Possible inference improvement in iterated assignment where the RHS uses splatting

[jishnub]

julia> function f(r)
       a, b, c = r..., nothing
       end
f (generic function with 1 method)

julia> @code_warntype f(1:2)
MethodInstance for f(::UnitRange{Int64})
  from f(r) in Main at REPL[1]:1
Arguments
  #self#::Core.Const(f)
  r::UnitRange{Int64}
Locals
  @_3::Int64
  c::Any
  b::Any
  a::Any
Body::Tuple
1 ─ %1 = Core.tuple(Main.nothing)::Core.Const((nothing,))
│   %2 = Core._apply_iterate(Base.iterate, Core.tuple, r, %1)::Tuple
│   %3 = Base.indexed_iterate(%2, 1)::Core.PartialStruct(Tuple{Any, Int64}, Any[Any, Core.Const(2)])
│        (a = Core.getfield(%3, 1))
│        (@_3 = Core.getfield(%3, 2))
│   %6 = Base.indexed_iterate(%2, 2, @_3::Core.Const(2))::Core.PartialStruct(Tuple{Any, Int64}, Any[Any, Core.Const(3)])
│        (b = Core.getfield(%6, 1))
│        (@_3 = Core.getfield(%6, 2))
│   %9 = Base.indexed_iterate(%2, 3, @_3::Core.Const(3))::Core.PartialStruct(Tuple{Any, Int64}, Any[Any, Core.Const(4)])
│        (c = Core.getfield(%9, 1))
└──      return %2

The RHS is inferred as a Tuple, although it's really a Tuple{Vararg{Union{Int, Nothing}}}

In this case the variables may at least be inferred as Union{Int, Nothing}. Ideally a and b may be inferred as Ints as the function will error if there are less than three elements on the right hand side.

julia> f(1:1)
ERROR: BoundsError: attempt to access Tuple{Int64, Nothing} at index [3]

Edit: A simpler example:

julia> function f(r)
       a, b, c = r..., 1
       end
f (generic function with 1 method)

julia> @code_warntype f(1:2)
Variables
  #self#::Core.Const(f)
  r::UnitRange{Int64}
  @_3::Int64
  c::Any
  b::Any
  a::Any

Body::Tuple
1 ─ %1 = Core.tuple(1)::Core.Const((1,))
│   %2 = Core._apply_iterate(Base.iterate, Core.tuple, r, %1)::Tuple
│   %3 = Base.indexed_iterate(%2, 1)::Core.PartialStruct(Tuple{Any, Int64}, Any[Any, Core.Const(2)])
│        (a = Core.getfield(%3, 1))
│        (@_3 = Core.getfield(%3, 2))
│   %6 = Base.indexed_iterate(%2, 2, @_3::Core.Const(2))::Core.PartialStruct(Tuple{Any, Int64}, Any[Any, Core.Const(3)])
│        (b = Core.getfield(%6, 1))
│        (@_3 = Core.getfield(%6, 2))
│   %9 = Base.indexed_iterate(%2, 3, @_3::Core.Const(3))::Core.PartialStruct(Tuple{Any, Int64}, Any[Any, Core.Const(4)])
│        (c = Core.getfield(%9, 1))
└──      return %2

In this case the variables must all be Ints