Fix to_bjdata() emitting the Draft-3-only 'B' marker in default Draft-2 mode

_ArrayType_ = "byte" mapped unconditionally to the BJData type marker
'B', regardless of the requested bjdata_version. 'B' is defined only by
BJData Draft 3; with the default version (draft2), this produced a
stream that is invalid for Draft 2 and, unlike every other
_ArrayType_, round-tripped back as a binary value instead of the
original annotated object.

Only accept "byte" / emit 'B' when bjdata_version selects Draft 3.
Under Draft 2, fall back to the same plain-object encoding used
elsewhere in this function for other invalid-annotation cases, so the
value round-trips correctly.

Fixes #5404.

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

include/nlohmann/detail/output/binary_writer.hpp

@@ -1681,6 +1681,16 @@ class binary_writer
         }
         CharType dtype = it->second;
 
+        // the 'B' (byte) marker is only defined by BJData Draft 3; emitting it
+        // under the default Draft 2 mode would produce a stream that Draft 2
+        // readers reject, so such an object falls back to a plain object
+        // encoding instead (see the "Binary values" section of the BJData
+        // documentation)
+        if (dtype == 'B' && bjdata_version != bjdata_version_t::draft3)
+        {
+            return true;
+        }
+
         key = "_ArraySize_";
         // the dimensions are written verbatim as the header length below, so a
         // value that is not an array cannot produce a valid one: null emits 'Z'

single_include/nlohmann/json.hpp

@@ -18689,6 +18689,16 @@ class binary_writer
         }
         CharType dtype = it->second;
 
+        // the 'B' (byte) marker is only defined by BJData Draft 3; emitting it
+        // under the default Draft 2 mode would produce a stream that Draft 2
+        // readers reject, so such an object falls back to a plain object
+        // encoding instead (see the "Binary values" section of the BJData
+        // documentation)
+        if (dtype == 'B' && bjdata_version != bjdata_version_t::draft3)
+        {
+            return true;
+        }
+
         key = "_ArraySize_";
         // the dimensions are written verbatim as the header length below, so a
         // value that is not an array cannot produce a valid one: null emits 'Z'

tests/src/unit-bjdata.cpp

@@ -2586,7 +2586,12 @@ TEST_CASE("BJData")
                 CHECK(json::to_bjdata(json::from_bjdata(v_d), true, true) == v_d);
                 CHECK(json::to_bjdata(json::from_bjdata(v_D), true, true) == v_D);
                 CHECK(json::to_bjdata(json::from_bjdata(v_C), true, true) == v_C);
-                CHECK(json::to_bjdata(json::from_bjdata(v_B), true, true) == v_B);
+                // v_B uses the Draft-3-only 'B' marker, so it round-trips only when
+                // Draft 3 is explicitly selected (see GitHub issue #5404); the
+                // default Draft 2 falls back to a plain object instead, covered by
+                // the "ndarray with _ArrayType_ "byte" is gated by the BJData draft
+                // version" section below
+                CHECK(json::to_bjdata(json::from_bjdata(v_B), true, true, json::bjdata_version_t::draft3) == v_B);
             }
 
             SECTION("ndarray with data not matching _ArrayType_ is written as an object")
@@ -2629,8 +2634,10 @@ TEST_CASE("BJData")
                 // the C++ API stores an int literal as number_integer, so _ArrayType_
                 // names the wire type rather than the storage. Both storages have to
                 // produce the same typed array for every type.
+                // "byte" is checked separately below since it additionally requires
+                // BJData Draft 3 to be selected explicitly (see GitHub issue #5404).
                 for (const char* type :
-                        {"uint8", "int8", "uint16", "int16", "uint32", "int32", "uint64", "int64", "char", "byte"
+                        {"uint8", "int8", "uint16", "int16", "uint32", "int32", "uint64", "int64", "char"
                         })
                 {
                     CAPTURE(type);
@@ -2641,6 +2648,14 @@ TEST_CASE("BJData")
                     CHECK(from_text == json::to_bjdata(json({{"_ArrayType_", type}, {"_ArraySize_", {2, 3}}, {"_ArrayData_", {1, 2, 3, 4, 5, 6}}})));
                 }
 
+                {
+                    const std::string text = R"({"_ArrayType_":"byte","_ArraySize_":[2,3],"_ArrayData_":[1,2,3,4,5,6]})";
+                    const auto from_text = json::to_bjdata(json::parse(text), true, true, json::bjdata_version_t::draft3);
+                    CHECK(from_text.at(0) == '[');
+                    CHECK(from_text == json::to_bjdata(json({{"_ArrayType_", "byte"}, {"_ArraySize_", {2, 3}}, {"_ArrayData_", {1, 2, 3, 4, 5, 6}}}),
+                    true, true, json::bjdata_version_t::draft3));
+                }
+
                 // negative values under a signed type behave the same way
                 const auto from_neg = json::to_bjdata(json::parse(R"({"_ArrayType_":"int32","_ArraySize_":[2],"_ArrayData_":[-5,7]})"));
                 CHECK(from_neg.at(0) == '[');
@@ -2823,6 +2838,36 @@ TEST_CASE("BJData")
                 CHECK(out_single_ok.at(0) == '[');
                 CHECK(json::from_bjdata(out_single_ok) == json({1.5f}));
             }
+
+            SECTION("ndarray with _ArrayType_ \"byte\" is gated by the BJData draft version")
+            {
+                // the 'B' (byte) marker used by _ArrayType_ "byte" is only defined
+                // by BJData Draft 3; Draft 2 (the default) has no such marker, so
+                // emitting it unconditionally produced a stream that a Draft 2
+                // reader could not parse as intended (see GitHub issue #5404).
+                // Two dimensions are used so that a successfully written ndarray
+                // round-trips back into the annotated object (a single dimension
+                // is, by the BJData ndarray convention, read back as a plain
+                // binary value rather than the annotated object, same as every
+                // other single-dimension ndarray of a non-"byte" type is read
+                // back as a plain array instead of the annotated object).
+                json const j_byte = json({{"_ArrayType_", "byte"}, {"_ArraySize_", {2, 3}}, {"_ArrayData_", {1, 2, 3, 4, 5, 6}}});
+
+                // default (Draft 2): falls back to a plain object and round-trips
+                const auto out_draft2 = json::to_bjdata(j_byte);
+                CHECK(out_draft2.at(0) == '{');
+                CHECK(json::from_bjdata(out_draft2) == j_byte);
+
+                // explicit Draft 2: same as the default
+                const auto out_draft2_explicit = json::to_bjdata(j_byte, true, true, json::bjdata_version_t::draft2);
+                CHECK(out_draft2_explicit.at(0) == '{');
+                CHECK(json::from_bjdata(out_draft2_explicit) == j_byte);
+
+                // Draft 3 explicitly selected: still uses the compact 'B' ndarray encoding
+                const auto out_draft3 = json::to_bjdata(j_byte, true, true, json::bjdata_version_t::draft3);
+                CHECK(out_draft3 == std::vector<uint8_t>({'[', '$', 'B', '#', '[', '$', 'i', '#', 'i', 2, 2, 3, 1, 2, 3, 4, 5, 6}));
+                CHECK(json::from_bjdata(out_draft3) == j_byte);
+            }
         }
     }