# Changelog

## v2.2.8 ? 2023 (Boost 1.84) [release]

### Enhancements:

- cmake 3.9 is now the minimum required for standalone Outcome. This fixes a long standing cmake issue with probing for standard library facilities. cmake 3.9 is what RHEL7 ships with, when RHEL7 EOLs we may raise the minimum cmake version at that point.

### Bug fixes:

- There was a bug in the Outcome C++ coroutine awaitables whereby we were over eagerly resuming execution of coroutines which return one of our awaitables. It is surprising how many years have passed before this was noticed, but it is now fixed. It is believed that this has been fixed without affecting ABI stability, however mixing old Outcome and new Outcome in the same binary without recompiling all the C++ coroutine code to use new Outcome will not fix the bug.

#291 - A Result or Outcome with `void` value type and move-only non-value type was only usable in const use cases, due to the lack of provision of non-const member functions in relevant observers injection layers for the `void` specialisation. The missing non-const member functions have now been added.

## v2.2.7 13th August 2023 (Boost 1.83) [release]

### Enhancements:

- Update the list of known compiler issues in the docs.

- Update Outcome.Experimental to match latest changes requested of `status_code` by WG21. This as usual will cause minor breakage due to LEWG renaming of things.

- Outcome previously took addresses of things not using `std::addressof()`, and until now nobody complained because custom `operator&` which doesn’t return an address is an abomination not used in much modern C++. But finally someone did complain, so for both normal Outcome and Experimental.Outcome, if you set `BOOST_OUTCOME_USE_STD_ADDRESSOF = 1`, Outcome will use `std::addressof()`

### Bug fixes:

#273 - Changes to other Boost libraries had caused Boost.Outcome’s test suite to fail to compile for some compiler and C++ language configurations in recent releases. Thanks to work contributed by @alandefreitas and @pdimov, Boost.Outcome now CI tests a wide range of compilers and configurations and it is believed all those corner case issues have been fixed or worked around, for the compilers and configurations within that CI matrix.

Standalone Outcome’s test suite was never affected, as it did not have Boost changing underneath it. Nevertheless, a few of the compiler parse bug workarounds will have improved compatibility there too for atyical toolchain choices.

- Experimental.Outcome now supports big endian architectures. Implementation for them simply wasn’t done before under the assumption that nobody would be using Experimental.Outcome on big endian architectures. Turns out that was a wrong assumption!

## v2.2.6 24th March 2023 (Boost 1.82) [release]

### Enhancements:

- Update to latest `status-code` in Experimental.Outcome, which relocates its header files and may cause some end user inclusions to need path changes.

### Bug fixes:

- Latest `status-code` fixes a number of nasty bugs, some mild build breakage in Experimental.Outcome may occur as a result. See its commit log for more information.

## v2.2.4 11th August 2022 (Boost 1.80) [release]

### Enhancements:

- Update to latest `status-code` in Experimental.Outcome, which has some breaking changes and important bug fixes.

- Add `generator<T, Executor = void>` to coroutine awaitables.

- Add optional `Executor` template parameter to all Outcome awaitable types for improved compatibility with third party software such as ASIO.

- To Experimental.Outcome add `clone()` for `basic_result` and `basic_outcome` types whose `EC` is a `status_code`. Erased status codes are move-only which makes the Result/Outcome type move-only, but they provide a cloning function, so this convenience function both clones the status code and propagates the spare storage so stack backtraces etc are also cloned.

- Add type constraints to `success()` and `failure()` to disable them if they aren’t available.

- Work around a bug in GCC’s C++ Coroutines implementation whereby one gets an ICE from `gimplify_expr` in any `BOOST_OUTCOME_CO_TRY` taking even a mildly complex expression, which obviously is a showstopper. The work around assigns the failure type to a stack temporary before `co_return`-ing that temporary. Thanks to RVO pre-17 and copy elision since, this should add no runtime overhead.

### Bug fixes:

#261
  Move assignable with move constructible not available did not work with `void`.

## v2.2.3 17th March 2022 (Boost 1.79) [release]

### Enhancements:

Standalone Outcome permanently locks ABI to v2.2.3 release
  From this release onwards, you are guaranteed that any code compiled with v2.2.3 Outcome or newer will link without issue with any code compiled with a different version of Outcome after v2.2.3. As a result, `BOOST_OUTCOME_UNSTABLE_VERSION` is no longer defined by default, and therefore `BOOST_OUTCOME_V2_NAMESPACE` is now hard coded to `outcome_v2` rather than a namespace which permutes per commit.

