Crash accessing a KeyPath to generic nested container

[mrbodich]

Description

When trying to access the key path of a static nested container type, the compiler crashes.
Important: Xcode IDE does not show any error for this code.

let resource = Utils.resource(\.Category, resource: \.nestedResource)

Reproduction

Try to compile or run this, the compiler crashes

1. Full version, required to work on the project

struct Resource {
    let name: String
}

enum Resources {
    enum Category {
        static let nestedResource = Resource(name: "test")
    }
}

struct Utils {
    static func resource<ResourceCategory>(
        _ type: KeyPath<Resources.Type, ResourceCategory.Type>,
        resource: KeyPath<ResourceCategory.Type, Resource>
    ) -> Resource {
        ResourceCategory.self[keyPath: resource]
    }
}

func testResourceGetter() {
    // 👇 This line crashes the compiler. Btw, Xcode editor does not show errors. 
    let resource = Utils.resource(\.Category, resource: \.nestedResource)
}

2. Simplified version

enum Resources {
    enum Category { }
}

struct Utils {
    static func resource<ResourceCategory>(_ type: KeyPath<Resources.Type, ResourceCategory.Type>) -> ResourceCategory.Type {
        Resources.self[keyPath: type]
    }
}

func testResourceGetter() {
     let resource = Utils.resource(\.Category)
}

Stack dump

error: compile command failed due to signal 6 (use -v to see invocation)
Assertion failed: (isa<To>(Val) && "cast<Ty>() argument of incompatible type!"), function cast, file Casting.h, line 578.
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.
Stack dump:
0.	Program arguments: /Users/{my_username}/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/bin/swift-frontend -frontend -c -primary-file generic_keypath_test.swift -target arm64-apple-macosx16.0 -Xllvm -aarch64-use-tbi -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -color-diagnostics -g -debug-info-format=dwarf -dwarf-version=5 -swift-version 6 -empty-abi-descriptor -resource-dir /Users/{my_username}/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir "/Users/{path_to_project}/swift-playground" -module-name my_binary -in-process-plugin-server-path /Users/{my_username}/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/host/libSwiftInProcPluginServer.dylib -plugin-path /Users/{my_username}/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Users/{my_username}/Library/Developer/Toolchains/swift-6.1.2-RELEASE.xctoolchain/usr/local/lib/swift/host/plugins -target-sdk-version 15.5 -target-sdk-name macosx15.5 -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/local/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -o /var/folders/pq/sqq5mbnx4810_9_5y0vw1scw0000gn/T/TemporaryDirectory.dURTVW/generic_keypath_test-1.o
1.	Apple Swift version 6.1.2 (swift-6.1.2-RELEASE)
2.	Compiling with the current language version
3.	While evaluating request TypeCheckSourceFileRequest(source_file "generic_keypath_test.swift")
4.	While evaluating request TypeCheckFunctionBodyRequest(my_binary.(file).testResourceGetter()@generic_keypath_test.swift:20:6)
5.	While type-checking statement at [generic_keypath_test.swift:20:27 - line:22:1] RangeText="{
    let resource = Utils.resource(\.Category, resource: \.nestedResource)
"
6.	While type-checking declaration 0xaf4fdba88 (at generic_keypath_test.swift:21:5)
7.	While evaluating request PatternBindingEntryRequest((unknown decl)@generic_keypath_test.swift:21:5, 0)
8.	While type-checking expression at [generic_keypath_test.swift:21:20 - line:21:73] RangeText="Utils.resource(\.Category, resource: \.nestedResource"
9.	While type-checking-target starting at generic_keypath_test.swift:21:26
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           0x0000000109ed3854 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x0000000109ed1f80 llvm::sys::RunSignalHandlers() + 112
2  swift-frontend           0x0000000109ed3e98 SignalHandler(int) + 292
3  libsystem_platform.dylib 0x000000018c7aa704 _sigtramp + 56
4  libsystem_pthread.dylib  0x000000018c7a088c pthread_kill + 296
5  libsystem_c.dylib        0x000000018c6a5898 abort + 124
6  libsystem_c.dylib        0x000000018c6a4acc err + 0
7  swift-frontend           0x0000000109f5be8c (anonymous namespace)::TypeMemberDiffFinder::visit(swift::ide::api::SDKNode*) (.cold.2) + 0
8  swift-frontend           0x0000000105af2908 (anonymous namespace)::ExprRewriter::buildKeyPathPropertyComponent(swift::constraints::SelectedOverload const&, swift::SourceLoc, swift::constraints::ConstraintLocator*, llvm::SmallVectorImpl<swift::KeyPathExpr::Component>&) + 488
9  swift-frontend           0x0000000105ae14f4 (anonymous namespace)::ExprWalker::walkToExprPost(swift::Expr*) + 17984
10 swift-frontend           0x000000010605d39c (anonymous namespace)::Traversal::doIt(swift::Expr*) + 988
11 swift-frontend           0x0000000106061694 swift::ArgumentList* llvm::function_ref<swift::ArgumentList* (swift::ArgumentList*)>::callback_fn<(anonymous namespace)::Traversal::doIt(swift::ArgumentList*)::'lambda'(swift::ArgumentList*)>(long, swift::ArgumentList*) + 264
12 swift-frontend           0x000000010605eecc swift::Pattern* (anonymous namespace)::Traversal::traverse<swift::Pattern>(swift::ASTWalker::PreWalkResult<swift::Pattern*>, llvm::function_ref<swift::Pattern* (swift::Pattern*)>, llvm::function_ref<swift::ASTWalker::PostWalkResult<swift::Pattern*> (swift::Pattern*)>) + 84
