`cabal repl --enable-multi-repl` failure when a package uses mixins.

[chreekat]

Context

Consider three packages: Alice, Bob, and Spike. Alice is a normal package. Bob is a normal package that depends on Spike. Spike depends on Alice like so:

-- Spike.cabal
...
library
  mixins:
    Alice hiding (Alice.Foo)

graph LR;
Bob --> Spike;
Spike -->|via mixin| Alice;

Loading

All packages are listed in cabal.project:

packages:
  ./Alice
  ./Bob
  ./Spike

Problem

If I run cabal repl --enable-multi-repl all, I get the following error:

Error:
    Mix-in refers to non-existent library 'Alice'
    (did you forget to add the package to build-depends?)
Error: [Cabal-7125]
Failed to build Spike-0.4.1 (which is required by exe:Bob  from Bob-0.1.6). The failure occurred during the configure step.

Gotchas

This is sometimes hard to reproduce. If I run cabal repl --enable-multi-repl Bob, Spike may get built successfully. Same with cabal repl --enable-multi-repl Spike, which usually works.

Furthermore, I don't understand it, but sometimes cabal repl decides it needs to rebuild dependencies. (Spuriously, in my view.) If Spike needs to get rebuilt, it'll likely fail again.

This example is taken from a much larger project with a deep tree of dependencies between the component packages. I don't know if it is easy to reproduce or not.

[chreekat]

The following is closer to the actual use case that triggers the bug.

Consider four packages, Alice, Bob, Carol, and Spike. Bob depends on Carol as well as Spike. Carol is a normal package. Everything else is the same as before.

graph LR;
Bob --> Spike;
Spike -->|via mixin| Alice;
Bob --> Carol

Loading

Works: cabal repl --enable-multi-repl Spike
Works: cabal repl --enable-multi-repl Bob
Works: cabal repl --enable-multi-repl Carol
Works: cabal repl --enable-multi-repl Bob Spike
Does not work: cabal repl --enable-multi-repl Bob Carol
Does not work: cabal repl --enable-multi-repl Bob Carol Spike
Does not work: cabal repl --enable-multi-repl Carol Spike (!)

For some reason, the bug only triggers when attempting to include Carol, who isn't even on the dependency path involving the mixin.