Fix from_json() for enums with underlying type bool

get_arithmetic_value() rejects boolean_t, so the default from_json()
for enums failed to compile for an enum whose underlying type is
bool (e.g. enum class Flag : bool { off, on }), even though the
matching to_json() serializes such enums as an unsigned number.

Read the underlying value through number_unsigned_t in that case,
matching what to_json() writes, then cast back to the underlying
type before constructing the enum.

Fixes #5671.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/conversions/from_json.hpp

@@ -167,9 +167,13 @@ template<typename BasicJsonType, typename EnumType,
          enable_if_t<std::is_enum<EnumType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, EnumType& e)
 {
-    typename std::underlying_type<EnumType>::type val;
+    using underlying_type = typename std::underlying_type<EnumType>::type;
+    // get_arithmetic_value() does not accept boolean_t; read the number that to_json() wrote instead
+    using value_type = typename std::conditional<std::is_same<underlying_type, typename BasicJsonType::boolean_t>::value,
+          typename BasicJsonType::number_unsigned_t, underlying_type>::type;
+    value_type val;
     get_arithmetic_value(j, val);
-    e = static_cast<EnumType>(val);
+    e = static_cast<EnumType>(static_cast<underlying_type>(val));
 }
 #endif  // JSON_DISABLE_ENUM_SERIALIZATION
 

single_include/nlohmann/json.hpp

@@ -5664,9 +5664,13 @@ template<typename BasicJsonType, typename EnumType,
          enable_if_t<std::is_enum<EnumType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, EnumType& e)
 {
-    typename std::underlying_type<EnumType>::type val;
+    using underlying_type = typename std::underlying_type<EnumType>::type;
+    // get_arithmetic_value() does not accept boolean_t; read the number that to_json() wrote instead
+    using value_type = typename std::conditional<std::is_same<underlying_type, typename BasicJsonType::boolean_t>::value,
+          typename BasicJsonType::number_unsigned_t, underlying_type>::type;
+    value_type val;
     get_arithmetic_value(j, val);
-    e = static_cast<EnumType>(val);
+    e = static_cast<EnumType>(static_cast<underlying_type>(val));
 }
 #endif  // JSON_DISABLE_ENUM_SERIALIZATION
 

tests/src/unit-conversions.cpp

@@ -1358,6 +1358,14 @@ TEST_CASE("value conversion")
         CHECK(json(value_1).get<c_enum>() == value_1);
         CHECK(json(cpp_enum::value_1).get<cpp_enum>() == cpp_enum::value_1);
     }
+
+    SECTION("get an enum with underlying type bool (#5671)")
+    {
+        enum class bool_enum : bool { off, on };
+
+        CHECK(json(bool_enum::off).get<bool_enum>() == bool_enum::off);
+        CHECK(json(bool_enum::on).get<bool_enum>() == bool_enum::on);
+    }
 #endif
 
     SECTION("more involved conversions")