Type instability when eachindex() is passed three arguments

[nalimilan]

The second function below suffers from lots of type instabilities, while the first one is fine. Is that expected?

julia> function f(y, mu)
           ll = 0.0
           for i in eachindex(y, mu)
               ll += 1.0
           end
           ll
       end
f (generic function with 1 method)

julia> function f(y, mu, wts)
           ll = 0.0
           for i in eachindex(y, mu, wts)
               ll += 1.0
           end
           ll
       end
f (generic function with 2 methods)

julia> @code_warntype f(1:3, 1:3, 1:3)
Variables:
  y::UnitRange{Int64}
  mu::UnitRange{Int64}
  wts::UnitRange{Int64}
  ll::Float64
  #s1::Any
  i::Any
  ##B#6980::Tuple{UnitRange{Int64},UnitRange{Int64}}
  ####B#6977#6981::Tuple{UnitRange{Int64},UnitRange{Int64}}
  ####B#6978#6982::Tuple{UnitRange{Int64},UnitRange{Int64}}
  ######xs#6976#6979#6983::Tuple{}

Body:
  begin  # none, line 2:
      ll = 0.0 # none, line 3: # abstractarray.jl, line 413: # abstractarray.jl, line 414: # abstractarray.jl, line 116: # abstractarray.jl, line 110: # abstractarray.jl, line 113: # abstractarray.jl, line 115: # abstractarray.jl, line 110: # abstractarray.jl, line 113: # abstractarray.jl, line 110: # abstractarray.jl, line 113: # abstractarray.jl, line 117: # abstractarray.jl, line 117: # abstractarray.jl, line 417: # range.jl, line 339: # int.jl, line 475: # number.jl, line 51: # essentials.jl, line 61: # int.jl, line 474: # tuple.jl, line 61: # tuple.jl, line 8: # tuple.jl, line 8:
      GenSym(3) = (Base.length)(mu::UnitRange{Int64})::Any
      GenSym(2) = (Base.length)(wts::UnitRange{Int64})::Any # operators.jl, line 103:
      GenSym(4) = (Base.max)((Base.max)((Base.box)(Int64,(Base.checked_sadd)((Base.box)(Int64,(Base.checked_ssub)((top(getfield))(y::UnitRange{Int64},:stop)::Int64,(top(getfield))(y::UnitRange{Int64},:start)::Int64)),1)),GenSym(3))::Any,GenSym(2))::Any # operators.jl, line 82:
      GenSym(0) = (Base.colon)(1,GenSym(4))::Any
      #s1 = (top(start))(GenSym(0))::Any
      unless (top(!))((top(done))(GenSym(0),#s1)::Any)::Any goto 1
      2: 
      GenSym(1) = (top(next))(GenSym(0),#s1)::Any
      i = (top(getfield))(GenSym(1),1)::Any
      #s1 = (top(getfield))(GenSym(1),2)::Any # none, line 4: # float.jl, line 204:
      ll = (Base.box)(Base.Float64,(Base.add_float)(ll::Float64,1.0))
      3: 
      unless (top(!))((top(!))((top(done))(GenSym(0),#s1)::Any)::Any)::Any goto 2
      1: 
      0:  # none, line 6:
      return ll::Float64
  end::Float64