Improve test to include a projection, per @huonw's suggestion.

src/test/compile-fail/associated-type-projection-from-multiple-supertraits.rs

@@ -38,4 +38,10 @@ fn dent_object<COLOR>(c: BoxCar<Color=COLOR>) {
     //~^ ERROR ambiguous associated type
 }
 
+fn paint<C:BoxCar>(c: C, d: C::Color) {
+    //~^ ERROR ambiguous associated type `Color` in bounds of `C`
+    //~| NOTE could derive from `Vehicle`
+    //~| NOTE could derive from `Box`
+}
+
 pub fn main() { }