Coverity claims:

 error #2443: defaulted default constructor cannot be constexpr
          because the corresponding implicitly declared default constructor
          would not be constexpr
    constexpr StatusCode() = default;

Independent of that there is little use of declaring the default
constructor as constexpr as it is calling the non-constexpr
default_category().

Charles Leggett authored 7 years ago and Marco Clemencic committed 7 years ago

cherry-pick of !599

gaudi_add_compile_test can be used to add tests that check for the
existence of (expected) compilation failures. Use this to implement a
unit test to check for StatusCode comilation failures.

Closes #14

See merge request !546

add support for categories (domains) to `StatusCode by using typed
enums and ideas from `std::error_code`.

See merge request !581

See merge request !579

See merge request !578

See merge request !572

See merge request !509

This reverts merge request !524

This reverts merge request !524

Closes #5

- Remove IRegistry interface, and define it (for now) as an alias to RegistryEntry
- Remove reference counting from RegistryEntry, replace with std::unique_ptr
- Add move constructor/assignment to RegistryEntry

See merge request !524

See merge request !526

The syntax of
```
  ToolHandle<IMyTool> m_tool;
  bool m_useToolFlag;
...
  ATH_CHECK( m_tool.retrieve( DisableTool{ !m_useToolFlag } ));
```

Add another TaggedBool to  make it a little clearer what are people's
thoughts on `DisableTool / EnableTool` vs.
`DisableToolIf / EnableToolIf`:
```
   ATH_CHECK( m_tool.retrieve( EnableTool{ m_useToolFlag } ));
```