protocol P { typealias E = Int }
func foo(arg: P) {
  print(arg.E.self) // Crash
}

Assertion failed: (isa<X>(Val) && "cast<Ty>() argument of incompatible type!"), function cast, file /Users/spestov/src/llvm/include/llvm/Support/Casting.h, line 255.
Stack dump:
0.  Program arguments: /Users/spestov/src/build/Ninja-RelWithDebInfoAssert/swift-macosx-x86_64/bin/swift -frontend -c -primary-file a.swift -target x86_64-apple-darwin18.2.0 -enable-objc-interop -color-diagnostics -module-name a -o /var/folders/wg/nmvf884n4b14lt0yw4khjsv00000gn/T/a-a5c34f.o 
1.  While type-checking 'foo(arg:)' (at a.swift:2:1)
2.  While type-checking statement at [a.swift:2:18 - line:4:1] RangeText="{
  print(arg.E.self)
"
3.  While type-checking expression at [a.swift:3:3 - line:3:19] RangeText="print(arg.E.self"
0  swift                    0x00000001080d9a78 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x00000001080d8cf5 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x00000001080da082 SignalHandler(int) + 258
3  libsystem_platform.dylib 0x00007fff5f09db3d _sigtramp + 29
4  libsystem_platform.dylib 0x0000000000003e88 _sigtramp + 2700501864
5  libsystem_c.dylib        0x00007fff5ef5b1c9 abort + 127
6  libsystem_c.dylib        0x00007fff5ef23868 basename_r + 0
7  swift                    0x00000001052ffb28 (anonymous namespace)::ExprRewriter::coerceObjectArgumentToType(swift::Expr*, swift::Type, swift::ValueDecl*, swift::AccessSemantics, swift::constraints::ConstraintLocatorBuilder) + 1080
8  swift                    0x00000001052eca8b (anonymous namespace)::ExprRewriter::buildMemberRef(swift::Expr*, swift::Type, swift::SourceLoc, swift::constraints::OverloadChoice, swift::DeclNameLoc, swift::Type, swift::constraints::ConstraintLocatorBuilder, swift::constraints::ConstraintLocatorBuilder, bool, swift::FunctionRefKind, swift::AccessSemantics, bool) + 2475