Inline namespaces do not permit per-symbol migration from ABI version vX to vY. Inline namespaces cannot be (re)declared inline in one context and non-inline in another context. This means future source-breaking-but-binary-compatible upgrades would require transitioning the entire inline ABI namespace from vX to vY, even if only a small selection of entities may motivate the upgrade.

To allow for incremental source-breaking-but-binary-compatible upgrades on a per-symbol basis (e.g. upgrading bsoncxx::foo from bsoncxx::vX::foo to bsoncxx::vY::foo), remove inline from the v_noabi namespace and instead use using-declarations to declare entities in the root library namespaces.

Current:

namespace bsoncxx {
inline namespace vX {

void foo(); // bsoncxx::foo CANNOT be upgraded to vY...
void bar();  // ... without forcibly upgrading bsoncxx::bar.

}  // namespace vX
}  // namespace bsoncxx

namespace bsoncxx {
namespace vY {

void foo(); // A newer ABI-incompatible interface.

}  // namespace vY
}  // namespace bsoncxx

Proposed:

namespace bsoncxx {
namespace vX {

void foo();
void bar();

}  // namespace vX
}  // namespace bsoncxx

namespace bsoncxx {
namespace vY {

void foo(); // A newer ABI-incompatible interface.

}  // namespace vY
}  // namespace bsoncxx

namespace bsoncxx {

using bsoncxx::vY::foo; // bsoncxx::foo CAN be upgraded to vX...
using bsoncxx::vX::bar; // ... without forcibly upgrading bsoncxx::bar.

}  // namespace bsoncxx

Changing inline namespace v_noabi into namespace v_noabi + using-declarations is a source-and-binary-compatible refactor and should not require an API major version bump.