### Bug fixes:

#255
  Fix enabling of move assignable was not calculated correctly.

## v2.2.2 8th December 2021 (Boost 1.78) [release]

### Enhancements:

#255
  Restore Experimental Outcome constexpr compatibility in C++ 20 which was an undocumented property of the Outcome v2.1 series, and which had been dropped in the v2.2 series.
GCC Coroutines support
  Coroutine support in GCCs after 10 is now correctly detected.

### Bug fixes:

- None.

## v2.2.1 13th August 2021 (Boost 1.77) [release]

### Bug fixes:

#251

  Fix failure to compile Boost.Outcome when the compiler declares support for C++ modules.

  Don’t use `[[likely]]` in GCCs before 9.

#251

  Make support for C++ modules opt-in.

## v2.2.0 16th April 2021 (Boost 1.76) [release]

BREAKING CHANGE As announced for a year and three Boost releases, Outcome v2.2 became the default, replacing v2.1.
  All v2.1 Outcome code will need to be upgraded as described in the v2.1 => v2.2 upgrade guide. This branch has a number of major breaking changes to Outcome v2.1, see the list of v2.2 major changes.

### Enhancements:

VS2019.8 compatibility
  VS2019.8 changed how to enable Coroutines, which caused Outcome to not compile on that compiler.
#237
  If on C++ 20, we now use C++ 20 `[[likely]]` instead of compiler-specific markup to indicate when TRY has likely success or failure.
BREAKING CHANGE #247

  Previously the value of `spare_storage(const basic_result|basic_outcome *) noexcept` was not propagated over `BOOST_OUTCOME_TRY`, which causes things like stack backtraces captured at the point of construction of an errored result to get dropped at every `TRY` point. This has been fixed by adding an optional `spare_storage` to `success_type<T>` and `failure_type<T>` , as well as to `auto success(T &&, ...)` and `auto failure(T &&, ...)` .

  You should not notice this in your code, except that where before spare storage values did not propagate through TRY, now they do, which is a breaking change.

### Bug fixes:

BREAKING CHANGE #244

  It came as a shock to learn that `BOOST_OUTCOME_TRY` had been broken since the inception of this library for certain corner case code:

      outcome::result<Foo>    get_foo();
      outcome::result<Foo>    filter1(outcome::result<Foo> &&);
      outcome::result<Foo> && filter2(outcome::result<Foo> &&);

      // This works fine, and always has
      BOOST_OUTCOME_TRY(auto v, filter1(get_foo()))

      // This causes UB due to result<Foo> being destructed before move of value into v
      BOOST_OUTCOME_TRY(auto v, filter2(get_foo()))

  Whilst reference passthrough filter functions are not common, they can turn up in highly generic code, where destruction before copy/move is not helpful.

  The cause is that TRY used to work by binding the result of the expression to an `auto &&unique`, testing if that unique if successful or not, and if successful then moving from `unique.value()` into the user’s output variable. If the expression returned is a prvalue, the Result’s lifetime is extended by the bound reference to outside of the statement, and all is good. If the expression returned is an xvalue or lvalue, then the lifetime extension does not exceed that of the statement, and the Result is destructed after the semicolon succeeding the assignment to `auto &&unique`.

  This bug has been fixed by TRY deducing the value category of its input expression as follows:

  - prvalues => `auto unique = (expr)` (breaking change)
  - xvalue => `auto unique = (expr)` (breaking change)
  - lvalue => `auto unique = (expr)` (breaking change)

  This ensures that xvalue and lvalue inputs do not cause unhelpfully early lifetime end, though it does silently change the behaviour of existing code which relied on rvalues and lvalues being passed through, as a new construct-move-destruct or construct-copy-destruct cycle is introduced to where there was none before. Also, before C++ 17, there is now an added copy/move for prvalue inputs, which does not affect runtime codegen due to Return Value Optimisation (RVO), but does cause Results containing non-copying non-moving types to fail to compile, which is a breaking change from beforehand.

