poor type inference in mapreduce

[oscardssmith]

In Julia Master (and 1.6)

julia> f(x) = (T=eltype(x); mapreduce(v->T(v), +, x))
julia> @code_warntype f([3,4,5])
MethodInstance for f(::Vector{Int64})
  from f(x) in Main at REPL[2]:1
Arguments
  #self#::Core.Const(f)
  x::Vector{Int64}
Locals
  #3::var"#3#4"{DataType}
  T::Type{Int64}
Body::Any
1 ─      (T = Main.eltype(x))
│   %2 = Main.:(var"#3#4")::Core.Const(var"#3#4")
│   %3 = Core.typeof(T::Core.Const(Int64))::Core.Const(DataType)
│   %4 = Core.apply_type(%2, %3)::Core.Const(var"#3#4"{DataType})
│        (#3 = %new(%4, T::Core.Const(Int64)))
│   %6 = #3::Core.Const(var"#3#4"{DataType}(Int64))
│   %7 = Main.mapreduce(%6, Main.:+, x)::Any
└──      return %7

Could we make type inference smarter here?

[tkf]

It looks like #23618 (which could be solved by #40985).

[simeonschaub]

Yes, it's a duplicate of #23618