Equivalent guard-case expressions produce different results in Swift Testing’s #expect (compiler crash)

[yakovmanshin]

Description

Two seemingly equivalent guard-case expressions result in different outcomes when they’re used inside an #expect call in Swift Testing:

// Compiles fine:
guard case .keyNotFound(let key, _) = error as? DecodingError else { return false }

// “Command SwiftCompile failed with a nonzero exit code:”
guard case DecodingError.keyNotFound(let key, _) = error else { return false }

I reported this in the Swift Testing repo. @stmontgomery suggested this could be caused by a compiler crash.

Crash backtrace

SwiftCompile normal x86_64 Compiling\ SomeAppTests.swift /Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift (in target 'SomeAppTests' from project 'SomeApp')
    cd /Users/UserName/path/to/SomeApp
    builtin-swiftTaskExecution -- /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -c -primary-file /Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift -emit-dependencies-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.d -emit-const-values-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.swiftconstvalues -emit-reference-dependencies-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.swiftdeps -serialize-diagnostics-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.dia -target x86_64-apple-ios18.0-simulator -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator18.0.sdk -I /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Products/Debug-iphonesimulator -I /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/lib -F /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Products/Debug-iphonesimulator -F /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks -F /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator18.0.sdk/Developer/Library/Frameworks -no-color-diagnostics -enable-testing -g -debug-info-format\=dwarf -dwarf-version\=5 -module-cache-path /Users/UserName/Library/Developer/Xcode/DerivedData/ModuleCache.noindex -profile-generate -profile-coverage-mapping -swift-version 5 -enforce-exclusivity\=checked -Onone -D DEBUG -serialize-debugging-options -const-gather-protocols-file /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests_const_extract_protocols.json -enable-experimental-feature DebugDescriptionMacro -enable-experimental-feature OpaqueTypeErasure -enable-bare-slash-regex -empty-abi-descriptor -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins/testing -validate-clang-modules-once -clang-build-session-file /Users/UserName/Library/Developer/Xcode/DerivedData/ModuleCache.noindex/Session.modulevalidation -Xcc -working-directory -Xcc /Users/UserName/path/to/SomeApp -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir /Users/UserName/path/to/SomeApp -Xcc -D_LIBCPP_HARDENING_MODE\=_LIBCPP_HARDENING_MODE_DEBUG -Xcc -ivfsstatcache -Xcc /Users/UserName/Library/Developer/Xcode/DerivedData/SDKStatCaches.noindex/iphonesimulator18.0-22A348-db63dc9361471f152f572502bdbfe70a.sdkstatcache -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/swift-overrides.hmap -Xcc -iquote -Xcc /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-generated-files.hmap -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-own-target-headers.hmap -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-all-target-headers.hmap -Xcc -iquote -Xcc /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-project-headers.hmap -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Products/Debug-iphonesimulator/include -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/DerivedSources-normal/x86_64 -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/DerivedSources/x86_64 -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/DerivedSources -Xcc -DDEBUG\=1 -module-name SomeAppTests -frontend-parseable-output -disable-clang-spi -target-sdk-version 18.0 -target-sdk-name iphonesimulator18.0 -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/lib/swift/host/plugins\#/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/local/lib/swift/host/plugins\#/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/swift-plugin-server -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/local/lib/swift/host/plugins -parse-as-library -o /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.o -index-unit-output-path /SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.o -index-store-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Index.noindex/DataStore -index-system-modules

Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.
Stack dump:
0.	Program arguments: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -c -primary-file /Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift -emit-dependencies-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.d -emit-const-values-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.swiftconstvalues -emit-reference-dependencies-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.swiftdeps -serialize-diagnostics-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.dia -target x86_64-apple-ios18.0-simulator -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator18.0.sdk -I /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Products/Debug-iphonesimulator -I /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/lib -F /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Products/Debug-iphonesimulator -F /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks -F /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator18.0.sdk/Developer/Library/Frameworks -no-color-diagnostics -enable-testing -g -debug-info-format=dwarf -dwarf-version=5 -module-cache-path /Users/UserName/Library/Developer/Xcode/DerivedData/ModuleCache.noindex -profile-generate -profile-coverage-mapping -swift-version 5 -enforce-exclusivity=checked -Onone -D DEBUG -serialize-debugging-options -const-gather-protocols-file /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests_const_extract_protocols.json -enable-experimental-feature DebugDescriptionMacro -enable-experimental-feature OpaqueTypeErasure -enable-bare-slash-regex -empty-abi-descriptor -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins/testing -validate-clang-modules-once -clang-build-session-file /Users/UserName/Library/Developer/Xcode/DerivedData/ModuleCache.noindex/Session.modulevalidation -Xcc -working-directory -Xcc /Users/UserName/path/to/SomeApp -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir /Users/UserName/path/to/SomeApp -Xcc -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG -Xcc -ivfsstatcache -Xcc /Users/UserName/Library/Developer/Xcode/DerivedData/SDKStatCaches.noindex/iphonesimulator18.0-22A348-db63dc9361471f152f572502bdbfe70a.sdkstatcache -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/swift-overrides.hmap -Xcc -iquote -Xcc /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-generated-files.hmap -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-own-target-headers.hmap -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-all-target-headers.hmap -Xcc -iquote -Xcc /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/SomeAppTests-project-headers.hmap -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Products/Debug-iphonesimulator/include -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/DerivedSources-normal/x86_64 -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/DerivedSources/x86_64 -Xcc -I/Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/DerivedSources -Xcc -DDEBUG=1 -module-name SomeAppTests -frontend-parseable-output -disable-clang-spi -target-sdk-version 18.0 -target-sdk-name iphonesimulator18.0 -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/local/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/swift-plugin-server -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/local/lib/swift/host/plugins -parse-as-library -o /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Build/Intermediates.noindex/SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.o -index-unit-output-path /SomeApp.build/Debug-iphonesimulator/SomeAppTests.build/Objects-normal/x86_64/SomeAppTests.o -index-store-path /Users/UserName/Library/Developer/Xcode/DerivedData/SomeApp-fluofojhgcixbzfjyazqcykrqoeb/Index.noindex/DataStore -index-system-modules
1.	Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
2.	Compiling with effective version 5.10
3.	While evaluating request TypeCheckSourceFileRequest(source_file "/Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift")
4.	While evaluating request TypeCheckFunctionBodyRequest(SomeAppTests.(file).SomeAppTests.someTest()@/Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift:9:16)
5.	While type-checking statement at [/Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift:9:27 - line:21:5] RangeText="{
        #expect {
            try someThrowingFunction()
        } throws: { error in
            // This works:
//            guard case .keyNotFound(let key, _) = error as? DecodingError else { return false }
            
            // This doesn’t:
            guard case DecodingError.keyNotFound(let key, _) = error else { return false }
            
            return key.stringValue == "expectedKey"
        }
    "
6.	While type-checking expression at [/Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift:10:9 - line:20:9] RangeText="#expect {
            try someThrowingFunction()
        } throws: { error in
            // This works:
//            guard case .keyNotFound(let key, _) = error as? DecodingError else { return false }
            
            // This doesn’t:
            guard case DecodingError.keyNotFound(let key, _) = error else { return false }
            
            return key.stringValue == "expectedKey"
        "
7.	While type-checking-target starting at /Users/UserName/path/to/SomeApp/SomeAppTests/SomeAppTests.swift:10:9
8.	While evaluating request ExpandMacroExpansionExprRequest(expression)
9.	While type checking expanded macro expression at [@__swiftmacro_12SomeAppTests0023SomeAppTestsswift_elFCffMX9_8_6expectfMf_.swift:1:1 - line:13:166] RangeText="Testing.__checkClosureCall(performing: {
            try someThrowingFunction()
        }, throws: { error in
            // This works:
//            guard case .keyNotFound(let key, _) = error as? DecodingError else { return false }

            // This doesn’t:
            guard case DecodingError.keyNotFound(let key, _) = error else {
                    return false
                }

            return key.stringValue == "expectedKey"
        }, expression: .__fromSyntaxNode("try someThrowingFunction()"), comments: [], isRequired: false, sourceLocation: Testing.SourceLocation.__here()).__expected("
10.	While type-checking expression at [@__swiftmacro_12SomeAppTests0023SomeAppTestsswift_elFCffMX9_8_6expectfMf_.swift:1:1 - line:13:166] RangeText="Testing.__checkClosureCall(performing: {
            try someThrowingFunction()
        }, throws: { error in
            // This works:
//            guard case .keyNotFound(let key, _) = error as? DecodingError else { return false }

            // This doesn’t:
            guard case DecodingError.keyNotFound(let key, _) = error else {
                    return false
                }

