Avoid msvc-14.0 constexpr errors in enum_to_string

.drone.jsonnet

@@ -370,12 +370,19 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
     ),
 
     linux_pipeline(
-        "Linux 25.04 Clang 20",
-        "cppalliance/droneubuntu2504:1",
+        "Linux 24.04 Clang 20",
+        "cppalliance/droneubuntu2404:1",
         { TOOLSET: 'clang', COMPILER: 'clang++-20', CXXSTD: '03,11,14,17,20,23,2c' },
         "clang-20",
     ),
 
+    linux_pipeline(
+        "Linux 25.10 Clang 21",
+        "cppalliance/droneubuntu2510:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-21', CXXSTD: '03,11,14,17,20,23,2c' },
+        "clang-21",
+    ),
+
     macos_pipeline(
         "MacOS 10.15 Xcode 12.2 UBSAN",
         { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,1z' } + ubsan,
@@ -386,6 +393,18 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
         { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,1z' } + asan,
     ),
 
+    macos_pipeline(
+        "MacOS 12.4 Xcode 13.2.1 UBSAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,17,20,2b' } + ubsan,
+        xcode_version = "13.2.1", osx_version = "monterey", arch = "arm64",
+    ),
+
+    macos_pipeline(
+        "MacOS 12.4 Xcode 13.2.1 ASAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,17,20,2b' } + asan,
+        xcode_version = "13.2.1", osx_version = "monterey", arch = "arm64",
+    ),
+
     macos_pipeline(
         "MacOS 12.4 Xcode 13.4.1 UBSAN",
         { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,17,20,2b' } + ubsan,
@@ -398,6 +417,18 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
         xcode_version = "13.4.1", osx_version = "monterey", arch = "arm64",
     ),
 
+    macos_pipeline(
+        "MacOS 14 Xcode 16.2.0 UBSAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,17,20,2b' } + ubsan,
+        xcode_version = "16.2.0", osx_version = "sonoma", arch = "arm64",
+    ),
+
+    macos_pipeline(
+        "MacOS 14 Xcode 16.2.0 ASAN",
+        { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '03,11,14,17,20,2b' } + asan,
+        xcode_version = "16.2.0", osx_version = "sonoma", arch = "arm64",
+    ),
+
     windows_pipeline(
         "Windows VS2015 msvc-14.0",
         "cppalliance/dronevs2015",

.github/workflows/ci.yml

@@ -67,6 +67,11 @@ jobs:
             container: ubuntu:24.04
             os: ubuntu-latest
             install: g++-14
+          - toolset: gcc-15
+            cxxstd: "03,11,14,17,20,23,2c"
+            container: ubuntu:25.10
+            os: ubuntu-latest
+            install: g++-15
           - toolset: clang
             compiler: clang++-3.9
             cxxstd: "03,11,14"
@@ -170,14 +175,26 @@ jobs:
             os: ubuntu-latest
             install: clang-19
           - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
-            os: macos-13
+            compiler: clang++-20
+            cxxstd: "03,11,14,17,20,23,2c"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-20
+          - toolset: clang
+            compiler: clang++-21
+            cxxstd: "03,11,14,17,20,23,2c"
+            container: ubuntu:25.10
+            os: ubuntu-latest
+            install: clang-21
           - toolset: clang
             cxxstd: "03,11,14,17,20,2b"
             os: macos-14
           - toolset: clang
             cxxstd: "03,11,14,17,20,2b"
             os: macos-15
+          - toolset: clang
+            cxxstd: "03,11,14,17,20,23,2c"
+            os: macos-26
 
     runs-on: ${{matrix.os}}
 
@@ -310,9 +327,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 
@@ -358,9 +375,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 
@@ -416,9 +433,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 

doc/describe/changes.adoc

@@ -8,6 +8,11 @@ https://www.boost.org/LICENSE_1_0.txt
 # Revision History
 :idprefix:
 
+## Changes in Boost 1.91.0
+
+* Under {cpp}20, described nested enums now work when the enclosing class is still incomplete. (Thanks to Julien Blanc.)
+* Made `enum_to_string` constexpr. (Thanks to Julien Blanc.)
+
 ## Changes in Boost 1.84.0
 
 * Added an overload of `enum_from_string` that takes a string view, to avoid

include/boost/describe/detail/config.hpp

@@ -45,4 +45,20 @@
 
 #define BOOST_DESCRIBE_INLINE_CONSTEXPR BOOST_DESCRIBE_INLINE_VARIABLE BOOST_DESCRIBE_CONSTEXPR_OR_CONST
 
+#if __cplusplus >= 202002L || ( defined(_MSVC_LANG) && _MSVC_LANG >= 202002L )
+
+# define BOOST_DESCRIBE_CXX20
+
+// Clang 13.0 is needed for unevaluated lambdas
+# if defined(__clang__) && __clang_major__ < 13
+#  undef BOOST_DESCRIBE_CXX20
+# endif
+
+// Apple Clang 13.1 is Clang 13.0
+# if defined(__clang__) && defined(__apple_build_version__) && __clang_major__ == 13 && __clang_minor__ < 1
+#  undef BOOST_DESCRIBE_CXX20
+# endif
+
+#endif
+
 #endif // #ifndef BOOST_DESCRIBE_DETAIL_CONFIG_HPP_INCLUDED

include/boost/describe/enum.hpp

@@ -43,6 +43,23 @@ template<class... T> auto enum_descriptor_fn_impl( int, T... )
     return list<enum_descriptor<T>...>();
 }
 
