If there is ~/.ghc/…/environments/default, cabal install fails.

[kindaro]

Describe the bug

Not sure how I arrived there, but since yesterday I cannot install any packages. It looks like
this:

% cabal v2-install optparse-applicative --lib
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: base-4.12.0.0/installed-4.12.0.0 (user goal)
[__1] trying: containers-0.6.2.1 (user goal)
[__2] next goal: ghc (user goal)
[__2] rejecting: ghc-8.6.5/installed-8.6.5 (conflict: containers==0.6.2.1, ghc
=> containers==0.6.0.1/installed-0.6.0.1)
[__2] trying: ghc-8.6.5
[__3] rejecting: ghc:-buildable (conflict: base==4.12.0.0/installed-4.12.0.0,
ghc -buildable => base<0)
[__3] rejecting: ghc:+buildable (manual flag can only be changed explicitly)
[__3] fail (backjumping, conflict set: base, ghc, ghc:buildable)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: containers, base, ghc, ghc:buildable
Try running with --minimize-conflict-set to improve the error message.

• If I remove the store, like rm ~/.cabal/store, the issue gets fixed.
• If I remove the default environment, like rm ~/.ghc/x86_64-linux-8.6.5/environments/default,
  I can install one library, then the file gets created again, I have to remove it again to
  install more things, and so on indefinitely.

The issue affects only a single version of GHC. I can install things with other GHC versions
without issue. All installations of the same version are affected at once.

To Reproduce

I have a 10 GB store that I can reliably play the issue out from. Tomorrow I am thinking to try and
cut packages from a copy of the store to reduce its volume while making sure the issue remains.

Expected behavior

A library or an executable is installed as usual.

System information

• Operating system: Arch Linux x86_64.
• Cabal: Across several versions, including current master.
• GHC: only 8.6.5. Other versions work fine.

Additional context

We had a conversation with folks on the IRC and it turns out someone else also experienced
this issue. Also see a longer log.

[phadej]

Duplicate of #5888