struct R {
  var i: Int!
}

@dynamicMemberLookup
struct S {
  subscript(dynamicMember kp: KeyPath<R, Int>) -> Void { () }
}

func foo(_ x: S) {
  x.i
}

Stack dump:

SIL verification failed: keypath value type should match value type of keypath pattern: leafTy == pattern->getValueType().subst(patternSubs)->getCanonicalType()
Verifying instruction:
->   %2 = keypath $KeyPath<R, Int>, (root $R; stored_property #R.i : $Optional<Int>) // users: %5, %4
     %4 = apply %3(%2, %0) : $@convention(method) (@guaranteed KeyPath<R, Int>, S) -> ()
     destroy_value %2 : $KeyPath<R, Int>          // id: %5
In function:
// foo(_:)
sil hidden [ossa] @$s4main3fooyyAA1SVF : $@convention(thin) (S) -> () {
// %0                                             // users: %4, %1
bb0(%0 : $S):
  debug_value %0 : $S, let, name "x", argno 1     // id: %1
  %2 = keypath $KeyPath<R, Int>, (root $R; stored_property #R.i : $Optional<Int>) // users: %5, %4
  // function_ref S.subscript.getter
  %3 = function_ref @$s4main1SV13dynamicMemberys7KeyPathCyAA1RVSiG_tcig : $@convention(method) (@guaranteed KeyPath<R, Int>, S) -> () // user: %4
  %4 = apply %3(%2, %0) : $@convention(method) (@guaranteed KeyPath<R, Int>, S) -> ()
  destroy_value %2 : $KeyPath<R, Int>             // id: %5
  %6 = tuple ()                                   // user: %7
  return %6 : $()                                 // id: %7
} // end sil function '$s4main3fooyyAA1SVF'

Stack dump:
0.  Program arguments: /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2019-11-29-a.xctoolchain/usr/bin/swift -frontend -emit-sil /Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift
1.  Apple Swift version 5.1.1-dev (Swift d2e1f09169)
2.  While emitting SIL for 'foo(_:)' (at /Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift:750:1)
3.  While silgen emitFunction SIL function "@$s4main3fooyyAA1SVF".
 for 'foo(_:)' (at /Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift:750:1)
4.  While verifying SIL function "@$s4main3fooyyAA1SVF".
 for 'foo(_:)' (at /Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift:750:1)
0  swift                    0x0000000105f6f6f5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x0000000105f6e905 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x0000000105f6fce6 SignalHandler(int) + 278
3  libsystem_platform.dylib 0x00007fff68536b5d _sigtramp + 29
4  swift                    0x0000000107e20000 (anonymous namespace)::DarwinX86AsmBackend::getCompactUnwindRegNum(unsigned int) const::CU64BitRegs + 236198
5  libsystem_c.dylib        0x00007fff683f66a6 abort + 127
6  swift                    0x0000000102b9df18 (anonymous namespace)::SILVerifier::_require(bool, llvm::Twine const&, std::__1::function<void ()> const&) + 616
7  swift                    0x0000000102bad4d0 swift::SILInstructionVisitor<(anonymous namespace)::SILVerifier, void>::visit(swift::SILInstruction*) + 51040
8  swift                    0x0000000102b9fafc (anonymous namespace)::SILVerifier::visitSILBasicBlock(swift::SILBasicBlock*) + 1484
9  swift                    0x0000000102b9a1d7 swift::SILFunction::verify(bool) const + 7703
10 swift                    0x000000010260b455 swift::Lowering::SILGenModule::postEmitFunction(swift::SILDeclRef, swift::SILFunction*) + 213
11 swift                    0x0000000102611bd0 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*)::$_0::operator()(swift::SILFunction*) const + 256
12 swift                    0x000000010260a281 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 609
13 swift                    0x000000010260f2d3 swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*) + 915
14 swift                    0x0000000102610229 swift::SILModule::constructSIL(swift::ModuleDecl*, swift::Lowering::TypeConverter&, swift::SILOptions&, swift::FileUnit*) + 313
15 swift                    0x00000001026105e1 swift::performSILGeneration(swift::ModuleDecl*, swift::Lowering::TypeConverter&, swift::SILOptions&) + 17
16 swift                    0x0000000102312f49 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 8713
17 swift                    0x000000010230fdc6 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3046
18 swift                    0x00000001022aa8ae main + 734
19 libdyld.dylib            0x00007fff683513d5 start + 1
20 libdyld.dylib            0x0000000000000004 start + 2546658352
fish: '/Library/Developer/Toolchains/s…' terminated by signal SIGABRT (Abort)

struct R {
  var i: Int!
}

@dynamicMemberLookup
struct S {
  subscript(dynamicMember kp: KeyPath<R, Int>) -> Int {
    return R()[keyPath: kp]
  }
}

func foo(_ x: S) {
  print(x.i) // prints: 0
}

foo(S())