function pointer as the enum base type

[mumba]

Idea: to make an enum type with function pointers as it's elements.
Problem: no more than one element can be inserted!

$echo '
void foo() {}
void bar() {}

enum Enum : void function() {
    FOO = &foo,
    BAR = &bar
}

int main() { return 0;	}' > enum.d
$dmd --help
Digital Mars D Compiler v2.014
Copyright (c) 1999-2008 by Digital Mars written by Walter Bright
Documentation: http://www.digitalmars.com/d/2.0/index.html
...
$dmd enum.d
enum.d(5): Error: Integer constant expression expected instead of (& bar) < (& foo)
enum.d(5): Error: Integer constant expression expected instead of (& bar) > (& foo)


I don't understand the error message. 
First - why integer constant expression expected, if it's supposed to be the function based enum? Second - instead of what? (& bar) expression, which is smaller than (& foo), or boolean expression (& bar) < (& foo)? 
Third - in both cases: if only one of these messages appeard - it would be just strange for me. But as they are together and both refer to the same line - I'm knocked out. What logic leads to such mutually exclusive pieces of information?

Please someone explain it to me.

By the way:

enum Enum : void function() {
    FOO = &foo
}

works fine.


cheers