Restrict generic ==(x::NCRingElem, y::NCRingElem) fallback

So far this method default to returning `false` if it failed
to promote the arguments into a common ring. That lead to
situations were comparisons between unrelated rings seemed
"possible" but returned surprising results.

Author: fingolfin

src/NCRings.jl

@@ -83,13 +83,17 @@ end
 
 *(x::NCRingElement, y::NCRingElem) = parent(y)(x)*y
 
+# general fallback for comparison of elements via promotion.
+# This is different from Julia's existing promotion logic because
+# it takes parents into account
 function ==(x::NCRingElem, y::NCRingElem)
-   fl, u, v = try_promote(x, y)
-   if fl
-     return u == v
-   else
-     return false
-   end
+  if typeof(x) === typeof(y)
+    # avoid infinite recursion: we only get here
+    # if a ring type "forgot" to implement ==
+  end
+  fl, u, v = try_promote(x, y)
+  fl && return u == v
+  throw(NotImplementedError(:(==), x, y))
  end
 
 ==(x::NCRingElem, y::NCRingElement) = x == parent(x)(y)