Cabal: Can't install a specific version of the package (with message "only already installed instances can be used")

Question

I've been trying to install some package:

PS C:\Users\Neo> cabal install template-haskell-2.15.0.0
Resolving dependencies...
cabal-3.6.2.0.exe: Could not resolve dependencies:
[__0] next goal: template-haskell (user goal)
[__0] rejecting: template-haskell-2.19.0.0, template-haskell-2.18.0.0,
template-haskell-2.17.0.0, template-haskell-2.16.0.0 (constraint from user
target requires ==2.15.0.0)
[__0] rejecting: template-haskell-2.15.0.0 (only already installed instances
can be used)
[__0] rejecting: template-haskell-2.14.0.0, template-haskell-2.13.0.0,
template-haskell-2.12.0.0, template-haskell-2.11.1.0,
template-haskell-2.11.0.0, template-haskell-2.10.0.0,
template-haskell-2.9.0.0, template-haskell-2.8.0.0, template-haskell-2.7.0.0,
template-haskell-2.6.0.0, template-haskell-2.5.0.0, template-haskell-2.4.0.1,
template-haskell-2.4.0.0, template-haskell-2.3.0.1, template-haskell-2.3.0.0,
template-haskell-2.2.0.0 (constraint from user target requires ==2.15.0.0)
[__0] fail (backjumping, conflict set: template-haskell)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: template-haskell

What does the message "only already installed instances can be used" actually mean?

Answer 1

Unlike most packages, template-haskell can't be installed, upgraded, or downgraded separately from GHC. If you really need to downgrade to template-haskell-2.15.0.0 , then you need to downgrade to GHC 8.8.4. https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/libraries/version-history has a chart that lets you check which GHC version corresponds to which library versions.