[API Compat] Adding a new value to a public enum is not detected as a breaking change

[bitbonk]

Is your feature request related to a problem? Please describe.

When adding a new value to an existing public enum, these changes are not detected as a source breaking changes by the baseline package validator. Such changes are not source compatible because consuming code would need to be adjusted to account for the new value, for example in a switch statement.

Describe the solution you'd like

Introduce a new validation check that fails when a new value to an existing public enum was added.

[dotnet-policy-service]

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage

[ViktorHofer]

Such changes are not source compatible because consuming code would need to be adjusted to account for the new value, for example in a switch statement.

See the definition of source compatibility here: https://github.com/dotnet/runtime/blob/main/docs/coding-guidelines/breaking-change-definitions.md#source-compatibility.

By definition, if a consumer needs to make changes to its code in order for it to build successfully

Adding a value to an enumeration doesn't fall into that bucket. The consumer's code still builds successfully but doesn't account for the new value. See https://github.com/dotnet/runtime/blob/main/docs/coding-guidelines/breaking-change-rules.md for more details. The addition of an enum value isn't document as a breaking change.