13 swift-frontend           0x000000010605ed04 (anonymous namespace)::Traversal::visitApplyExpr(swift::ApplyExpr*) + 108
14 swift-frontend           0x000000010605d09c (anonymous namespace)::Traversal::doIt(swift::Expr*) + 220
15 swift-frontend           0x000000010605cfb4 swift::Expr::walk(swift::ASTWalker&) + 32
16 swift-frontend           0x0000000105ad8814 (anonymous namespace)::ExprWalker::rewriteTarget(swift::constraints::SyntacticElementTarget) + 636
17 swift-frontend           0x0000000105ad84c8 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::constraints::SyntacticElementTarget) + 632
18 swift-frontend           0x0000000105d0061c swift::TypeChecker::typeCheckTarget(swift::constraints::SyntacticElementTarget&, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::DiagnosticTransaction*) + 384
19 swift-frontend           0x0000000105d00440 swift::TypeChecker::typeCheckExpression(swift::constraints::SyntacticElementTarget&, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::DiagnosticTransaction*) + 196
20 swift-frontend           0x0000000105d01658 swift::TypeChecker::typeCheckBinding(swift::Pattern*&, swift::Expr*&, swift::DeclContext*, swift::Type, swift::PatternBindingDecl*, unsigned int, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 124
21 swift-frontend           0x0000000105d018a0 swift::TypeChecker::typeCheckPatternBinding(swift::PatternBindingDecl*, unsigned int, swift::Type, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 264
22 swift-frontend           0x0000000105dd1e3c swift::PatternBindingEntryRequest::evaluate(swift::Evaluator&, swift::PatternBindingDecl*, unsigned int) const + 1932
23 swift-frontend           0x00000001060ef478 swift::PatternBindingEntryRequest::OutputType swift::Evaluator::getResultUncached<swift::PatternBindingEntryRequest, swift::PatternBindingEntryRequest::OutputType swift::evaluateOrDefault<swift::PatternBindingEntryRequest>(swift::Evaluator&, swift::PatternBindingEntryRequest, swift::PatternBindingEntryRequest::OutputType)::'lambda'()>(swift::PatternBindingEntryRequest const&, swift::PatternBindingEntryRequest::OutputType swift::evaluateOrDefault<swift::PatternBindingEntryRequest>(swift::Evaluator&, swift::PatternBindingEntryRequest, swift::PatternBindingEntryRequest::OutputType)::'lambda'()) + 228
24 swift-frontend           0x00000001060c050c swift::PatternBindingDecl::getCheckedPatternBindingEntry(unsigned int) const + 124
25 swift-frontend           0x0000000105d49424 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 2100
26 swift-frontend           0x0000000105d48bd8 swift::TypeChecker::typeCheckDecl(swift::Decl*) + 116
27 swift-frontend           0x0000000105dca540 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 152
28 swift-frontend           0x0000000105dc8d94 bool (anonymous namespace)::StmtChecker::typeCheckStmt<swift::BraceStmt>(swift::BraceStmt*&) + 136
29 swift-frontend           0x0000000105dc8788 (anonymous namespace)::StmtChecker::typeCheckBody(swift::BraceStmt*&) + 32
30 swift-frontend           0x0000000105dc84e0 swift::TypeCheckFunctionBodyRequest::evaluate(swift::Evaluator&, swift::AbstractFunctionDecl*) const + 1056
31 swift-frontend           0x000000010615b4a0 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'()) + 240
32 swift-frontend           0x00000001060d6d40 swift::AbstractFunctionDecl::getTypecheckedBody() const + 112
33 swift-frontend           0x00000001061fec00 swift::SourceFile::typeCheckDelayedFunctions() + 96
34 swift-frontend           0x0000000105e0fb34 swift::TypeCheckSourceFileRequest::evaluate(swift::Evaluator&, swift::SourceFile*) const + 244
35 swift-frontend           0x0000000105e11394 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'()) + 232
36 swift-frontend           0x0000000105e0fa18 swift::performTypeChecking(swift::SourceFile&) + 84
37 swift-frontend           0x0000000104e1e994 bool llvm::function_ref<bool (swift::SourceFile&)>::callback_fn<swift::CompilerInstance::performSema()::$_0>(long, swift::SourceFile&) + 16
38 swift-frontend           0x0000000104e18438 swift::CompilerInstance::forEachFileToTypeCheck(llvm::function_ref<bool (swift::SourceFile&)>) + 68
39 swift-frontend           0x0000000104e183d4 swift::CompilerInstance::performSema() + 76
40 swift-frontend           0x0000000104bf08a8 withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) + 60
41 swift-frontend           0x0000000104be650c performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 716
42 swift-frontend           0x0000000104be5d28 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2228
43 swift-frontend           0x00000001049c96b0 swift::mainEntry(int, char const**) + 3064
44 dyld                     0x000000018c3e9924 start + 6400

Expected behavior

Attached code has to compile, both versions 1 and 2.
Because what works to access the Type, does not work for KeyPath.
Container.staticProperty - ОК, \.staticProperty - ОК
Container.Category - ОК, \.Category - WRONG

Environment

1. Swift 6.1.2
2. 6.2-snapshot-2025-07-17

MacOS 26.0 Beta (25A5306g)

Additional information

Tested with Swift 6.1.2 and Swift 6.2-snapshot-2025-07-17. Swift 6.2 does not give any helpful stack trace but also crashed, attaching swift 6.1.2 stack trace below.

[felipemfpeixoto]

Hello! This looks like an interesting issue, I would like to give it a try!