$ cat t.swift 
struct S {
  var foo: Int
  func bar() -> Int { return 0}
}

@dynamicMemberLookup
struct A<T> {
  var obj: T
  init(_ obj: T) { self.obj = obj }
  subscript<U>(dynamicMember member: KeyPath<T, U>) -> U {
    return obj[keyPath: member]
  }
}

func test(x: A<S>) {
  _ = x.foo // OK
  _ = x.bar() // CRASH
}

$ swiftc -typecheck t.swift
Assertion failed: (isa<X>(Val) && "cast<Ty>() argument of incompatible type!"), function cast, file /Users/blangmuir/src/s/llvm/include/llvm/Support/Casting.h, line 255.
Stack dump:
0.  Program arguments: /Users/blangmuir/src/s/build/Ninja-RelWithDebInfoAssert/swift-macosx-x86_64/bin/swift -frontend -typecheck -primary-file t.swift -target x86_64-apple-darwin19.0.0 -enable-objc-interop -color-diagnostics -module-name t 
1.  Swift version 5.0-dev (LLVM 082dec2e22, Swift 24139eae61)
2.  While type-checking 'test(x:)' (at t.swift:15:1)
3.  While type-checking statement at [t.swift:15:20 - line:18:1] RangeText="{
  _ = x.foo // OK
  _ = x.bar() // CRASH
"
4.  While type-checking expression at [t.swift:17:3 - line:17:13] RangeText="_ = x.bar("
0  swift                    0x00000001055671e5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x00000001055664a5 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x00000001055677c8 SignalHandler(int) + 264
3  libsystem_platform.dylib 0x00007fff632cf8bd _sigtramp + 29
4  libsystem_platform.dylib 0x0000000110a7ec08 _sigtramp + 2910516072
5  libsystem_c.dylib        0x00007fff6318b8b6 abort + 127
6  libsystem_c.dylib        0x00007fff631544e9 basename_r + 0
7  swift                    0x00000001023ba3c1 (anonymous namespace)::ExprRewriter::buildKeyPathPropertyComponent(swift::constraints::SelectedOverload const&, swift::SourceLoc, swift::constraints::ConstraintLocator*) + 529
8  swift                    0x00000001023b9388 (anonymous namespace)::ExprRewriter::buildKeyPathDynamicMemberIndexExpr(swift::BoundGenericType*, swift::SourceLoc, swift::constraints::ConstraintLocator*) + 1368
9  swift                    0x00000001023b72c9 (anonymous namespace)::ExprRewriter::buildDynamicMemberLookupRef(swift::Expr*, swift::Expr*, swift::SourceLoc, swift::SourceLoc, swift::constraints::SelectedOverload const&, swift::constraints::ConstraintLocator*) + 265
10 swift                    0x00000001023af37e swift::ASTVisitor<(anonymous namespace)::ExprRewriter, swift::Expr*, void, void, void, void, void>::visit(swift::Expr*) + 10366
11 swift                    0x00000001023ac91d (anonymous namespace)::ExprWalker::walkToExprPost(swift::Expr*) + 29
12 swift                    0x000000010280de4b (anonymous namespace)::Traversal::visitApplyExpr(swift::ApplyExpr*) + 123
13 swift                    0x000000010280bfc7 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 2871
14 swift                    0x000000010280ae64 swift::Expr::walk(swift::ASTWalker&) + 84
15 swift                    0x00000001023a45e6 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::Expr*, swift::Type, bool, bool) + 534
16 swift                    0x00000001024e973a swift::TypeChecker::typeCheckExpressionImpl(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener&, swift::constraints::ConstraintSystem*) + 1066
17 swift                    0x00000001024e92ff swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem*) + 63
18 swift                    0x0000000102589f06 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 614
19 swift                    0x0000000102588e88 bool (anonymous namespace)::StmtChecker::typeCheckStmt<swift::BraceStmt>(swift::BraceStmt*&) + 136
20 swift                    0x0000000102587911 swift::TypeChecker::typeCheckFunctionBodyUntil(swift::FuncDecl*, swift::SourceLoc) + 401
21 swift                    0x000000010258853f swift::TypeChecker::typeCheckAbstractFunctionBody(swift::AbstractFunctionDecl*) + 271
22 swift                    0x00000001025a9ef2 typeCheckFunctionsAndExternalDecls(swift::SourceFile&, swift::TypeChecker&) + 322
23 swift                    0x00000001025aaa9b swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 1019
24 swift                    0x0000000101d07e26 swift::CompilerInstance::parseAndTypeCheckMainFileUpTo(swift::SourceFile::ASTStage_t, swift::PersistentParserState&, swift::DelayedParsingCallbacks*, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>) + 502
25 swift                    0x0000000101d06ab3 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 339
26 swift                    0x0000000101d062a7 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 615
27 swift                    0x0000000101ad0965 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1605
28 swift                    0x0000000101acf2c2 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2978
29 swift                    0x0000000101a76029 main + 729
30 libdyld.dylib            0x00007fff630de5dd start + 1
31 libdyld.dylib            0x000000000000000b start + 2633112111
[1]    85246 abort      bin/swiftc -typecheck t.swift