fix inconsistent BroadcastFunction behavior (JuliaLang#40330)

fixes JuliaLang#40309

base/broadcast.jl

@@ -1300,7 +1300,13 @@ macro __dot__(x)
  esc(__dot__(x))
 end
 
-@inline broadcasted_kwsyntax(f, args...; kwargs...) = broadcasted((args...)->f(args...; kwargs...), args...)
+@inline function broadcasted_kwsyntax(f, args...; kwargs...)
+ if isempty(kwargs) # some BroadcastStyles dispatch on `f`, so try to preserve its type
+ return broadcasted(f, args...)
+ else
+ return broadcasted((args...) -> f(args...; kwargs...), args...)
+ end
+end
 @inline function broadcasted(f, args...)
  args′ = map(broadcastable, args)
  broadcasted(combine_styles(args′...), f, args′...)

test/broadcast.jl

@@ -1015,3 +1015,7 @@ end
  @test a_ == dropdims(a .* c, dims=(findall(==(1), size(c))...,))
  end
 end
+
+# issue 40309
+@test Base.broadcasted_kwsyntax(+, [1], [2]) isa Broadcast.Broadcasted{<:Any, <:Any, typeof(+)}
+@test Broadcast.BroadcastFunction(+)(2:3, 2:3) === 4:2:6