More TDPL overloads

[Andrej Mitrovic]

Page 373 adds another operator overload for use with integral numbers (CheckedInt op Int). But it conflicts with the previos template (CheckedInt op CheckedInt):

module binary_ops;

import std.stdio : writeln;
import std.traits;
import std.exception;

unittest
{
    auto foo = CheckedInt!(int)(5);

    foo + 4;
}

void main()
{
}
 
struct CheckedInt(N) if (isIntegral!N)
{
    private N value;
    public int x;
    
    this(N value)
    {
        this.value = value;
    }
    
    // Operation with raw numbers
    CheckedInt opBinary(string op)(N rhs) if (isIntegral!N)
    {
        return opBinary!op(CheckedInt(rhs));
    }
    
    // addition
    CheckedInt opBinary(string op)(CheckedInt rhs) if (op == "+")
    {
        auto result = value + rhs.value;
        
        enforce(rhs.value >= 0 ? result >= value : result < value);
        return CheckedInt(result);
    }
}

I get back:

binary_ops.d(34): Error: template instance opBinary!(op) matches more than one template declaration, 
binary_ops.d(32):opBinary(string op) if (isIntegral!(N)) and
binary_ops.d(38):opBinary(string op) if (op == "+")

One is taking an integral by using a constraint, the other specifically a CheckedInt type. Any clues how they both match?

If I remove the first operator overload template then I can't compile, so where's the ambiguity?