Throw type_error.321 when serializing discarded values to binary formats (#5761)

* Throw type_error.321 when serializing discarded values to binary formats

The CBOR, MessagePack, UBJSON, BJData, and BSON writers silently
skipped the payload of a value_t::discarded value nested in an array
or object, while still writing its slot in the element/member count
(and, for BSON, its entry header), producing a binary document whose
declared size does not match what was actually written.

Throw type_error.321 instead, for a discarded value anywhere in the
tree, including at the top level.

Rewritten from the original PR against the current (non-recursive
option aside) binary_writer.hpp, which has changed substantially since
this was first proposed; the out_of_range.412 MessagePack size check
and unrelated test reformatting from that PR are dropped as out of
scope here.

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

* Document and test type_error.321 for discarded binary values

Add docs for the new exception (home/exceptions.md and the Exceptions
sections of to_cbor/to_msgpack/to_ubjson/to_bjdata/to_bson) and test
coverage for a discarded value nested in an array or object, nested
deeper, and (for UBJSON/BJData) inside an optimized same-type array,
for each of CBOR, MessagePack, UBJSON, BJData, and BSON. Adjust the
three pre-existing "discarded" tests that asserted the old silent
behavior (empty/short output) to expect type_error.321 instead.

Co-authored-by: ameliabarnabyhub <312084480+ameliabarnabyhub@users.noreply.github.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: ameliabarnabyhub <ameliabarnabyhub@users.noreply.github.com>
Co-authored-by: ameliabarnabyhub <312084480+ameliabarnabyhub@users.noreply.github.com>

tests/src/unit-bson.cpp

@@ -149,6 +149,54 @@ TEST_CASE("BSON")
             json const j = std::vector<int> {1, 2, 3, 4, 5, 6, 7};
             CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is array", json::type_error&);
         }
+
+        SECTION("discarded")
+        {
+            json const j = json::value_t::discarded;
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is discarded", json::type_error&);
+        }
+    }
+
+    SECTION("discarded values nested in a container cannot be serialized to BSON")
+    {
+        json const discarded = json::value_t::discarded;
+
+        SECTION("as an object value")
+        {
+            json j;
+            j["a"] = 1;
+            j["b"] = discarded;
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to BSON", json::type_error&);
+#else
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
+#endif
+        }
+
+        SECTION("in an array that is an object value")
+        {
+            json j;
+            j["a"] = json::array({1, discarded, 2});
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/a/1) cannot serialize discarded value to BSON", json::type_error&);
+#else
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
+#endif
+        }
+
+        SECTION("nested deeper (array in object in object)")
+        {
+            json inner_array = {1, discarded};
+            json middle_object;
+            middle_object["x"] = inner_array;
+            json j;
+            j["outer"] = middle_object;
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/outer/x/1) cannot serialize discarded value to BSON", json::type_error&);
+#else
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
+#endif
+        }
     }
 
     SECTION("keys containing code-point U+0000 cannot be serialized to BSON")