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Niels Lohmann
989b8ed841
ordered_map::emplace(key, value) took the mapped value only by T&&, an
rvalue reference rather than a forwarding reference, so
ordered_json::emplace("a", value) failed to compile whenever value was
an lvalue or a const lvalue, even though the same call compiles for
json (whose object_t is std::map, with a variadic emplace). Turn the
value parameter into a separately-deduced forwarding reference,
constrained with std::is_constructible so the overloads still only
accept something convertible to the mapped type. std::map-compatible
semantics are unchanged: emplace still does nothing if the key already
exists.
Open PR #5609 also touches ordered_map.hpp (moving values on vector
growth); this change only touches the two emplace() overloads and
should not conflict.
Fixes #5673.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>