            return key.stringValue == "expectedKey"
        }, expression: .__fromSyntaxNode("try someThrowingFunction()"), comments: [], isRequired: false, sourceLocation: Testing.SourceLocation.__here()).__expected("
11.	While type-checking-target starting at @__swiftmacro_12SomeAppTests0023SomeAppTestsswift_elFCffMX9_8_6expectfMf_.swift:13:155
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           0x00000001099cd088 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 40
1  swift-frontend           0x00000001099cae59 llvm::sys::RunSignalHandlers() + 233
2  swift-frontend           0x00000001099cd646 SignalHandler(int) + 278
3  libsystem_platform.dylib 0x00007ff81a538e9d _sigtramp + 29
4  libsystem_platform.dylib 000000000000000000 _sigtramp + 18446603370139251072
5  swift-frontend           0x0000000104cb4c92 swift::ExportContext::forFunctionBody(swift::DeclContext*, swift::SourceLoc) + 258
6  swift-frontend           0x0000000104cc8b6c (anonymous namespace)::StmtAvailabilityWalker::walkToPatternPre(swift::Pattern*) + 60
7  swift-frontend           0x000000010521ad25 (anonymous namespace)::Traversal::doIt(llvm::MutableArrayRef<swift::StmtConditionElement> const&) + 197
8  swift-frontend           0x000000010521df9f (anonymous namespace)::Traversal::visit(swift::Stmt*) + 1327
9  swift-frontend           0x0000000104c2797f swift::performStmtDiagnostics(swift::Stmt const*, swift::DeclContext*) + 9695
10 swift-frontend           0x0000000104a402f9 (anonymous namespace)::SyntacticElementSolutionApplication::visit(swift::Stmt*, bool) + 7657
11 swift-frontend           0x0000000104a4003e (anonymous namespace)::SyntacticElementSolutionApplication::visit(swift::Stmt*, bool) + 6958
12 swift-frontend           0x0000000104a3c9c2 (anonymous namespace)::SyntacticElementSolutionApplication::apply() + 130
13 swift-frontend           0x0000000104a3c823 swift::constraints::ConstraintSystem::applySolutionToBody(swift::constraints::Solution&, swift::AnyFunctionRef, swift::DeclContext*&, std::__1::function<std::__1::optional<swift::constraints::SyntacticElementTarget> (swift::constraints::SyntacticElementTarget)>) + 419
14 swift-frontend           0x00000001049e01d7 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::constraints::SyntacticElementTarget) + 8359
15 swift-frontend           0x0000000104d015b2 swift::TypeChecker::typeCheckTarget(swift::constraints::SyntacticElementTarget&, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 770
16 swift-frontend           0x0000000104d0117f swift::TypeChecker::typeCheckExpression(swift::constraints::SyntacticElementTarget&, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 431
17 swift-frontend           0x0000000104daa466 swift::expandMacroExpr(swift::MacroExpansionExpr*) + 1526
18 swift-frontend           0x0000000104da9e37 swift::ExpandMacroExpansionExprRequest::evaluate(swift::Evaluator&, swift::MacroExpansionExpr*) const + 439
19 swift-frontend           0x00000001049fa0ed swift::ExpandMacroExpansionExprRequest::OutputType swift::Evaluator::getResultUncached<swift::ExpandMacroExpansionExprRequest, swift::ExpandMacroExpansionExprRequest::OutputType swift::evaluateOrDefault<swift::ExpandMacroExpansionExprRequest>(swift::Evaluator&, swift::ExpandMacroExpansionExprRequest, swift::ExpandMacroExpansionExprRequest::OutputType)::'lambda'()>(swift::ExpandMacroExpansionExprRequest const&, swift::ExpandMacroExpansionExprRequest::OutputType swift::evaluateOrDefault<swift::ExpandMacroExpansionExprRequest>(swift::Evaluator&, swift::ExpandMacroExpansionExprRequest, swift::ExpandMacroExpansionExprRequest::OutputType)::'lambda'()) + 733
20 swift-frontend           0x00000001049f9d57 swift::ExpandMacroExpansionExprRequest::OutputType swift::Evaluator::getResultCached<swift::ExpandMacroExpansionExprRequest, swift::ExpandMacroExpansionExprRequest::OutputType swift::evaluateOrDefault<swift::ExpandMacroExpansionExprRequest>(swift::Evaluator&, swift::ExpandMacroExpansionExprRequest, swift::ExpandMacroExpansionExprRequest::OutputType)::'lambda'(), (void*)0>(swift::ExpandMacroExpansionExprRequest const&, swift::ExpandMacroExpansionExprRequest::OutputType swift::evaluateOrDefault<swift::ExpandMacroExpansionExprRequest>(swift::Evaluator&, swift::ExpandMacroExpansionExprRequest, swift::ExpandMacroExpansionExprRequest::OutputType)::'lambda'()) + 311
21 swift-frontend           0x00000001049f77c1 (anonymous namespace)::ExprRewriter::walkToExprPost(swift::Expr*) + 47121
22 swift-frontend           0x00000001049e84d5 (anonymous namespace)::ExprWalker::walkToExprPost(swift::Expr*) + 21
23 swift-frontend           0x00000001049e1595 (anonymous namespace)::ExprWalker::rewriteTarget(swift::constraints::SyntacticElementTarget) + 4037
24 swift-frontend           0x00000001049e0072 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::constraints::SyntacticElementTarget) + 8002
25 swift-frontend           0x0000000104d015b2 swift::TypeChecker::typeCheckTarget(swift::constraints::SyntacticElementTarget&, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 770
26 swift-frontend           0x0000000104d0117f swift::TypeChecker::typeCheckExpression(swift::constraints::SyntacticElementTarget&, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 431
27 swift-frontend           0x0000000104e079cd (anonymous namespace)::StmtChecker::typeCheckASTNode(swift::ASTNode&) + 413
28 swift-frontend           0x0000000104e0d32c swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 252
29 swift-frontend           0x0000000104e0ac1a bool (anonymous namespace)::StmtChecker::typeCheckStmt<swift::BraceStmt>(swift::BraceStmt*&) + 314
30 swift-frontend           0x0000000104e094ae swift::TypeCheckFunctionBodyRequest::evaluate(swift::Evaluator&, swift::AbstractFunctionDecl*) const + 1646
31 swift-frontend           0x000000010534780d swift::TypeCheckFunctionBodyRequest::OutputType swift::Evaluator::getResultUncached<swift::TypeCheckFunctionBodyRequest, swift::TypeCheckFunctionBodyRequest::OutputType swift::evaluateOrDefault<swift::TypeCheckFunctionBodyRequest>(swift::Evaluator&, swift::TypeCheckFunctionBodyRequest, swift::TypeCheckFunctionBodyRequest::OutputType)::'lambda'()>(swift::TypeCheckFunctionBodyRequest const&, swift::TypeCheckFunctionBodyRequest::OutputType swift::evaluateOrDefault<swift::TypeCheckFunctionBodyRequest>(swift::Evaluator&, swift::TypeCheckFunctionBodyRequest, swift::TypeCheckFunctionBodyRequest::OutputType)::'lambda'()) + 781
32 swift-frontend           0x00000001052acec7 swift::AbstractFunctionDecl::getTypecheckedBody() const + 151
33 swift-frontend           0x0000000104e61109 swift::TypeCheckSourceFileRequest::evaluate(swift::Evaluator&, swift::SourceFile*) const + 969
34 swift-frontend           0x0000000104e68e44 swift::TypeCheckSourceFileRequest::OutputType swift::Evaluator::getResultUncached<swift::TypeCheckSourceFileRequest, swift::TypeCheckSourceFileRequest::OutputType swift::evaluateOrDefault<swift::TypeCheckSourceFileRequest>(swift::Evaluator&, swift::TypeCheckSourceFileRequest, swift::TypeCheckSourceFileRequest::OutputType)::'lambda'()>(swift::TypeCheckSourceFileRequest const&, swift::TypeCheckSourceFileRequest::OutputType swift::evaluateOrDefault<swift::TypeCheckSourceFileRequest>(swift::Evaluator&, swift::TypeCheckSourceFileRequest, swift::TypeCheckSourceFileRequest::OutputType)::'lambda'()) + 772
35 swift-frontend           0x0000000104e60d21 swift::performTypeChecking(swift::SourceFile&) + 337
36 swift-frontend           0x0000000103bd0069 swift::CompilerInstance::performSema() + 281
37 swift-frontend           0x00000001037a6f86 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 918
38 swift-frontend           0x00000001037a5d58 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 4392
39 swift-frontend           0x00000001037186a7 swift::mainEntry(int, char const**) + 8103
40 dyld                     0x00007ff81a1762cd start + 1805

