@dynamicMemberLookup
struct S {
  subscript(dynamicMember member: Substring) -> Int { 0 }
}

_ = \S.okay

Unhandled coercion
UNREACHABLE executed at /Users/hamishknight/Desktop/swift-dev/swift/lib/Sema/CSApply.cpp:6405!
Stack dump:
0.  Program arguments: ./swift -frontend -typecheck /Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -debug-diagnostic-names -disable-objc-attr-requires-foundation-module
1.  Swift version 5.1.1-dev (Swift 7edbd9327e)
2.  While type-checking statement at [/Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift:701:1 - line:701:8] RangeText="_ = \S."
3.  While type-checking expression at [/Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift:701:1 - line:701:8] RangeText="_ = \S."
0  swift                    0x0000000105d0fd75 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x0000000105d0ece8 llvm::sys::RunSignalHandlers() + 248
2  swift                    0x0000000105d10376 SignalHandler(int) + 278
3  libsystem_platform.dylib 0x00007fff602c5b5d _sigtramp + 29
4  swift                    0x0000000107c55000 (anonymous namespace)::DarwinX86AsmBackend::getCompactUnwindRegNum(unsigned int) const::CU64BitRegs + 231410
5  libsystem_c.dylib        0x00007fff601856a6 abort + 127
6  swift                    0x0000000106f429de llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) + 462
7  swift                    0x000000010264c01f (anonymous namespace)::ExprRewriter::coerceToType(swift::Expr*, swift::Type, swift::constraints::ConstraintLocatorBuilder, llvm::Optional<swift::Pattern*>) + 9679
8  swift                    0x000000010265f0c0 (anonymous namespace)::ExprRewriter::coerceCallArguments(swift::Expr*, swift::AnyFunctionType*, swift::ConcreteDeclRef, swift::ApplyExpr*, llvm::ArrayRef<swift::Identifier>, bool, swift::constraints::ConstraintLocatorBuilder) + 3104
9  swift                    0x0000000102663304 (anonymous namespace)::ExprRewriter::buildKeyPathSubscriptComponent(swift::constraints::SelectedOverload&, swift::SourceLoc, swift::Expr*, llvm::ArrayRef<swift::Identifier>, swift::constraints::ConstraintLocator*) + 1396
10 swift                    0x0000000102653f1c swift::ASTVisitor<(anonymous namespace)::ExprRewriter, swift::Expr*, void, void, void, void, void>::visit(swift::Expr*) + 24092
11 swift                    0x000000010264d976 (anonymous namespace)::ExprWalker::walkToExprPost(swift::Expr*) + 22
12 swift                    0x0000000102b2dc92 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 2226
13 swift                    0x0000000102b2ce51 swift::Expr::walk(swift::ASTWalker&) + 81
14 swift                    0x00000001026496b5 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::Expr*, swift::Type, bool, bool) + 453
15 swift                    0x00000001027a8012 swift::TypeChecker::typeCheckExpressionImpl(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener&, swift::constraints::ConstraintSystem*) + 946
16 swift                    0x00000001027a7c4b swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem*) + 107
17 swift                    0x000000010284611b swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 7003
18 swift                    0x0000000102843120 bool (anonymous namespace)::StmtChecker::typeCheckStmt<swift::BraceStmt>(swift::BraceStmt*&) + 128
19 swift                    0x0000000102843225 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) + 181
20 swift                    0x000000010286f07e swift::performTypeChecking(swift::SourceFile&, unsigned int) + 366
21 swift                    0x0000000101f99ccd swift::CompilerInstance::parseAndTypeCheckMainFileUpTo(swift::SourceFile::ASTStage_t) + 413
22 swift                    0x0000000101f98972 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 418
23 swift                    0x0000000101f981dd swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 621
24 swift                    0x0000000101d7ad06 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 2022
25 swift                    0x0000000101d795a6 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3046
26 swift                    0x0000000101d1409e main + 734
27 libdyld.dylib            0x00007fff600e03d5 start + 1
28 libdyld.dylib            0x0000000000000008 start + 2683436084
fish: './swift -frontend -typecheck /U…' terminated by signal SIGABRT (Abort)