Can't infer block return type even though there is nothing to infer

[panhania]

Consider the following snippet:

class Tree

  def initialize(@value : Int32, @children : Array(Tree))
  end

  def sum : Int32
    @value + @children.each.map(&.sum).sum
  end

end

An attempt to use Tree#sum results in the following error:

can't infer block return type, try to cast the block body with `as`. See: https://github.com/crystal-lang/crystal/wiki/Compiler-error-messages#cant-infer-block-return-type

    @value + @children.each.map(&.sum).sum
                            ^~~

We can fix it by doing what the compiler suggests, that is be replacing map(&.sum) with

map { |child| child.sum.as(Int32) }

However, this seems completely unnecessary - the type of Tree#sum is specified as Int32, so the compiler has nothing to infer and the cast should do nothing.