  If one wishes rvalues or lvalues to be passed through, one can avail of a new TRY syntax based on preprocessor overloading:

  - `BOOST_OUTCOME_TRY((refspec, varname), expr)`
  - `BOOST_OUTCOME_TRYV2(refspec, expr)`

  Here `refspec` is the storage to be used for both the internal temporary unique, AND `varname`. So if you write:

      Foo &&foo;
      BOOST_OUTCOME_TRY((auto &&, v), filter2(foo))

  … then the internal unique is declared as `auto &&unique = (filter2(foo))`, and the output variable is declared as `auto &&v = std::move(unique).assume_value()`. This passes through the rvalue referencing, and completely avoids copies and moves of `Foo`. If you wish to not extract the value but also specify unique storage, there is a new `BOOST_OUTCOME_TRYV2(refspec, expr)`.

  My thanks to KamilCuk from https://stackoverflow.com/questions/66069152/token-detection-within-a-c-preprocessor-macro-argument for all their help in designing the new overloaded TRY syntax. My thanks also to vasama for reporting this issue and working through how best to fix it with me.

#249

  The preprocessor logic for choosing when to use `bool` with `concept` on GCC was yet again refactored. This should fix those choices of GCC configuration which caused failure due to the wrong combination being chosen.

## v2.1.5 11th December 2020 (Boost 1.75) [release]

### Enhancements:

The ADL discovered event hooks have been replaced with policy-specified event hooks instead
  This is due to brittleness (where hooks would quietly self-disable if somebody changed something), compiler bugs (a difference in compiler settings causes the wrong hooks, or some but not all hooks, to get discovered), and end user difficulty in using them at all. The policy-specified event hooks can be told to default to ADL discovered hooks for backwards compatibility: set `BOOST_OUTCOME_ENABLE_LEGACY_SUPPORT_FOR` to less than `220` to enable emulation.
Improve configuring `BOOST_OUTCOME_GCC6_CONCEPT_BOOL`
  Older GCCs had boolean based concepts syntax, whereas newer GCCs are standards conforming. However the precise logic of when to use legacy and conforming syntax was not well understood, which caused Outcome to fail to compile depending on what options you pass to GCC. The new logic always uses the legacy syntax if on GCC 8 or older, otherwise we use conforming syntax if and only if GCC is in C++ 20 mode or later. This hopefully will resolve the corner case build failures on GCC.

### Bug fixes:

Boost.Outcome should now compile with `BOOST_NO_EXCEPTIONS` defined
  Thanks to Emil, maintainer of Boost.Exception, making a change for me, Boost.Outcome should now compile with C++ exceptions globally disabled. You won’t be able to use `boost::exception_ptr` as it can’t be included if C++ exceptions are globally disabled.
#236
  In the Coroutine support the `final_suspend()` was not `noexcept`, despite being required to be so in the C++ 20 standard. This has been fixed, but only if your compiler implements `noop_coroutine`. Additionally, if `noop_coroutine` is available, we use the much more efficient coroutine handle returning variant of `await_suspend()` which should significantly improve codegen and context switching performance.

## v2.1.4 14th August 2020 (Boost 1.74) [release]

### Enhancements:

BREAKING CHANGE `void` results and outcomes no longer default construct types during explicit construction
  Previously if you explicitly constructed a `result<T>` from a non-errored `result<void>`, it default constructed `T`. This was found to cause unhelpful surprise, so it has been disabled.
New macro `BOOST_OUTCOME_ENABLE_LEGACY_SUPPORT_FOR`
  The macro `BOOST_OUTCOME_ENABLE_LEGACY_SUPPORT_FOR` can be used to enable aliasing of older naming and features to newer naming and features when using a newer version of Outcome.
Concepts now have snake case style naming instead of camel case style
  When Outcome was first implemented, it was thought that C++ 20 concepts were going to have camel case style. This was changed before the C++ 20 release, and Outcome’s concepts have been renamed similarly. This won’t break any code in Outcome v2.1, as compatibility aliases are provided. However code compiled against Outcome v2.2 will need to be upgraded, unless `BOOST_OUTCOME_ENABLE_LEGACY_SUPPORT_FOR` is set to less than `220`.
Concepts now live in `BOOST_OUTCOME_V2_NAMESPACE::concepts` namespace
  Previously concepts lived in the `convert` namespace, now they live in their own namespace.
New concepts `basic_result<T>` and `basic_outcome<T>` added
  End users were finding an unhelpful gap in between `is_basic_result<T>` and `value_or_error<T>` where they wanted a concept that matched types which were `basic_result`, but not exactly one of those. Concepts filling that gap were added.
Operation `TRY` works differently from Outcome v2.2 onwards
  This is a severely code breaking change which change the syntax of how one uses `BOOST_OUTCOME_TRY()`. A regular expression suitable for upgrading code can be found in the list of changes between Outcome v2.1 and v2.2.

