Instantiate vs Resolve: Which Approach Safely Recovers a Corrupted Julia Environment?

Goal

After a failed Julia package upgrade, the user wants to restore a functional environment that previously compiled successfully.

Constraints

During the upgrade, the Manifest.toml was modified to include an impossible or incompatible version entry for a package, causing subsequent using statements to error. The user has a backup of the original Manifest but does not know whether the recorded versions are still available on the registry.

Recovery Options

Two documented Pkg commands can be used: Pkg.instantiate() reinstalls the exact versions listed in the Manifest, while Pkg.resolve() recomputes a compatible set of versions based on the Project.toml constraints, potentially selecting newer releases.

Questions

• Will Pkg.instantiate() always restore the pre‑upgrade state, or can it propagate existing incompatibilities?
• Does Pkg.resolve() guarantee a working configuration, and how does it handle the impossible version entry?
• Under what circumstances should one prefer instantiate over resolve, or vice versa, when recovering from a corrupted environment?