It seems that during function partial ordering the compiler thinks that an int-based enum is implicitly convertible to char:

---------
module a;

void foo(char) { }

---------
module b;

enum MyEnum : int { _ }

void foo(MyEnum) { }

---------
module main;

import a;
import b;

void main()
{
    foo(char.init);
    foo(MyEnum.init); // [1]
}

// 1) Error: b.foo at b.d(5) conflicts with a.foo at a.d(3)