### Bug fixes:

#224
  The clang Apple ships in Xcode 11.4 (currently the latest) has not been patched with the fixes to LLVM clang that fix `noexcept(std::is_constructible<T, void>)` failing to compile which I originally submitted years ago. So give up waiting on Apple to fix their clang, add a workaround to Outcome.
Use of `void` in `T` or `E` caused `noexcept(false)`
  Direct traits examination of `void` was causing nothrow detection to return false, fixed.
Spare storage could not be used from within no-value policy classes
  Due to an obvious brain fart when writing the code at the time, the spare storage APIs had the wrong prototype which prevented them working from within policy classes. Sorry.

## v2.1.3 29th April 2020 (Boost 1.73) [release]

### Enhancements:

Performance of Outcome-based code compiled by clang has been greatly improved

  The previous implementation of Outcome’s status bitfield confused clang’s optimiser, which caused low quality codegen. Unlike most codegen issues, this was noticeably in empirical benchmarks of real world code, as was shown by P1886 Error speed benchmarking.

  The safe part of the `better_optimisation` Outcome v2.2.0 future branch was merged to Outcome v2.1.3 which includes a new status bitfield implementation. This appears to not confuse clang’s optimiser, and clang 9 produces code which routinely beats GCC 9’s code for various canned use cases.

Precompiled headers are automatically enabled on new enough cmake’s for standalone Outcome

  If on cmake 3.16 or later, its new precompiled headers build support is used to tell consumers of the `outcome::hl` cmake target to precompile Outcome, if and only if `outcome_IS_DEPENDENCY` is false. `outcome_IS_DEPENDENCY` is set by Outcome’s CMakeLists.txt if it detects that it was included using `add_subdirectory()`, so for the vast majority of Outcome end users, the use of precompiled headers will NOT be enabled.

  Exported targets do NOT request precompilation of headers, as it is assumed that importers of the Outcome cmake targets will configure their own precompiled headers which incorporate Outcome.

Installability is now CI tested per commit

  Due to installability of standalone Outcome (e.g. `make install`) breaking itself rather more frequently than is ideal, installability is now tested on CI per commit.

Coroutines support has been documented

  The coroutines support added in v2.1.2 has now been properly documented.

### Bug fixes:

#214
  Newer Concepts implementing compilers were unhappy with the early check for destructibility of `T` and `E`, so removed template constraints, falling back to static assert which runs later in the type instantiation sequence.
#215
  For standalone Outcome, `CMAKE_TOOLCHAIN_FILE` is now passed through during dependency superbuild. This should solve build issues for some embedded toolchain users.
#220
  A false positive undefined behaviour sanitiser failure in some use cases of Experimental Outcome was worked around to avoid the failure message.
#221
  Restored compatibility with x86 on Windows, which was failing with link errors. It was quite surprising that this bug was not reported sooner, but obviously almost nobody is using Outcome with x86 on Windows.
#223
  Fix a segfault in Debug builds only when cloning a `status_code_ptr` in Experimental.Outcome only.