Reproduction

I was able to reproduce the issue in multiple iOS projects, including a newly-created one.

The following code causes the crash:

#expect {
    try someThrowingFunction()
} throws: { error in
    guard case DecodingError.keyNotFound(let key, _) = error else { return false }
    return key.stringValue == "expectedKey"
}

Complete file

@testable import SomeApp

import Testing

func someThrowingFunction() throws { }

struct SomeAppTests {
    
    @Test func someTest() {
        #expect {
            try someThrowingFunction()
        } throws: { error in
            // This works:
//            guard case .keyNotFound(let key, _) = error as? DecodingError else { return false }
            
            // This doesn’t:
            guard case DecodingError.keyNotFound(let key, _) = error else { return false }
            
            return key.stringValue == "expectedKey"
        }
    }
    
}

Expected behavior

The two guard-case expressions can be used interchangeably.

Environment

swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
Target: x86_64-apple-macosx15.0

• macOS 15.0 (24A335)
• Xcode 16.0 (16A242)

Additional information

• Unexpected build errors with guard-case expressions #76770

[tshortli]

Thanks for the report, I'm able to reproduce the issue.

[tshortli]

This reproduces with swiftlang-6.0.0.9.10 but doesn't reproduce with a compiler built from main. I won't be able to identify to a PR that fixed it without more investigation, but there has been a lot of work in this area on main in general. If you have a chance, please try with a recent development snapshot of Swift.