add more test and remove cat_nested

`cat_nested` failed to infer in some cases.
It has been inserted into `make_makeargs`, So I remove it.

base/broadcast.jl

@@ -314,9 +314,8 @@ some cases.
 """
 function flatten(bc::Broadcasted{Style}) where {Style}
  isflat(bc) && return bc
- # concatenate the nested arguments into {a, b, c, d}
- # args = cat_nested(bc)
- # build a function `makeargs` that takes a "flat" argument list and
+ # 1. concatenate the nested arguments into {a, b, c, d}
+ # 2. build a function `makeargs` that takes a "flat" argument list and
  # and creates the appropriate input arguments for `f`, e.g.,
  # makeargs = (w, x, y, z) -> (w, g(x, y), z)
  #
@@ -329,8 +328,7 @@ function flatten(bc::Broadcasted{Style}) where {Style}
  @inline function (args::Vararg{Any,N})
  f(makeargs(args...)...)
  end
- newf = _make(args)
- return Broadcasted{Style}(newf, args, bc.axes)
+ return Broadcasted{Style}(_make(args), args, bc.axes)
  end
 end
 
@@ -340,18 +338,13 @@ _isflat(args::NestedTuple) = false
 _isflat(args::Tuple) = _isflat(tail(args))
 _isflat(args::Tuple{}) = true
 
-cat_nested(t::Broadcasted, rest...) = (cat_nested(t.args...)..., cat_nested(rest...)...)
-cat_nested(t::Any, rest...) = (t, cat_nested(rest...)...)
-cat_nested() = ()
-
 """
  make_makeargs(args::Tuple, t::Tuple) -> Function, Tuple
 
 Each element of `t` is one (consecutive) node in a broadcast tree.
 `args` contains the rest arguments on the "right" side of `t`.
 The jobs of `make_makeargs` are:
- 1. append the flattened arguments in `t` at the beginning of `args`, i.e.
- `(cat_nested(t)..., args...)`
+ 1. append the flattened arguments in `t` at the beginning of `args`.
  2. return a function that takes in flattened argument list and returns a
  tuple (each entry corresponding to an entry in `t`, having evaluated
  the corresponding element in the broadcast tree).

test/broadcast.jl

@@ -772,16 +772,19 @@ end
 
 # issue #27988: inference of Broadcast.flatten
 using .Broadcast: Broadcasted
-let
+let _cat_nested(bc) = Broadcast.flatten(bc).args
  bc = Broadcasted(+, (Broadcasted(*, (1, 2)), Broadcasted(*, (Broadcasted(*, (3, 4)), 5))))
- @test @inferred(Broadcast.cat_nested(bc)) == (1,2,3,4,5)
+ @test @inferred(_cat_nested(bc)) == (1,2,3,4,5)
  @test @inferred(Broadcast.materialize(Broadcast.flatten(bc))) == @inferred(Broadcast.materialize(bc)) == 62
  bc = Broadcasted(+, (Broadcasted(*, (1, Broadcasted(/, (2.0, 2.5)))), Broadcasted(*, (Broadcasted(*, (3, 4)), 5))))
- @test @inferred(Broadcast.cat_nested(bc)) == (1,2.0,2.5,3,4,5)
+ @test @inferred(_cat_nested(bc)) == (1,2.0,2.5,3,4,5)
  @test @inferred(Broadcast.materialize(Broadcast.flatten(bc))) == @inferred(Broadcast.materialize(bc)) == 60.8
  # 1 .* 1 .- 1 .* 1 .^2 .+ 1 .* 1 .+ 1 .^ 3
- bc = Base.Broadcast.Broadcasted(+, (Base.Broadcast.Broadcasted(+, (Base.Broadcast.Broadcasted(-, (Base.Broadcast.Broadcasted(*, (1, 1)), Base.Broadcast.Broadcasted(*, (1, Base.Broadcast.Broadcasted(Base.literal_pow, (Base.RefValue{typeof(^)}(^), 1, Base.RefValue{Val{2}}(Val{2}()))))))), Base.Broadcast.Broadcasted(*, (1, 1)))), Base.Broadcast.Broadcasted(Base.literal_pow, (Base.RefValue{typeof(^)}(^), 1, Base.RefValue{Val{3}}(Val{3}())))))
+ bc = Broadcasted(+, (Broadcasted(+, (Broadcasted(-, (Broadcasted(*, (1, 1)), Broadcasted(*, (1, Broadcasted(Base.literal_pow, (Ref(^), 1, Ref(Val(2)))))))), Broadcasted(*, (1, 1)))), Broadcasted(Base.literal_pow, (Base.RefValue{typeof(^)}(^), 1, Base.RefValue{Val{3}}(Val{3}())))))
  @test @inferred(Broadcast.materialize(Broadcast.flatten(bc))) == @inferred(Broadcast.materialize(bc)) == 2
+ # @. 1 + 1 * (1 + 1 + 1 + 1)
+ bc = Broadcasted(+, (1, Broadcasted(*, (1, Broadcasted(+, (1, 1, 1, 1))))))
+ @test @inferred(_cat_nested(bc)) == (1,1,1,1,1,1) # `cat_nested` failed to infer this
 end
 
 let