g++-16 on OSX: test_flags-cpp26 Exception: SegFault

[ClausKlein]

bash-5.3$ git describe --tag --dirty
v0.9.8-54-g00d372c
bash-5.3$ git branch
  develop
  feature/format-cmake-files
  feature/module_target
* master
  module_target
bash-5.3$ 

cmake -S . -B build/release -G Ninja -D CMAKE_BUILD_TYPE=Release -D CMAKE_CXX_STANDARD=26
ninja -C build/release
ninja -C build/release test

bash-5.3$ gmake env | egrep '^C' 
CMAKE_CONFIG_TYPE = Release
CMAKE_PREFIX_PATH = /Users/clausklein/.local
CMAKE_GENERATOR = Ninja
CURDIR = /Users/clausklein/Workspace/cpp/magic_enum
CMAKE_INSTALL_PREFIX = /Users/clausklein/.local
CMAKE_CONFIGURATION_TYPES = Release;Debug
CPM_USE_LOCAL_PACKAGES = NO
CLASSPATH = /usr/local/opt/antlr/antlr-4.13.0-complete.jar:
CMAKE_EXPORT_COMPILE_COMMANDS = YES
CMAKE_CXX_COMPILER_LAUNCHER = ccache
CMAKE_C_COMPILER_LAUNCHER = ccache
CPM_SOURCE_CACHE = /Users/clausklein/.cache/CPM
CXX = g++-16
bash-5.3$ ctest --test-dir build/release --rerun-failed --output-on-failure
Test project /Users/clausklein/Workspace/cpp/magic_enum/build/release
    Start 18: test_flags-cpp26
1/1 Test #18: test_flags-cpp26 .................***Exception: SegFault  0.01 sec
[doctest] doctest version is "2.5.2"
[doctest] run with "--help" for options
===============================================================================
/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:123:
TEST CASE:  enum_cast
  string

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:123: FATAL ERROR: test case CRASHED: SIGSEGV - Segmentation violation signal

===============================================================================
/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:123:
TEST CASE:  enum_cast

DEEPEST SUBCASE STACK REACHED (DIFFERENT FROM THE CURRENT ONE):
  string

===============================================================================
[doctest] test cases:  2 |  1 passed | 1 failed | 23 skipped
[doctest] assertions: 22 | 22 passed | 0 failed |
[doctest] Status: FAILURE!


0% tests passed, 1 tests failed out of 1

Total Test time (real) =   0.02 sec

The following tests FAILED:
	 18 - test_flags-cpp26 (SEGFAULT)
Errors while running CTest
bash-5.3$ 

[ClausKlein]

bash-5.3$ build/release/test/test_flags-cpp26 -tc enum_cast -s
[doctest] doctest version is "2.5.2"
[doctest] run with "--help" for options
===============================================================================
/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:110:
TEST CASE:  enum_reflected

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:111: SUCCESS: REQUIRE( enum_reflected<Color>(Color::RED) ) is correct!
  values: REQUIRE( true )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:112: SUCCESS: REQUIRE( enum_reflected<Color, as_flags<>>(Color::BLUE) ) is correct!
  values: REQUIRE( true )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:113: SUCCESS: REQUIRE( enum_reflected<Directions>(Directions::Left) ) is correct!
  values: REQUIRE( true )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:114: SUCCESS: REQUIRE( enum_reflected<Directions>(Directions::Right) ) is correct!
  values: REQUIRE( true )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:115: SUCCESS: REQUIRE( enum_reflected<Directions>(std::uint64_t{1} << 62) ) is correct!
  values: REQUIRE( true )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:116: SUCCESS: REQUIRE_FALSE( enum_reflected<Directions>(Directions::NoDirection) ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:117: SUCCESS: REQUIRE_FALSE( enum_reflected<Directions>(Directions::Left | Directions::Down) ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:118: SUCCESS: REQUIRE_FALSE( enum_reflected<Directions>(std::uint64_t{3}) ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:119: SUCCESS: REQUIRE_FALSE( enum_reflected<Directions, as_common<>>(Directions::Left) ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:120: SUCCESS: REQUIRE_FALSE( enum_contains<Directions>(std::uint64_t{1} << 62) ) is correct!
  values: REQUIRE_FALSE( false )

===============================================================================
/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:123:
TEST CASE:  enum_cast
  string

