913df154 start + 2476

Expected behavior

the code should compile

Environment

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx14.0

Additional information

No response

[Rajveer100]

I tried to debug this issue:

void SILGenModule::useConformance(ProtocolConformanceRef conformanceRef) {
  // If the conformance is invalid, crash deterministically even in noassert
  // builds.
  if (conformanceRef.isInvalid()) {
    // When lazy type checking is enabled, a conformance may only be diagnosed
    // as invalid during SILGen. Ignore it instead of asserting.
    auto &ctx = getASTContext();
    if (ctx.TypeCheckerOpts.EnableLazyTypecheck && ctx.hadError())
      return;

    llvm::report_fatal_error("Invalid conformance in type-checked AST");
  }

Seems like the underlying structure is a union with this form:

typedef llvm::PointerUnion<ProtocolDecl *, ProtocolConformance *, PackConformance *> UnionType;

In the Xcode debugger, this seems to be empty (char[8] = ""). Also, if I remove the condition from the protocol:

protocol P {}

Even though the code compiles as mentioned in the description of the issue, this debug statement shows there is an invalid conformance:

bool PackConformance::isInvalid() const {
  return llvm::any_of(getPatternConformances(),
                      [&](const auto ref) {
    if (ref.isInvalid())
      llvm::errs() << "An invalid conformance exists." << "\n";
    return ref.isInvalid();
  });
}

../build/Ninja-RelWithDebInfoAssert/swift-macosx-arm64/bin/swiftc -sdk /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk debug.swift
An invalid conformance exists.
An invalid conformance exists.

[Rajveer100]

@AnthonyLatsis
Why are we forcefully crashing here when it's obvious that the conformance is invalid, we should instead throw an error with a fix-it:

protocol P: AnyObject where Self: Base {}

Any thoughts?

[AnthonyLatsis]

Which conformance do you think is invalid and why? protocol P: AnyObject where Self: Base is equivalent to protocol P: Base.

We are crashing because we should not be lowering an AST containing an invalid conformance. It is the job of semantic analysis to diagnose invalid conformances and compute conformances correctly. I don’t see anything wrong with the code, so I think we are incorrectly computing this conformance somewhere.