+#if defined(BOOST_DESCRIBE_CXX20)
+
+template<auto V, auto N> struct enum_desc
+{
+    static constexpr auto value() noexcept { return V; }
+    static constexpr auto name() noexcept { return N(); }
+};
+
+#define BOOST_DESCRIBE_ENUM_BEGIN(E) \
+    inline decltype( boost::describe::detail::enum_descriptor_fn_impl( 0
+
+#define BOOST_DESCRIBE_ENUM_ENTRY(E, e) , boost::describe::detail::enum_desc<E::e, []{ return #e; }>{}
+
+#define BOOST_DESCRIBE_ENUM_END(E) ) ) boost_enum_descriptor_fn( E** ) { return {}; }
+
+#else
+
 #define BOOST_DESCRIBE_ENUM_BEGIN(E) \
     inline auto boost_enum_descriptor_fn( E** ) \
     { return boost::describe::detail::enum_descriptor_fn_impl( 0
@@ -53,6 +70,8 @@ template<class... T> auto enum_descriptor_fn_impl( int, T... )
 
 #define BOOST_DESCRIBE_ENUM_END(E) ); }
 
+#endif
+
 } // namespace detail
 
 #if defined(_MSC_VER) && !defined(__clang__)

include/boost/describe/enum_to_string.hpp

@@ -1,7 +1,7 @@
 #ifndef BOOST_DESCRIBE_ENUM_TO_STRING_HPP_INCLUDED
 #define BOOST_DESCRIBE_ENUM_TO_STRING_HPP_INCLUDED
 
-// Copyright 2020, 2021 Peter Dimov
+// Copyright 2020, 2021, 2025 Peter Dimov
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -22,16 +22,34 @@ namespace boost
 namespace describe
 {
 
-template<class E, class De = describe_enumerators<E>>
-char const * enum_to_string( E e, char const* def ) noexcept
+namespace detail
 {
-    char const * r = def;
 
-    mp11::mp_for_each<De>([&](auto D){
+// [&](auto D){ if( e == D.value ) r = D.name; }
+// except constexpr under C++14
 
-        if( e == D.value ) r = D.name;
+template<class E> struct ets_lambda
+{
+    E e;
+    char const** pr;
 
-    });
+    template<class D> constexpr void operator()( D d ) const noexcept
+    {
+        if( e == d.value ) *pr = d.name;
+    }
+};
+
+} // namespace detail
+
+template<class E, class De = describe_enumerators<E>>
+#if !( defined(_MSC_VER) && _MSC_VER == 1900 )
+constexpr
+#endif
+char const* enum_to_string( E e, char const* def ) noexcept
+{
+    char const* r = def;
+
+    mp11::mp_for_each<De>( detail::ets_lambda<E>{ e, &r } );
 
     return r;
 }

test/Jamfile

@@ -85,6 +85,10 @@ run pedantic_members_test.cpp
 
 run enum_from_string_test2.cpp ;
 
+compile nested_enum_test2.cpp ;
+
+compile enum_to_string_test_cx.cpp ;
+
 # examples
 
 obj describe_cxx14 : describe_cxx14.cpp ;

test/enum_to_string_test_cx.cpp

@@ -0,0 +1,32 @@
+// Copyright 2021, 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/describe/enum_to_string.hpp>
+#include <boost/describe/enum.hpp>
+#include <boost/describe/detail/cx_streq.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(BOOST_DESCRIBE_CXX14)
+
+BOOST_PRAGMA_MESSAGE("Skipping test because C++14 is not available")
+
+#elif defined(_MSC_VER) && _MSC_VER == 1900
+
+BOOST_PRAGMA_MESSAGE("Skipping test because _MSC_VER == 1900")
+
+#else
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+BOOST_DEFINE_ENUM_CLASS(E, v1, v2, v3)
+
+using boost::describe::enum_to_string;
+using boost::describe::detail::cx_streq;
+
+STATIC_ASSERT( cx_streq( enum_to_string( E::v1, "" ), "v1" ) );
+STATIC_ASSERT( cx_streq( enum_to_string( E::v2, "" ), "v2" ) );
+STATIC_ASSERT( cx_streq( enum_to_string( E::v3, "" ), "v3" ) );
+STATIC_ASSERT( cx_streq( enum_to_string( (E)17, "def" ), "def" ) );
+
+#endif // !defined(BOOST_DESCRIBE_CXX14)

test/nested_enum_test2.cpp

@@ -0,0 +1,24 @@
+// Copyright 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/describe/enumerators.hpp>
+#include <boost/describe/enum.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if !defined(BOOST_DESCRIBE_CXX20)
+
+BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_DESCRIBE_CXX20 isn't defined")
+
+#else
+
+struct X
+{
+    enum E { v1 };
+    BOOST_DESCRIBE_NESTED_ENUM(E, v1)
+
+    using L = boost::describe::describe_enumerators<E>;
+};
+
+#endif