Local function forward reference of `lazy` var crashes in SILGen

[hamishknight]

The following hits an assert in SILGen:

do {
  func foo() -> Any { x }
  lazy var x = foo()
}

Assertion failed: (existing->second == context && "closure shouldn't be emitted with different capture type expansion contexts"), function setCaptureTypeExpansionContext, file TypeLowering.cpp, line 5381.
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.
Stack dump:
0.      Program arguments: /Users/hamish/src/swift-dev/build/Release/swift-macosx-arm64/bin/swift-frontend -typecheck /Users/hamish/src/swift-test-arena/swift-test-arena/main.swift -sdk /Users/hamish/src/MacOSX.sdk -debug-diagnostic-names -emit-silgen
1.      Swift version 6.3-dev (LLVM 4d15cb92d0756cd, Swift 8c1e0c83ec0408e)
2.      Compiling with effective version 5.10
3.      While evaluating request ASTLoweringRequest(Lowering AST to SIL for module main)
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           0x000000010667be60 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x0000000106679b68 llvm::sys::RunSignalHandlers() + 172
2  swift-frontend           0x000000010667c948 SignalHandler(int, __siginfo*, void*) + 344
3  libsystem_platform.dylib 0x000000019678e744 _sigtramp + 56
4  libsystem_pthread.dylib  0x0000000196784888 pthread_kill + 296
5  libsystem_c.dylib        0x000000019668a808 abort + 124
6  libsystem_c.dylib        0x0000000196689a3c err + 0
7  swift-frontend           0x00000001066cbfd4 swift::Lowering::TypeConverter::setLoweredAddresses() (.cold.8) + 0
8  swift-frontend           0x000000010118d440 swift::Lowering::TypeConverter::setCaptureTypeExpansionContext(swift::SILDeclRef, swift::SILModule&) + 436
9  swift-frontend           0x0000000100e0f340 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 128
10 swift-frontend           0x0000000101da9a78 swift::AbstractStorageDecl::visitExpectedOpaqueAccessors(llvm::function_ref<void (swift::AccessorKind)>) const + 68
11 swift-frontend           0x0000000101da9a1c swift::AbstractStorageDecl::visitEmittedAccessors(llvm::function_ref<void (swift::AccessorDecl*)>) const + 160
12 swift-frontend           0x0000000100e13cf0 swift::Lowering::SILGenModule::visitEmittedAccessors(swift::AbstractStorageDecl*, llvm::function_ref<void (swift::AccessorDecl*)>) + 48
13 swift-frontend           0x0000000100e6b330 swift::Lowering::SILGenFunction::visitVarDecl(swift::VarDecl*) + 160
14 swift-frontend           0x0000000100f0e6c4 swift::ASTVisitor<(anonymous namespace)::StmtEmitter, void, void, void, void, void, void>::visit(swift::Stmt*) + 5388
15 swift-frontend           0x0000000100f0d1ac swift::Lowering::SILGenFunction::emitStmt(swift::Stmt*) + 24
16 swift-frontend           0x0000000100f1adb0 swift::Lowering::SILGenTopLevel::visitTopLevelCodeDecl(swift::TopLevelCodeDecl*) + 156
17 swift-frontend           0x0000000100f19608 swift::Lowering::SILGenTopLevel::visitSourceFile(swift::SourceFile*) + 100
18 swift-frontend           0x0000000100f18e44 swift::Lowering::SILGenModule::emitEntryPoint(swift::SourceFile*, swift::SILFunction*) + 776
19 swift-frontend           0x0000000100f1a744 swift::Lowering::SILGenModule::emitEntryPoint(swift::SourceFile*) + 268
20 swift-frontend           0x0000000100e144dc swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*) + 780
21 swift-frontend           0x0000000100e14760 swift::ASTLoweringRequest::evaluate(swift::Evaluator&, swift::ASTLoweringDescriptor) const + 608
22 swift-frontend           0x0000000100f0cbd0 swift::SimpleRequest<swift::ASTLoweringRequest, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>> (swift::ASTLoweringDescriptor), (swift::RequestFlags)17>::evaluateRequest(swift::ASTLoweringRequest const&, swift::Evaluator&) + 136
23 swift-frontend           0x0000000100e18764 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'())::'lambda'()::operator()() const + 84
24 swift-frontend           0x0000000100e18680 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'()) + 320
25 swift-frontend           0x0000000100e14a34 swift::performASTLowering(swift::ModuleDecl*, swift::Lowering::TypeConverter&, swift::SILOptions const&, swift::IRGenOptions const*) + 120
26 swift-frontend           0x0000000100837b10 swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*, llvm::ArrayRef<char const*>) + 2712
27 swift-frontend           0x00000001008475c4 withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) + 164
28 swift-frontend           0x000000010083b19c performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*, llvm::ArrayRef<char const*>) + 556
29 swift-frontend           0x0000000100838d54 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2448
30 swift-frontend           0x00000001005d3804 swift::mainEntry(int, char const**) + 3244
31 dyld                     0x00000001963c5d54 start + 7184

[jamieQ]

note: appears to crash on the 6.2 dev branch (678ec6e as of today), but not the current 6.2 release on godbolt: https://swift.godbolt.org/z/1jfMv6q1K. not sure if that means it's a recent regression or if it just doesn't show up in the release build due to assert config.

[slavapestov]

@jamieQ it's probably the latter. The type expansion context determines the lowering of opaque result types and the directness of resilient types, so you might be able to contrive a scenario where something goes wrong at runtime too if you give the getter the right set of captures, but it might be somewhat challenging.

[jamieQ]

@slavapestov yeah i think you're correct – i double checked with the 6.2, 6.1 and 5.10 open source macos distributions (via swiftly) which IIUC are configured with assertions enabled, and the assertion fails on all of them.