func f<T>(_ x: KeyPath<String?, T>) -> T { "1"[keyPath: x] }

_ = f(\.?.count)
_ = f(\String?.?.count)

func f<T>(_ x: KeyPath<String?, T>) -> T { "1"[keyPath: x] }

_ = f(\.!.count)
_ = f(\String?.!.count)

1.  Apple Swift version 5.3 (swiftlang-1200.0.16.9 clang-1200.0.22.5)
2.  While evaluating request TypeCheckSourceFileRequest(source_file "/Users/lucianoalmeida/Documents/Programming/Test Area/swiftc/swiftc/main.swift")
3.  While type-checking statement at [/Users/lucianoalmeida/Documents/Programming/Test Area/swiftc/swiftc/main.swift:25:1 - line:25:23] RangeText="_ = f(\String?.!.count"
4.  While type-checking expression at [/Users/lucianoalmeida/Documents/Programming/Test Area/swiftc/swiftc/main.swift:25:1 - line:25:23] RangeText="_ = f(\String?.!.count"
0  swift                    0x0000000107fdef65 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x0000000107fddf65 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x0000000107fdf51f SignalHandler(int) + 111
3  libsystem_platform.dylib 0x00007fff72d035fd _sigtramp + 29
4  swift                    0x0000000104eb1739 llvm::Optional<swift::Type> llvm::function_ref<llvm::Optional<swift::Type> (swift::TypeBase*)>::callback_fn<substType(swift::Type, llvm::function_ref<swift::Type (swift::SubstitutableType*)>, llvm::function_ref<swift::ProtocolConformanceRef (swift::CanType, swift::Type, swift::ProtocolDecl*)>, swift::SubstOptions)::$_25>(long, swift::TypeBase*) + 745
5  swift                    0x000000010484e3b0 (anonymous namespace)::SetExprTypes::walkToExprPost(swift::Expr*) + 688
6  swift                    0x0000000104d33879 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 297
7  swift                    0x0000000104d35db3 (anonymous namespace)::Traversal::visitApplyExpr(swift::ApplyExpr*) + 211
8  swift                    0x000000010483ca25 (anonymous namespace)::ExprRewriter::finishApply(swift::ApplyExpr*, swift::Type, swift::constraints::ConstraintLocatorBuilder, swift::constraints::ConstraintLocatorBuilder) + 12309
9  swift                    0x000000010484677d (anonymous namespace)::ExprRewriter::visitApplyExpr(swift::ApplyExpr*) + 477
10 swift                    0x000000010481efb2 (anonymous namespace)::ExprWalker::walkToExprPost(swift::Expr*) + 18
11 swift                    0x0000000104d35161 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 6673
12 swift                    0x0000000104815664 (anonymous namespace)::ExprWalker::rewriteTarget(swift::constraints::SolutionApplicationTarget) + 324
13 swift                    0x0000000104815380 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::constraints::SolutionApplicationTarget) + 6928
14 swift                    0x00000001049fc1cb swift::TypeChecker::typeCheckExpression(swift::constraints::SolutionApplicationTarget&, bool&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 1307
15 swift                    0x00000001049fbbf8 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 360
16 swift                    0x0000000104ae14cb swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 9115
17 swift                    0x0000000104adcc7a bool (anonymous namespace)::StmtChecker::typeCheckStmt<swift::BraceStmt>(swift::BraceStmt*&) + 314
18 swift                    0x0000000104adce45 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) + 261
19 swift                    0x0000000104b19bb8 swift::TypeCheckSourceFileRequest::evaluate(swift::Evaluator&, swift::SourceFile*) const + 744
20 swift                    0x0000000104b1c9c9 llvm::Expected<swift::TypeCheckSourceFileRequest::OutputType> swift::Evaluator::getResultUncached<swift::TypeCheckSourceFileRequest>(swift::TypeCheckSourceFileRequest const&) + 953
21 swift                    0x0000000104b19744 swift::TypeCheckSourceFileRequest::OutputType swift::evaluateOrDefault<swift::TypeCheckSourceFileRequest>(swift::Evaluator&, swift::TypeCheckSourceFileRequest, swift::TypeCheckSourceFileRequest::OutputType) + 164
22 swift                    0x0000000103c9cc47 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 6439
23 swift                    0x0000000103b5eee1 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 6849
24 swift                    0x0000000103ae3a97 main + 1255
25 libdyld.dylib            0x00007fff72b0acc9 start + 1
26 libdyld.dylib            0x0000000000000050 start + 18446603338591982472
error: Segmentation fault: 11 (in target 'swiftc' from project 'swiftc')