Solver backjumps too far when linked packages have different flag assignments

[grayjay]

I wrote a test case that reproduces the problem: grayjay@14e9d32 The solver doesn't find a solution.

The type D.C.Dependency.Modular.Dependency.Var can represent a flag variable. When Var is used in a conflict set, the name of the flag is usually simplified to "flag" to simplify the logic dealing with dependencies between flags. However, when a flag choice causes a linked dependency to have an inconsistent flag value, the solver creates a conflict set containing non-simplified Vars. The mismatch in flag names prevents the solver from finding the flag variables in the conflict set during backjumping. It backtracks past those variables, skipping the other possible flag assignments.

I'm interested in working on this. Does it make sense to create a SimpleVar type that does not contain the flag name and is only used in conflict sets?

[23Skidoo]

/cc @kosmikus

[kosmikus]

/cc @edsko

[kosmikus]

Also going to try to look at this.

[edsko]

I'm going to take a look at this now.

[grayjay]

Fixed by #3357.