xo-flatstring: bugfixes + utest expansion

docs/lessons.rst

@@ -11,7 +11,7 @@ One hurdle we've created for ourselves, is we need both gcc and clang to agree
 that an expression can be computed at compile-time;
 otherwise will get false alarms in our IDE (raised by LSP running in the background, which relies on clang).
 
-Must Fully Initialize Memory
+Must fully initialize memory
 ----------------------------
 
 Struggled for a while with the implementation of :ref:xo::flatstring_concat
@@ -42,3 +42,57 @@ Correction is to prove to clang that every memory address owned by an empty ``fl
     flatstring::flatstring<N>() {
         std::fill_n(value_, N, '\0');
     }
+
+Still need equality comparison alongside spaceship operator
+-----------------------------------------------------------
+
+Had the impression that spaceship operator for :ref:xo::flatstring would be sufficient
+to get all six comparison operators:
+
+.. code-block:: cpp
+
+    template <std::size_t N1,
+              std::size_t N2>
+    constexpr auto
+    operator<=>(const flatstring<N1> & s1,
+                const flatstring<N2> & s2) noexcept
+    {
+        return (std::string_view(s1) <=> std::string_view(s2));
+    }
+
+We observe this is not the case,  at least with gcc 13.1;  need to separately define :ref:xo::operator==
+
+.. code-block:: cpp
+
+    template <std::size_t N1,
+              std::size_t N2>
+    constexpr bool
+    operator==(const flatstring<N1> & s1,
+               const flatstring<N2> & s2) noexcept
+    {
+        return ((s1 <=> s2) == std::strong_ordering::equal);
+    }
+
+Constexpr strict about pointer arithmetic
+-----------------------------------------
+
+Initially attempted to implement :ref:xo::flatstring reverse iterators using char pointers.
+
+Notice there's an assymetry between reverse iterators and forward iterators.
+We can (and do) implement forward iterators using char pointers.
+The natural value of ``flatstring::end()`` is a char pointer referring to just past the end of
+the string,  i.e. to its null terminator.  From the compiler's perspective,  this is an ordinary
+char pointer,  just like other iterator values.
+
+For reverse iterators this isn't the case.  The natural value for ``flatstring::rend()`` might
+seem to be a char pointer referring to just before the first character in the string.
+However this is no longer a valid pointer address -- dereferencing would be undefined behavior.
+
+In particular,  with this implementation, gcc demotes ``flatstring::rend()`` to non-constexpr
+
+Workaround is to implement a shim iterator class, where representation is pointer to the
+character just after the one the iterator position; iterator's ``operator*`` adjusts pointer before
+dereferencing.
+
+This works because gcc can observe that we never dereference a reverse iterator with pointer value
+at the beginning of a flatstring.