FetchContent default shallow clone depth and tag propagation

Goal

Reduce network bandwidth and disk usage in CI pipelines by enabling shallow clones of external projects declared with FetchContent_Declare. The current default performs a full clone, which can be costly for large repositories.

Unresolved Decision

There is no documented option to set a default shallow clone depth for FetchContent. Developers must manually specify GIT_SHALLOW or wrap the declaration in a custom function. Additionally, the propagation of GIT_TAG and other fetch options to nested FetchContent calls is not guaranteed; each subproject typically re‑declares its own fetch settings.

Questions

• Should CMake introduce a default GIT_SHALLOW depth for FetchContent_Declare to simplify CI usage?
• How can nested FetchContent calls reliably inherit GIT_TAG and other fetch parameters without explicit re‑declaration?
• What would be the backward compatibility impact if a default shallow depth were added in a future release?