Given this code:
```
enum class X : unsigned;
X a{1};
```
gdb has an unfortunate behavior of reporting "incomplete type" and
refusing to show the underlying value of `a`, even when `a` is in scope.
Adding `enum class X : unsigned {}` fixes this and allows gdb to show
the value.
Compiler error messages are generally better when reporting a misuse
that fails a requires() versus reporting a misuse that fails a
std::enable_if. In some cases, this also makes the code clearer, and
avoids the need for dummy template parameters as a place to invoke
std::enable_if.
Use a single constructor that accepts anything convertible to both of
the required types, rather than special constructors for:
- Accepting a qualified_segment
- Accepting a variant of susegment with compatible const qualifiers
- Accepting a type T that converts to qualified_segment
This reduces the number of constructors to consider, which improves
error messages when an invalid input is used.
Iterating over it returns each side number in turn. This allows
converting many loops of the form:
```
for (int i = 0; i < MAX_SIDES_PER_SEGMENT; ++i)
```
to the compact form:
```
for (const auto i : MAX_SIDES_PER_SEGMENT)
```
The compact form brings the usual benefit of range-based for: delegating
iteration to the compiler prevents the loop body from skipping a step,
and makes clear in the code that this is the case.
Define separate enum values for rotation data in both the high bits,
where it is usually kept, and the low bits, where it is sometimes used
for math or comparisons.
Define an enum value to represent the composite of the index and the
rotation, since the composite is not suitable for use as an array
subscript. Add helper functions to extract the component pieces.
Various functions need to access both `shared_segment` and
`unique_segment` data. Using `segment &` for this blocks eliminating
the `segment` type. Add `susegment` and type aliases to it.
`susegment` records a reference to a `shared_segment` and a
`unique_segment` together, so that users cannot accidentally mismatch
`shared_segment` #1 with `unique_segment` #2 when passing references
down to a function which needs both `shared_segment` and
`unique_segment`.
