Avoid msvc-14.0 constexpr errors in enum_to_string

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/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

@@ -87,6 +87,8 @@ 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)