This code fails to compile

import std.utf;

struct S
{
    string s;
    string str() { return s; }
    alias str this;
}

void main()
{
    auto s = S("hello");
    auto range = byCodeUnit(s);
}


Similarly, this fails to compile

import std.utf;

enum E { a = "hello" }

void main()
{
    auto s = E.a;
    auto range = byCodeUnit(s);
}

Also, this compiles but uses the alias rather than the range API and thus fails the assertion:

import std.range.primitives;
import std.utf;

static struct RangeAndStringish
{
    string data;

    string s;
    alias s this;

    bool empty() { return data.empty; }
    char front() { return data[0]; }
    void popFront() { data = data[1 .. $]; }
}

void main()
{
    auto fn = RangeAndStringish("test.d", "other");
    auto x = fn.byCodeUnit();
    assert(x.front == 't');
}

I suppose that it could be argued that this last case is doing the right thing - or at least that it's ambiguous, but normally, an implicit conversion is only supposed to be used if the type itself doesn't work directly, and in this case, the type itself would work without the implicit conversion.