The sync.Once instance can be a package var or a member of a struct, and the returned values must be closure-captured.

Patterns should be considered:

The instance is a package var:
- Once.Do returns 1 value by closure.
- Once.Do returns 2 values by closure.
- If Once.Do returns more than 2 values, we should leave out the third and all subsequent values, as sync.OnceValues can return at most 2 values.
The instance is a struct member:

Basically this is the same as a package var, but we should update the corresponding member type from sync.Once to the return type of sync.OnceValues.

Return values

The returned values should be updated accordingly—typically by deleting unused ones, as in https://go.dev/cl/728860.

The closure-captured variable is a struct. As we may break the original behavior.
sync.Once.Do does not return any values.
The instance is a struct member whose type is not defined locally. We may not be able to update the member type.
The returned value is a package var and is either created by a function call or is partially initialized. We would change the initialization timing.