struct S {
  subscript(x: Int...) -> Int { 0 }
}

let k = \S.[1, 2, 3]

Assertion failed: (hashable.isAbstract() || hashable.getConcrete()->getType()->isEqual(formalTy)), function lowerKeyPathSubscriptIndexPatterns, file /Users/hamishknight/Desktop/swift-dev/swift/lib/SILGen/SILGenExpr.cpp, line 3356.
Stack dump:
0.  Program arguments: ./swift -frontend -emit-silgen /Users/hamishknight/Desktop/newnew/newnew/main.swift -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -debug-diagnostic-names
1.  Swift version 5.1.1-dev (Swift c0c247e7b5)
0  swift                    0x0000000107311e55 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x0000000107310e58 llvm::sys::RunSignalHandlers() + 248
2  swift                    0x0000000107312448 SignalHandler(int) + 264
3  libsystem_platform.dylib 0x00007fff6b473b1d _sigtramp + 29
4  libsystem_platform.dylib 0x0000000000004de0 _sigtramp + 2495156960
5  libsystem_c.dylib        0x00007fff6b349a08 abort + 120
6  libsystem_c.dylib        0x00007fff6b348cc2 err + 0
7  swift                    0x0000000107417113 swift::Lowering::SILGenModule::emitKeyPathComponentForDecl(swift::SILLocation, swift::GenericEnvironment*, swift::ResilienceExpansion, unsigned int&, bool&, swift::SubstitutionMap, swift::AbstractStorageDecl*, llvm::ArrayRef<swift::ProtocolConformanceRef>, swift::CanType, bool) (.cold.17) + 35
8  swift                    0x00000001036e587d swift::Lowering::SILGenModule::emitKeyPathComponentForDecl(swift::SILLocation, swift::GenericEnvironment*, swift::ResilienceExpansion, unsigned int&, bool&, swift::SubstitutionMap, swift::AbstractStorageDecl*, llvm::ArrayRef<swift::ProtocolConformanceRef>, swift::CanType, bool) + 5309
9  swift                    0x00000001036fa5f1 (anonymous namespace)::RValueEmitter::visitKeyPathExpr(swift::KeyPathExpr*, swift::Lowering::SGFContext) + 1569
10 swift                    0x00000001036ee03a swift::ASTVisitor<(anonymous namespace)::RValueEmitter, swift::Lowering::RValue, void, void, void, void, void, swift::Lowering::SGFContext>::visit(swift::Expr*, swift::Lowering::SGFContext) + 154
11 swift                    0x00000001036e0eff swift::Lowering::SILGenFunction::emitExprInto(swift::Expr*, swift::Lowering::Initialization*, llvm::Optional<swift::SILLocation>) + 303
12 swift                    0x00000001036d2dbf swift::Lowering::SILGenFunction::emitPatternBinding(swift::PatternBindingDecl*, unsigned int) + 335
13 swift                    0x00000001036d2e9d swift::Lowering::SILGenFunction::visitPatternBindingDecl(swift::PatternBindingDecl*) + 45
14 swift                    0x0000000103687112 swift::Lowering::SILGenModule::visitTopLevelCodeDecl(swift::TopLevelCodeDecl*) + 498
15 swift                    0x0000000103687633 swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*) + 915
16 swift                    0x0000000103688589 swift::SILModule::constructSIL(swift::ModuleDecl*, swift::Lowering::TypeConverter&, swift::SILOptions&, swift::FileUnit*) + 313
17 swift                    0x0000000103688941 swift::performSILGeneration(swift::ModuleDecl*, swift::Lowering::TypeConverter&, swift::SILOptions&) + 17
18 swift                    0x0000000103389d3b performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 8155
19 swift                    0x0000000103386de6 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3046
20 swift                    0x000000010332c259 main + 729
21 libdyld.dylib            0x00007fff6b272405 start + 1
22 libdyld.dylib            0x0000000000000007 start + 2497240067
fish: './swift -frontend -emit-silgen…' terminated by signal SIGABRT (Abort)