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Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions

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basic_json can be constructed from std::tuple<json&>, which it turns into
a one-element array. Because of this, std::tuple picks its converting
constructor that converts the whole source tuple instead of the
element-wise one. As a result, std::tuple<const json&> built from
std::forward_as_tuple(j) binds to a temporary (a compile error with libc++,
a dangling reference with other standard libraries), and std::tuple<json>
built the same way holds [j] instead of a copy of j.

The new opt-in macro JSON_DISABLE_TUPLE_REFERENCE_CONVERSION (CMake option
JSON_DisableTupleReferenceConversion) removes the conversion from a
one-element tuple holding a reference to the same basic_json type, so
std::tuple converts element-wise. It is off by default, so existing
behavior is unchanged. It does not change any function body and therefore
is not part of the ABI tag.

Fixes #2226

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-27 17:17:51 +02:00
parent f682cd2ef1
commit c5f1694b01
17 changed files with 296 additions and 3 deletions
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cmake/ci.cmake
+14
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@@ -276,6 +276,20 @@ add_custom_target(ci_test_disableenumserialization
COMMENT "Compile and test with enum serialization disabled"
)
###############################################################################
# Disable conversion from a one-element tuple of a JSON reference.
###############################################################################
add_custom_target(ci_test_disabletuplereferenceconversion
COMMAND ${CMAKE_COMMAND}
-DCMAKE_BUILD_TYPE=Debug -GNinja
-DJSON_BuildTests=ON -DJSON_FastTests=ON -DJSON_DisableTupleReferenceConversion=ON
-S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_disabletuplereferenceconversion
COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_disabletuplereferenceconversion
COMMAND cd ${PROJECT_BINARY_DIR}/build_disabletuplereferenceconversion && ${CMAKE_CTEST_COMMAND} --parallel ${N} --output-on-failure
COMMENT "Compile and test with tuple reference conversion disabled"
)
###############################################################################
# Skip the multiple-inclusion library version check.
###############################################################################