hpack-convert fails to build with GHC-8.2.1 and Cabal-2.0

[sjakobi]

First some warnings (GHC's memory usage really blows up here):

    [ 7 of 12] Compiling Hpack.Config     ( src/Hpack/Config.hs, .stack-work/dist/x86_64-linux/Cabal-2.0.0.2/build/Hpack/Config.o )
    
    /tmp/stack31554/hpack-convert-1.0.1/src/Hpack/Config.hs:122:19: warning: [-Wsimplifiable-class-constraints]
        • The constraint ‘HasTypeName a’ matches an instance declaration
          instance [safe] (GHC.Generics.Datatype d, Generic a,
                           Rep a ~ GHC.Generics.M1 GHC.Generics.D d m) =>
                          HasTypeName a
            -- Defined at src/Hpack/GenericsUtil.hs:21:10
          This makes type inference for inner bindings fragile;
            either use MonoLocalBinds, or simplify it using the instance
        • In the type signature:
            genericToJSON_ :: forall a.
                              (Generic a, GToJSON Zero (Rep a), HasTypeName a) => a -> Value
        |
    122 | genericToJSON_ :: forall a. (Generic a, GToJSON Zero (Rep a), HasTypeName a) => a -> Value
        |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    
    /tmp/stack31554/hpack-convert-1.0.1/src/Hpack/Config.hs:143:22: warning: [-Wsimplifiable-class-constraints]
        • The constraint ‘HasTypeName a’ matches an instance declaration
          instance [safe] (GHC.Generics.Datatype d, Generic a,
                           Rep a ~ GHC.Generics.M1 GHC.Generics.D d m) =>
                          HasTypeName a
            -- Defined at src/Hpack/GenericsUtil.hs:21:10
          This makes type inference for inner bindings fragile;
            either use MonoLocalBinds, or simplify it using the instance
        • In the type signature:
            genericParseJSON_ :: forall a.
                                 (Generic a, GFromJSON Zero (Rep a), HasTypeName a) =>
                                 Value -> Parser a
        |
    143 | genericParseJSON_ :: forall a. (Generic a, GFromJSON Zero (Rep a), HasTypeName a) => Value -> Parser a
        |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    
    /tmp/stack31554/hpack-convert-1.0.1/src/Hpack/Config.hs:166:11: warning: [-Wsimplifiable-class-constraints]
        • The constraint ‘HasTypeName a’ matches an instance declaration
          instance [safe] (GHC.Generics.Datatype d, Generic a,
                           Rep a ~ GHC.Generics.M1 GHC.Generics.D d m) =>
                          HasTypeName a
            -- Defined at src/Hpack/GenericsUtil.hs:21:10
          This makes type inference for inner bindings fragile;
            either use MonoLocalBinds, or simplify it using the instance
        • When checking the class method:
            fieldNames :: forall a. HasFieldNames a => Proxy a -> [FieldName]
          In the class declaration for ‘HasFieldNames’
        |
    166 |   default fieldNames :: (HasTypeName a, Selectors (Rep a)) => Proxy a -> [String]
        |           ^^^^^^^^^^
    
    /tmp/stack31554/hpack-convert-1.0.1/src/Hpack/Config.hs:503:10: warning: [-Wsimplifiable-class-constraints]
        • The constraint ‘HasTypeName (Section a)’
            matches an instance declaration
          instance [safe] (GHC.Generics.Datatype d, Generic a,
                           Rep a ~ GHC.Generics.M1 GHC.Generics.D d m) =>
                          HasTypeName a
            -- Defined at src/Hpack/GenericsUtil.hs:21:10
          This makes type inference for inner bindings fragile;
            either use MonoLocalBinds, or simplify it using the instance
        • In the context: (Generic (Section a),
                           GToJSON Zero (Rep (Section a)), HasTypeName (Section a), ToJSON a)
          While checking an instance declaration
          In the instance declaration for ‘ToJSON (Section a)’
        |
    503 | instance (Generic (Section a), GToJSON Zero (Rep (Section a)), HasTypeName (Section a),
        |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

Then the compile failure:

    [ 9 of 12] Compiling Hpack.Convert    ( src/Hpack/Convert.hs, .stack-work/dist/x86_64-linux/Cabal-2.0.0.2/build/Hpack/Convert.o )
    
    /tmp/stack31554/hpack-convert-1.0.1/src/Hpack/Convert.hs:52:34: error:
        Not in scope: data constructor ‘Cabal.FlagName’
        Perhaps you meant one of these:
          variable ‘Cabal.flagName’ (imported from Distribution.PackageDescription),
          variable ‘Cabal.mkFlagName’ (imported from Distribution.PackageDescription),
          variable ‘Cabal.unFlagName’ (imported from Distribution.PackageDescription)
        Neither ‘Distribution.InstalledPackageInfo’, ‘Distribution.Package’, ‘Distribution.PackageDescription’, ‘Distribution.PackageDescription.Parse’, ‘Distribution.Text’ nor ‘Distribution.Version’ exports ‘FlagName’.
       |
    52 |                              let Cabal.FlagName fn = flagName
       |                                  ^^^^^^^^^^^^^^
    
    /tmp/stack31554/hpack-convert-1.0.1/src/Hpack/Convert.hs:183:17: error:
        Not in scope: data constructor ‘Cabal.FlagName’
        Perhaps you meant one of these:
          variable ‘Cabal.flagName’ (imported from Distribution.PackageDescription),
          variable ‘Cabal.mkFlagName’ (imported from Distribution.PackageDescription),
          variable ‘Cabal.unFlagName’ (imported from Distribution.PackageDescription)
        Neither ‘Distribution.InstalledPackageInfo’, ‘Distribution.Package’, ‘Distribution.PackageDescription’, ‘Distribution.PackageDescription.Parse’, ‘Distribution.Text’ nor ‘Distribution.Version’ exports ‘FlagName’.
        |
    183 |     Cabal.Flag (Cabal.FlagName fl) -> "flag(" ++ fl ++ ")"
        |                 ^^^^^^^^^^^^^^