## v2.1.2 11th December 2019 (Boost 1.72) [release]

### Enhancements:

Improved compatibility with cmake tooling
  Standalone outcome is now `make install`-able, and cmake `find_package()` can find it. Note that you must separately install and `find_package()` Outcome’s dependency, quickcpplib, else `find_package()` of Outcome will fail.
Non-permissive parsing is now default in Visual Studio
  The default targets in standalone Outcome’s cmake now enable non-permissive parsing. This was required partially because VS2019 16.3’s quite buggy Concepts implementation is unusuable in permissive parsing mode. Even then, lazy ADL two phase lookup is broken in VS2019 16.3 with `/std:latest`, you may wish to use an earlier language standard.
Breaking change!
  The git submodule mechanism used by standalone Outcome of specifying dependent libraries has been replaced with a cmake superbuild of dependencies mechanism instead. Upon cmake configure, an internal copy of quickcpplib will be git cloned, built and installed into the build directory from where an internal `find_package()` uses it. This breaks the use of the unconfigured Outcome repo as an implementation of Outcome, one must now do one of:
  1. Add Outcome as subdirectory to cmake build.
  2. Use cmake superbuild (i.e. `ExternalProject_Add()`) to build and install Outcome into a local installation.
  3. Use one of the single header editions.
Breaking change!
  For standalone Outcome, the current compiler is now checked for whether it will compile code containing C++ Concepts, and if it does, all cmake consumers of Outcome will enable C++ Concepts. Set the cmake variable `BOOST_OUTCOME_C_CONCEPTS_FLAGS` to an empty string to prevent auto detection and enabling of C++ Concepts support occurring.
`BOOST_OUTCOME_TRY` operation now hints to the compiler that operation will be successful
  P1886 Error speed benchmarking showed that there is considerable gain in very small functions by hinting to the compiler whether the expression is expected to be successful or not. `BOOST_OUTCOME_TRY` previously did not hint to the compiler at all, but now it does. A new suite of macros `BOOST_OUTCOME_TRY_FAILURE_LIKELY` hint to the compiler that failure is expected. If you wish to return to the previously unhinted behaviour, define `BOOST_OUTCOME_TRY_LIKELY(expr)` to `(!!expr)`.
#199
  Support for C++ Coroutines has been added. This comes in two parts, firstly there is now an `BOOST_OUTCOME_CO_TRY()` operation suitable for performing the `TRY` operation from within a C++ Coroutine. Secondly, in the header `outcome/coroutine_support.hpp` there are implementations of `eager<OutcomeType>` and `lazy<OutcomeType>` which let you more naturally and efficiently use `basic_result` or `basic_outcome` from within C++ Coroutines – specifically, if the result or outcome will construct from an exception pointer, exceptions thrown in the coroutine return an errored or excepted result with the thrown exception instead of throwing the exception through the coroutine machinery (which in current compilers, has a high likelihood of blowing up the program). Both `eager<T>` and `lazy<T>` can accept any `T` as well. Both have been tested and found working on VS2019 and clang 9.
#210
  `make_error_code()` and `make_exception_ptr()` are now additionally considered for compatible copy and move conversions for `basic_result<>`. This lets you construct a `basic_result<T, E>` into a `basic_result<T, error_code>`, where `E` is a custom type which has implemented the ADL discovered free function `error_code make_error_code(E)`, but is otherwise unrelated to `error_code`. The same availability applies for `exception_ptr` with `make_exception_ptr()` being the ADL discovered free function. `basic_outcome<>` has less support for this than `basic_result<>` in order to keep constructor count down, but it will accept via this mechanism conversions from `basic_result<>` and `failure_type<>`.

### Bug fixes:

#184
  The detection of `[[nodiscard]]` support in the compiler was very mildly broken.

## v2.1.1 19th August 2019 (Boost 1.71) [release]

### Enhancements:

#184
  As per request from Boost release managers, relocated `version.hpp` and `revision.hpp` into detail, and added the Boost licence boilerplate to the top of every source file which was missing one (I think). Also took the opportunity to run the licence restamping script over all Outcome, so copyright dates are now up to date.
#185
  Add FAQ item explaining issue #185, and why we will do nothing to fix it right now.
#189
  Refactored the `BOOST_OUTCOME_TRY` implementation to use more clarified customisation points capable of accepting very foreign inputs. Removed the `std::experimental::expected<T, E>` specialisations, as those are no longer necessary. Fixed the documentation for the customisation points which previously claimed that they are ADL discovered, which they are not. Added a recipe describing how to add in support for foreign input types.
#183
  Added a separate `motivation/plug_error_code` specifically for Boost.

