/* Backwards Compatibility */ partially fill in

1 files changed, 26 insertions, 0 deletions

diff --git a/GLEP:73.mw b/GLEP:73.mw
index edd2ff0..6372a2d 100644
--- a/GLEP:73.mw
+++ b/GLEP:73.mw
@@ -296,6 +296,32 @@ For most of the real-life use cases, two iterations should be able to solve all
  c? ( d ) b? ( c ) a? ( b )
 
 ==Backwards Compatibility==
+===New constraints vs non-compliant package managers===
+This GLEP preserves full syntax compatibility with the existing package managers. The constraints written for auto-solving will still work correctly in the package managers not supporting it, resulting in regular REQUIRED_USE mismatch. Furthermore, the extended semantic meaning can result in improved readability of constraints, and therefore the messages issued by the package managers. Users aware of the auto-solving rules will have a suggested algorithm for satisfying REQUIRED_USE.
+
+The only potential danger is that the auto-solving will result in more extensive use of REQUIRED_USE and less concern for whether they are satisfied by default, resulting in more frequent REQUIRED_USE mismatches. Avoiding this problem should be done on policy level, requiring the developers not to rely purely on auto-solving through a migration period.
+
+===Old constraints vs auto-solving===
+Most of the existing REQUIRED_USE constraints are already compatible with auto-solving. There are three problematic cases:
+
+# constraints that are disallowed per [[#Restrictions_on_REQUIRED_USE_format|the restrictions on REQUIRED_USE format]],
+# constraints that can not be solved by the algorithm,
+# constraints that give sub-optimal (non-preferred) solutions.
+
+While the impact and details differ for each case, it can be commonly noted that all of them can be reliably fixed before implementing auto-solving, and — as noted above — the fixes will not break existing package managers.
+
+====Constraints disallowed in this GLEP====
+For simplification, this GLEP will reject some of the REQUIRED_USE forms that are valid per the PMS. They will be rejected for all combinations of USE flags that do not satisfy the constraint. However, this is not a major issue for three reasons:
+
+# The unsupported constraints are extremely rare, of low value and fixing them improves readability. As listed in [[#Restrictions_for_allowed_REQUIRED_USE_syntax|rationale for the restrictions]], there were a total of 8 packages being affected at the time of writing, and fixing them was already in progress.
+# The constraints are only rejected for auto-solving but are still supported for REQUIRED_USE verification. The package manager will therefore just report the unsolvable REQUIRED_USE to the user, making this not a regression from the previous state.
+# This GLEP does not strictly disallow the package manager from solving those constraints, only does not specify the solutions for them. Therefore, the package managers may implement custom extensions to solve them. However, they should still warn that this is non-portable and unreadable.
+
+====Constraints that can not be solved====
+Not all valid REQUIRED_USE constraints can be reliably solved. There are two major cases for that:
+
+# Constraints that toggle flags that caused previous conditions not to apply. Solving those may require more than one iteration of the solving algorithm. However, they usually can be fixed easily by reordering.
+# Constraints that have conflicts between flags. Solving those will result in repeated results where the constraint is unsatisfied. With multi-iteration solving, they can cause infinite loops.
 
 ==Reference Implementation==