```
class A {}
RedBlackTree!(A, "cast(void*)a < cast(void*)b") tree;
```

This used to work, but now no longer works because of the addition of rbtree.toString.
With a nice instantiation error:

/Library/D/dmd/src/phobos/std/format.d(2916): Error: template instance std.format.formatObject!(void delegate(const(char)[]), const(A), char) does not match template declaration formatObject(Writer, T, Char)(ref Writer w, ref T val, ref FormatSpec!Char f) if (hasToString!(T, Char))
/Library/D/dmd/src/phobos/std/format.d(2649): Error: template instance std.format.formatValue!(void delegate(const(char)[]), const(A), char) error instantiating
/Library/D/dmd/src/phobos/std/format.d(2435):        instantiated from here: formatElement!(void delegate(const(char)[]), const(A), char)
/Library/D/dmd/src/phobos/std/format.d(3142):        instantiated from here: formatRange!(void delegate(const(char)[]), RBRange!(const(RBNode!(A))*), char)
/Library/D/dmd/src/phobos/std/container/rbtree.d(1679):        instantiated from here: formatValue!(void delegate(const(char)[]), RBRange!(const(RBNode!(A))*), char)
/Users/johan/ldc/johan/ddmd/root/array.d(217):        instantiated from here: RedBlackTree!(A, "cast(void*)a < cast(void*)b", false)