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Fix container input_adapter SFINAE for lvalue-only ADL begin/end (#111)

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The container overload of json::parse(c) / accept(c) / sax_parse(c, ...)
silently dropped from overload resolution for user types whose ADL
begin(T&) / end(T&) accepted only non-const lvalue references
(a legitimate pattern matching std::begin semantics). This was because
the detection code used std::declval<ContainerType>() which synthesized
an rvalue, and the rvalue failed to bind to lvalue-only ADL functions.

Fix by making both the outer input_adapter(ContainerType&&) and the
factory's create(ContainerType&&) forwarding references, preserving the
caller's value category and constness via reference collapsing. This
ensures detection (std::declval) and actual use (std::forward) always
match without needing decay/remove_reference.

- Rewrite input_adapters.hpp container overload with forwarding refs
- Add regression tests for lvalue-only non-const ADL begin/end
- Add regression test for rvalue containers (no breakage)
- Update API docs (parse, accept, sax_parse, from_*) to clarify
  that begin/end must match std::begin/std::end semantics
- Add version history notes for 3.13.0
- Regenerate amalgamation

Second-order effect: binary_reader.hpp's internal call to
input_adapter(number_vector) now deduces iterator vs const_iterator
based on the lvalue; functionally harmless (iterator_input_adapter is
iterator-type-agnostic), verified via unit-ubjson/unit-bjdata tests.

Closes remaining limitation from #4354 / PR #5218 (todo 106).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-09 21:22:17 +02:00
parent d0de6a9111
commit fc7fde6910
10 changed files with 34 additions and 17 deletions
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single_include/nlohmann/json.hpp
+5 -4
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@@ -7354,18 +7354,19 @@ struct container_input_adapter_factory< ContainerType,
{
using adapter_type = decltype(input_adapter(begin(std::declval<ContainerType>()), end(std::declval<ContainerType>())));
static adapter_type create(const ContainerType& container)
static adapter_type create(ContainerType&& container)
{
return input_adapter(begin(container), end(container));
return input_adapter(begin(std::forward<ContainerType>(container)), end(std::forward<ContainerType>(container)));
}
};
} // namespace container_input_adapter_factory_impl
template<typename ContainerType>
typename container_input_adapter_factory_impl::container_input_adapter_factory<ContainerType>::adapter_type input_adapter(const ContainerType& container)
auto input_adapter(ContainerType&& container)
-> typename container_input_adapter_factory_impl::container_input_adapter_factory<ContainerType>::adapter_type
{
return container_input_adapter_factory_impl::container_input_adapter_factory<ContainerType>::create(container);
return container_input_adapter_factory_impl::container_input_adapter_factory<ContainerType>::create(std::forward<ContainerType>(container));
}
// specialization for std::string