Default types vs deriving

Summary

In !12179 (comment 551772) we added the following code:

default ()

data  Maybe a  =  Nothing | Just a
  deriving ( Eq  -- ^ @since base-2.01

           --, Ord -- ^ @since base-2.01
           )

-- ???
-- A non-standalone instance will slurp the interface file for GHC.Num.Integer.
  -- During simplifyInstanceContexts, a call to GHC.Tc.Utils.Env.tcGetDefaultTys
  -- apparently sees mb_defaults = Nothing and thus tries to bring in the
  -- default "default" types, including Integer.  This seems wrong.
deriving instance Ord a => Ord (Maybe a) -- ^ @since base-2.01

Both standalone and non-standalone instance declarations should "see" the same default types (none here because of the default ()).

Steps to reproduce

Reproduce with:

{-# LANGUAGE NoImplicitPrelude #-}

module M where

import GHC.Classes
default ()

data Foo a = Nothing | Just a
  deriving (Eq, Ord)

-- deriving instance Eq a => Eq (Foo a)
-- deriving instance Ord a => Ord (Foo a) 

Build with:

strace -f -e trace=file ghc M.hs -fforce-recomp -fno-code 2>&1 | grep ghc-bignum

We use -fno-code because the bignum interfaces seem to be loaded during codegen.

Result with standalone deriving:

❯ strace -f -e trace=file ghc-9.8 M.hs -fforce-recomp -fno-code 2>&1 | grep ghc-bignum
openat(AT_FDCWD, "/home/hsyl20/.ghcup/ghc/9.8.1-alpha1/lib/ghc-9.8.0.20230727/bin/../lib/x86_64-linux-ghc-9.8.0.20230727/libHSghc-bignum-1.3-0df6-ghc9.8.0.20230727.so", O_RDONLY|O_CLOEXEC) = 3

Result with non-standalone deriving:

❯ strace -f -e trace=file ghc-9.8 M.hs -fforce-recomp -fno-code 2>&1 | grep ghc-bignum
openat(AT_FDCWD, "/home/hsyl20/.ghcup/ghc/9.8.1-alpha1/lib/ghc-9.8.0.20230727/bin/../lib/x86_64-linux-ghc-9.8.0.20230727/libHSghc-bignum-1.3-0df6-ghc9.8.0.20230727.so", O_RDONLY|O_CLOEXEC) = 3
[pid 172062] stat("/home/hsyl20/.ghcup/ghc/9.8.1-alpha1/lib/ghc-9.8.0.20230727/lib/../lib/x86_64-linux-ghc-9.8.0.20230727/ghc-bignum-1.3-0df6/GHC/Num/Integer.hi", {st_mode=S_IFREG|0644, st_size=47534, ...}) = 0
[pid 172062] openat(AT_FDCWD, "/home/hsyl20/.ghcup/ghc/9.8.1-alpha1/lib/ghc-9.8.0.20230727/lib/../lib/x86_64-linux-ghc-9.8.0.20230727/ghc-bignum-1.3-0df6/GHC/Num/Integer.hi", O_RDONLY|O_NOCTTY|O_NONBLOCK) = 13

Expected behavior

Bignum interfaces shouldn't be loaded with non-standalone deriving.

Environment

• GHC version used: 9.6, 9.8