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:126: SUCCESS: REQUIRE( cr.value() == Color::RED ) is correct!
  values: REQUIRE( 1 == 1 )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:127: SUCCESS: REQUIRE( enum_cast<Color&>("GREEN").value() == Color::GREEN ) is correct!
  values: REQUIRE( 2 == 2 )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:128: SUCCESS: REQUIRE( enum_cast<Color>("blue", [](char lhs, char rhs) { return std::tolower(lhs) == std::tolower(rhs); }).value() == Color::BLUE ) is correct!
  values: REQUIRE( 4 == 4 )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:129: SUCCESS: REQUIRE_FALSE( enum_cast<Color&>("blue|RED", [](char lhs, char rhs) { return std::tolower(lhs) == std::tolower(rhs); }).has_value() ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:130: SUCCESS: REQUIRE_FALSE( enum_cast<Color&>("GREEN|RED").has_value() ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:131: SUCCESS: REQUIRE_FALSE( enum_cast<Color&>("GREEN|RED|RED").has_value() ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:132: SUCCESS: REQUIRE_FALSE( enum_cast<Color&>("GREEN|RED|None").has_value() ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:133: SUCCESS: REQUIRE_FALSE( enum_cast<Color>("None").has_value() ) is correct!
  values: REQUIRE_FALSE( false )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:135: SUCCESS: REQUIRE( enum_flags_cast<Color&>("GREEN").value() == Color::GREEN ) is correct!
  values: REQUIRE( 2 == 2 )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:136: SUCCESS: REQUIRE( enum_flags_cast<Color>("blue", '|', [](char lhs, char rhs) { return std::tolower(lhs) == std::tolower(rhs); }).value() == Color::BLUE ) is correct!
  values: REQUIRE( 4 == 4 )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:137: SUCCESS: REQUIRE( enum_flags_cast<Color&>("blue|RED", '|', [](char lhs, char rhs) { return std::tolower(lhs) == std::tolower(rhs); }).value() == (Color::BLUE | Color::RED) ) is correct!
  values: REQUIRE( 5 == 5 )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:138: SUCCESS: REQUIRE( enum_flags_cast<Color&>("GREEN|RED").value() == (Color::GREEN | Color::RED) ) is correct!
  values: REQUIRE( 3 == 3 )

/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:123: FATAL ERROR: test case CRASHED: SIGSEGV - Segmentation violation signal

===============================================================================
/Users/clausklein/Workspace/cpp/magic_enum/test/test_flags.cpp:123:
TEST CASE:  enum_cast

DEEPEST SUBCASE STACK REACHED (DIFFERENT FROM THE CURRENT ONE):
  string

===============================================================================
[doctest] test cases:  2 |  1 passed | 1 failed | 23 skipped
[doctest] assertions: 22 | 22 passed | 0 failed |
[doctest] Status: FAILURE!
Segmentation fault: 11     build/release/test/test_flags-cpp26 -tc enum_cast -s
bash-5.3$ 

[ClausKlein]

Same on linux with g++-17:

63825224d609 /src # ctest --test-dir build/release --rerun-failed --output-on-failure
Test project /src/build/release
    Start 18: test_flags-cpp26
1/1 Test #18: test_flags-cpp26 .................***Failed    0.01 sec
[doctest] doctest version is "2.5.2"
[doctest] run with "--help" for options
===============================================================================
/src/test/test_flags.cpp:123:
TEST CASE:  enum_cast
  string

/src/test/test_flags.cpp:139: FATAL ERROR: REQUIRE( enum_flags_cast<Color&>("GREEN|RED|RED").value() == (Color::GREEN | Color::RED) ) THREW exception: "bad optional access"

===============================================================================
/src/test/test_flags.cpp:261:
TEST CASE:  enum_contains
  string

/src/test/test_flags.cpp:359: FATAL ERROR: REQUIRE( enum_flags_contains<Color>("GREEN|RED|RED") ) is NOT correct!
  values: REQUIRE( false )

===============================================================================
[doctest] test cases:  25 |  23 passed | 2 failed | 0 skipped
[doctest] assertions: 550 | 548 passed | 2 failed |
[doctest] Status: FAILURE!


0% tests passed, 1 tests failed out of 1

Total Test time (real) =   0.02 sec

The following tests FAILED:
	 18 - test_flags-cpp26 (Failed)
Errors while running CTest
63825224d609 /src # g++ --version
g++ (Gentoo 17.0.9999 p, commit 6ff3934e1b39a90cf0a405c1ec18b24a6246acb5) 17.0.0 20260611 (experimental) 7cfe4a5f97ec98622c85c071f9b093631c0fd167
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

63825224d609 /src # 

[Neargye]

Thanks for the report. I reproduced this independently. This is a GCC bug, not a macOS, magic_enum, or doctest-specific problem.

The reported crash happens because an optimized runtime call to enum_flags_cast("GREEN|RED|RED") incorrectly returns a disengaged std::optional, and the following .value() accesses it.

I reduced it to this standard-library-only test case:

#include <iostream>
#include <optional>
#include <string_view>

constexpr std::optional<int> parse(std::string_view value) {
  while (!value.empty()) {
    const auto pos = value.find('|');
    if (pos == std::string_view::npos || value.substr(0, pos) != "A") {
      return {};
    }
    value.remove_prefix(pos + 1);
  }
  return 1;
}

int main() {
  std::cout << parse("A|A|A|").has_value() << '\n';
}

g++ -std=c++26 -O1 repro.cpp && ./a.out

The expected output is 1, but GCC prints 0. I reproduced it with GCC 14.4, 15.3, 16.1, and GCC 17 trunk. C++23 and C++26 -O0 produce the correct result; C++26 -O1, -O2, -O3, and -Os produce the wrong result. Forced constant evaluation and a non-constexpr version are also correct. The sanitizers and libstdc++ assertion/debug modes report no undefined behavior.

I have prepared an upstream GCC report, but GCC Bugzilla account creation requires manual approval and my account is still pending. If somebody here already has a GCC Bugzilla account, could you please file this testcase and post the bug URL here? Otherwise, I will file it as soon as my account is approved.

For now, I am adding a narrow GCC 14+ C++26 workaround that preserves constant evaluation, and I am keeping the affected C++26 paths covered by regression tests in CI.