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There seems to be an issue with the ANN pragma, with fatal compilation errors.

To reproduce, setup two modules as such:

module Test where

import Weights (Weight(..))

{-# ANN mainTest (Weight 2) #-}
mainTest :: IO ()
mainTest = return ()

{-# LANGUAGE DeriveDataTypeable #-}
module Weights (Weight(..)) where

import Data.Data (Data(..))

newtype Weight = Weight Integer deriving Data

Compiling with GHC version 8.2.2 yields:

λ ghc Test.hs
[1 of 2] Compiling Weights          ( Weights.hs, Weights.o )
[2 of 2] Compiling Test             ( Test.hs, Test.o )
Test.hs:5:1: fatal:
    cannot find object file ‘./Weights.dyn_o’
    while linking an interpreted expression

The issue disappears with -dynamic or -dynamic-too:

λ ghc -dynamic Test.hs
[1 of 2] Compiling Weights          ( Weights.hs, Weights.o )
[2 of 2] Compiling Test             ( Test.hs, Test.o )

Issue persist in HEAD, GHC version 8.5.20180219, but again disappears with -dynamic-too or -dynamic:

λ ../../ghc/inplace/bin/ghc-stage2 -dynamic-too Test.hs
[1 of 2] Compiling Weights          ( Weights.hs, Weights.o )
[2 of 2] Compiling Test             ( Test.hs, Test.o )

Remove the ANN pragma, and the compiler behaves just fine with both GHC versions. Please tell me if I can provide more information. Cheers.