### Bug fixes:

-
  `BOOST_OUTCOME_VERSION_MINOR` hadn’t been updated to 1.
#181
  Fix issue #181 where Outcome didn’t actually implement the strong swap guarantee, despite being documented as doing so.
#190
  Fix issue #190 in Boost edition where unit test suite was not runnable from the Boost release distro.
#182
  Fix issue #182 where `trait::is_exception_ptr_available<T>` was always true, thus causing much weirdness, like not printing diagnostics and trying to feed everything to `make_exception_ptr()`.
#194
  Fix issue #192 where the `std::basic_outcome_failure_exception_from_error()` was being defined twice for translation units which combine standalone and Boost Outcome’s.

## v2.1 12th Apr 2019 (Boost 1.70) [release]

- #180

  - `success()` and `failure()` now produce types marked `[[nodiscard]]`.

- `include/outcome/outcome.natvis` is now namespace permuted like the rest of Outcome, so debugging Outcome based code in Visual Studio should look much prettier than before.

- #162

  - `.has_failure()` was returning false at times when it should have returned true.

- #152

  - GCC 5 no longer can compile Outcome at all due to https://stackoverflow.com/questions/45607450/gcc5-nested-variable-template-is-not-a-function-template. Added explicit version trap for GCC 5 to say it can not work. Note this is not a breaking change, GCC 5 was never supported officially in any v2 Outcome.

- #150

  - BREAKING CHANGE `result<T, E>`, `boost_result<T, E>` and `std_result<T, E>` no longer implement hard UB on fetching a value from a valueless instance if `E` is a UDT, they now fail to compile with a useful error message. If you wish hard UB, use `unchecked<T, E>`, `boost_unchecked<T, E>` or `std_unchecked<T, E>` instead.

- #140

  - Fixed a nasty corner case bug where value type’s without a copy constructor but with a move constructor would indicate via traits that copy construction was available. Thanks to Microsoft’s compiler team for reporting this issue.

- Added experimental `status_result` and `status_outcome` based on experimental `status_code`.

- Boost edition is now 100% Boost, so defaults for `result` and `outcome` are `boost::system::error_code::errc_t` and `boost::exception_ptr`. Moreover, the test suite in the Boost edition now exclusively tests the Boost edition. One can, of course, freely use the standalone edition with Boost, and the Boost edition with `std` types.

- Renamed ADL discovered customisation point `throw_as_system_error_with_payload()` to `outcome_throw_as_system_error_with_payload()`.

- #135

  - Added much clearer compile failure when user tries `result<T, T>` or `outcome` where two or more types are identical. Thanks to Andrzej Krzemieński for suggesting a technique which combines SFINAE correctness with the remaining ability for `result<T, T>` etc to be a valid type, but not constructible.

- #67

  - Fixed one of the oldest long open bugs in Outcome, that the noexcept unit tests failed on OS X for an unknown reason.

- #115

  - Outcome did not construct correctly from `failure_type`.

- Inexplicably outcome’s error + exception constructor had been removed. Nobody noticed during the Boost peer review, which is worrying seeing as that constructor is needed for one of the main advertised features to Boost!

- #107 and #116

  - `operator==` and `operator!=` now become disabled if the value, error and exception types do not implement the same operator.
  - Relatedly, both comparison operators simply didn’t work right. Fixed.

- #109

  - `swap()` now has correct `noexcept` calculation and now correctly orders the swaps to be whichever is the throwing swap first.

- Added reference dump of v2.1 ABI so we can check if ABI breakage detection works in the next set of changes, plus Travis job to check ABI and API compatibility per commit.

- #124

  - `BOOST_OUTCOME_TRY` is now overloaded and selects `void` or `auto` edition according to input parameter count.

- #120

  - Fix generation of double underscored temporary variables in `BOOST_OUTCOME_UNIQUE_NAME`, which is UB.

- #110

  - Separated `result` from its hard coded dependency on the `<system_error>` header.
  - Renamed `result` and `outcome` to `basic_result` and `basic_outcome`.
  - Renamed `result.hpp` into `basic_result.hpp`.
  - Moved `<system_error>` and `<exception>` dependent code into new `std_result.hpp` and `std_outcome.hpp` header files.
  - Added `boost_result.hpp` and `boost_outcome.hpp` which use Boost.System and Boost.Exception (these are `result.hpp` and `outcome.hpp` in the Boost edition).

## v2.0 18th Jan 2018 [release]

- Boost peer review edition. This is what was reviewed.
- Changelog from v1 can be found in the release notes for this release.

Last revised: November 17, 2023 at 08:58:24 UTC