juliatypes.jl: algorithm improvement

it can now conclude that (@U T (A{T},T)) == (@U T @U S<:T (A{T},S))
inspired by the discussion in JuliaLang#8974

[ci skip]

examples/juliatypes.jl

@@ -301,10 +301,14 @@ function issub(a::Var, b::Var, env)
  if aa.right
  # this is a bit odd, but seems necessary to make this case work:
  # (@UnionAll x<:T<:x RefT{RefT{T}}) == RefT{@UnionAll x<:T<:x RefT{T}}
- bb.right && return issub(bb.ub, bb.lb, env) && issub(aa.ub, aa.lb, env)
+ bb.right && return issub(bb.ub, bb.lb, env)
  return var_lt(a, b, env)
  else
- #!bb.right && return issub(aa.ub, bb.lb, env) # shortcut check ∀a,b . a<:b
+ if !bb.right # check ∀a,b . a<:b
+ # the bounds of left-side variables never change, and can only lead
+ # to other left-side variables, so using || here is safe.
+ return issub(aa.ub, b, env) || issub(a, bb.lb, env)
+ end
  return var_gt(b, a, env)
  end
 end
@@ -627,6 +631,11 @@ function test_3()
  (@UnionAll T @UnionAll S inst(PairT,T,S)))
  @test issub_strict((@UnionAll T @UnionAll S>:T inst(PairT,T,S)),
  (@UnionAll T @UnionAll S inst(PairT,T,S)))
+
+ @test isequal_type((@UnionAll T tupletype(inst(RefT,T), T)),
+ (@UnionAll T @UnionAll S<:T tupletype(inst(RefT,T),S)))
+ @test isequal_type((@UnionAll T tupletype(inst(RefT,T), T)),
+ (@UnionAll T @UnionAll T<:S<:T tupletype(inst(RefT,T),S)))
 end
 
 # level 4: Union