$ cabal --version cabal-install version 1.20.0.2 using version 1.20.0.1 of the Cabal library

$ cabal repl ... src/full/Agda/TypeChecking/Serialise.hs:8:16: Warning: -O conflicts with --interactive; -O ignored.

<no location info>: Failing due to -Werror.

It seems a Cabal issue (see https://github.com/haskell/cabal/issues/1946 ).

The problem seems to be that there's an {-# OPTIONS_GHC -O2 #-} pragma in Serialise.

I will update the Cabal issue with this information.

Removing -Werror I could run cabal repl, but it uses almost 4 GB of memory.

The issue is fixed using the following command:

$ cabal repl --ghc-options=-Wwarn