ambiguous: fix a rare case of comparison

This used to do a weaker test than should have been required here. This
now aligns with the implementation in gf.c for the same fast path check.

base/reflection.jl

@@ -1789,7 +1789,9 @@ function isambiguous(m1::Method, m2::Method; ambiguous_bottom::Bool=false)
  end
  end
  # if ml-matches reported the existence of an ambiguity over their
- # intersection, see if both m1 and m2 may be involved in it
+ # intersection, see if both m1 and m2 seem to be involved in it
+ # (if one was fully dominated by a different method, we want to will
+ # report the other ambiguous pair)
  have_m1 = have_m2 = false
  for match in ms
  match = match::Core.MethodMatch
@@ -1814,20 +1816,13 @@ function isambiguous(m1::Method, m2::Method; ambiguous_bottom::Bool=false)
  minmax = m
  end
  end
- if minmax === nothing
+ if minmax === nothing || minmax == m1 || minmax == m2
  return true
  end
  for match in ms
  m = match.method
- m === minmax && continue
- if match.fully_covers
- if !morespecific(minmax.sig, m.sig)
- return true
- end
- else
- if morespecific(m.sig, minmax.sig)
- return true
- end
+ if !morespecific(minmax.sig, m.sig)
+ return true
  end
  end
  return false
@@ -1842,12 +1837,12 @@ function isambiguous(m1::Method, m2::Method; ambiguous_bottom::Bool=false)
  if ti2 <: m1.sig && ti2 <: m2.sig
  ti = ti2
  elseif ti != ti2
- # TODO: this would be the correct way to handle this case, but
+ # TODO: this would be the more correct way to handle this case, but
  # people complained so we don't do it
- # inner(ti2) || return false
- return false # report that the type system failed to decide if it was ambiguous by saying they definitely aren't
+ #inner(ti2) || return false # report that the type system failed to decide if it was ambiguous by saying they definitely are
+ return false # report that the type system failed to decide if it was ambiguous by saying they definitely are not
  else
- return false # report that the type system failed to decide if it was ambiguous by saying they definitely aren't
+ return false # report that the type system failed to decide if it was ambiguous by saying they definitely are not
  end
  end
  inner(ti) || return false