struct imaginary {}

struct complex {
    complex opAdd(complex s) { return this; }
    complex opAdd_r(T)(T t) if (is(T == imaginary)) { return this; }
}

void main() {
    complex s;
    s = s + s;
}

yields the error

Error: template complex.opAdd_r(T) if (is(T == imaginary)) does not match any function template declaration
Error: template complex.opAdd_r(T) if (is(T == imaginary)) cannot deduce template function from argument types !()(complex)

even though opAdd_r is only defined for T == imaginary. 

This may be by design: calling a constrained template function with template arguments that don't satisfy the constraints also doesn't give you an "undefined identifier" error, but rather a message like the one above, mentioning the function template declaration. However, it makes using constrained templates for operator overloading impossible.

I'd expect a non-matching template operator overload to be ignored if there are other matches.