Build error with GHC 8.4.1-alpha1

[asr]

GHC 8.4.1-alpha1 was announced. While testing Agda with this version of GHC, I got the following error on hashtables (master branch):

$ cabal install
...
src/Data/HashTable/ST/Basic.hs:491:10: error:
    • No instance for (Semigroup Slot)
        arising from the superclasses of an instance declaration
    • In the instance declaration for ‘Monoid Slot’
    |
491 | instance Monoid Slot where
    |          ^^^^^^^^^^^
Failed to install hashtables-1.2.2.1

Blocking agda/agda#2878.

[asr]

Note that this error is reported as a warning in GHC 8.0.2/8.2.2 using the -Wsemigroup flag:

$ cabal install --force-reinstalls --ghc-options='-Wsemigroup -Werror'

So, fixing this warning on GHC 8.0.2/8.2.2 fix the error on GHC 8.4.1-alpha1.

[asr]

For knowing the warnings in GHC 8.0.2/8.2.2, I realised it's better to use the -Wcompat flag:

$ cabal install --force-reinstalls --ghc-options='-Wcompat -Werror'

[DanBurton]

Bump. ghc-8.4.1 is out now; same issue.

[simonmichael]

Two PRs have been pending for a while: #46, #47. @gregorycollins, any chance you could merge and ship a quick release ?

[mgmeier]

hashtables conforms to the SMP as of release 1.2.3.0, closing this issue now.