No existing code uses "default:" for marking the default enum value
because currently this syntax is invalid. So this is not a breaking change.
Before this proposal:
ExternalLibrary::Enum e{};    // e == 0

After this proposal (and ExternalLibrary update): 
ExternalLibrary::Enum e{};    // e == some default value


On Fri, Mar 24, 2023 at 5:17 PM Andrey Semashev via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
On 3/24/23 18:05, Oleksandr Koval via Std-Proposals wrote:
> Actually, allowing default values can break
> existing code because right now `Enum e{};` always means e = 0, after
> your change the value will be different.

No existing code uses "default:" for marking the default enum value
because currently this syntax is invalid. So this is not a breaking change.

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Regards,
Oleksandr Koval.