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Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions (#5598)

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* Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions

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>

* Convert one-element tuples to arrays on every compiler

to_json for std::tuple assigns a braced list, j = { std::get<Idx>(t)... }.
With a single element that is itself a basic_json, Apple clang 15 and 16
treat j = {x} as a copy of x, so std::tuple<json>{true} became true
instead of [true]. The macOS jobs (Xcode 15.1, 16.1) failed the new
checks in unit-disable-tuple-reference-conversion and unit-regression2.

The one-element overload that already handles
JSON_BRACE_INIT_COPY_SEMANTICS builds the array (or object, for a
[string, value] element) explicitly, the same way the initializer-list
constructor does. Use it unconditionally. The output is unchanged on
compilers that already wrapped the element.

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

* Skip json reference tuple tests on clang < 4 and GCC < 5

ci_test_compilers_gcc_old (4.8) and ci_test_compilers_clang (3.4) could
not compile the new tuple tests. Creating a std::tuple of basic_json
references, e.g. std::forward_as_tuple(j), makes these compilers
instantiate basic_json's conversion operator for libstdc++'s internal
tuple bases, which fails hard. This happens with and without
JSON_DISABLE_TUPLE_REFERENCE_CONVERSION, so it is a limitation of these
compilers, not of the new option.

Tested with the CI images: clang 3.4 to 3.9 and GCC 4.8 and 4.9 fail,
clang 4, 5, and 6 and GCC 5 and 6 compile all cases. Skip only the
checks that create such tuples; the is_constructible checks still run.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 20:53:56 +02:00
committed by GitHub
parent 6a073dbae4
commit 3b6ae43c53
18 changed files with 334 additions and 15 deletions
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docs/docset/docSet.sql
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@@ -210,6 +210,7 @@ INSERT INTO searchIndex(name, type, path) VALUES ('JSON_CATCH_USER', 'Macro', 'a
INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DIAGNOSTICS', 'Macro', 'api/macros/json_diagnostics/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DIAGNOSTIC_POSITIONS', 'Macro', 'api/macros/json_diagnostic_positions/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DISABLE_ENUM_SERIALIZATION', 'Macro', 'api/macros/json_disable_enum_serialization/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('JSON_DISABLE_TUPLE_REFERENCE_CONVERSION', 'Macro', 'api/macros/json_disable_tuple_reference_conversion/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('JSON_HAS_CPP_11', 'Macro', 'api/macros/json_has_cpp_11/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('JSON_HAS_CPP_14', 'Macro', 'api/macros/json_has_cpp_11/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('JSON_HAS_CPP_17', 'Macro', 'api/macros/json_has_cpp_11/index.html');