Throw type_error.321 for discarded values in to_bon8

write_bon8_value silently skipped discarded values, but the array/object
count marker still counted them, so [1, discarded] produced 82 91: a marker
announcing two elements followed by one. Throw type_error.321 like the other
binary writers do, at any nesting level.

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

docs/mkdocs/docs/api/basic_json/to_bon8.md

@@ -41,6 +41,8 @@ With (2), the bytes written before the exception remain in the output adapter.
   above 9223372036854775807, which BON8 cannot represent
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if `j` contains a string that is not
   valid UTF-8
+- Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is
+  discarded; example: `"cannot serialize discarded value to BON8"`
 
 ## Complexity
 

docs/mkdocs/docs/home/exceptions.md

@@ -810,7 +810,7 @@ does not list an enumerator and it is therefore converted like the first listed
 A discarded value (one created by [`parse()`](../api/basic_json/parse.md) with a callback that returns `false` for the
 value, or by default-constructing a [`basic_json`](../api/basic_json/index.md) with
 [`value_t::discarded`](../api/basic_json/value_t.md)) was passed to a binary serialization function, either directly or
-nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, or BSON.
+nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, BSON, or BON8.
 
 !!! failure "Example message"
 

include/nlohmann/detail/output/binary_writer.hpp

@@ -809,6 +809,8 @@ class binary_writer
 
     /*!
     @param[in] j  JSON value to serialize
+
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_bon8(const BasicJsonType& j)
     {
@@ -2570,6 +2572,8 @@ class binary_writer
     @param[in] j                JSON value to serialize
     @param[in,out] string_open  whether the output ends with a non-empty
                                 string that has not been terminated with 0xFF
+
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_bon8_value(const BasicJsonType& j, bool& string_open)
     {
@@ -2678,7 +2682,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "BON8");
         }
     }
 

single_include/nlohmann/json.hpp

@@ -22426,6 +22426,8 @@ class binary_writer
 
     /*!
     @param[in] j  JSON value to serialize
+
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_bon8(const BasicJsonType& j)
     {
@@ -24187,6 +24189,8 @@ class binary_writer
     @param[in] j                JSON value to serialize
     @param[in,out] string_open  whether the output ends with a non-empty
                                 string that has not been terminated with 0xFF
+
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_bon8_value(const BasicJsonType& j, bool& string_open)
     {
@@ -24295,7 +24299,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "BON8");
         }
     }
 

tests/src/unit-bon8.cpp

@@ -82,9 +82,56 @@ TEST_CASE("BON8")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to BON8
             const json j = json::value_t::discarded;
-            CHECK(json::to_bon8(j).empty());
+            CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] cannot serialize discarded value to BON8", json::type_error&);
+        }
+
+        SECTION("discarded values nested in a container")
+        {
+            const json discarded = json::value_t::discarded;
+
+            SECTION("in a small array")
+            {
+                const json j = {1, discarded};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to BON8", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] cannot serialize discarded value to BON8", json::type_error&);
+#endif
+            }
+
+            SECTION("as a small object value")
+            {
+                json j;
+                j["a"] = discarded;
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] (/a) cannot serialize discarded value to BON8", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] cannot serialize discarded value to BON8", json::type_error&);
+#endif
+            }
+
+            SECTION("in a large array")
+            {
+                const json j = {1, 2, 3, 4, 5, discarded};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] (/5) cannot serialize discarded value to BON8", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] cannot serialize discarded value to BON8", json::type_error&);
+#endif
+            }
+
+            SECTION("as a large object value")
+            {
+                json j = {{"a", 1}, {"b", 2}, {"c", 3}, {"d", 4}, {"e", 5}};
+                j["f"] = discarded;
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] (/f) cannot serialize discarded value to BON8", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bon8(j), "[json.exception.type_error.321] cannot serialize discarded value to BON8", json::type_error&);
+#endif
+            }
         }
 
         SECTION("null")