compile error with GHC 7.0

[hvr]

See also http://matrix.hackage.haskell.org/package/xmlgen#GHC-7.0/xmlgen-0.6.0.0

/opt/ghc/7.0.4/bin/ghc --make -package-name xmlgen-0.6.0.0 -hide-all-packages -fbuilding-cabal-package -no-user-package-conf -package-conf /tmp/extra14722836196/.cabal-sandbox/x86_64-linux-ghc-7.0.4-packages.conf.d -i -idist/dist-sandbox-bc1ce4eb/build -isrc -idist/dist-sandbox-bc1ce4eb/build/autogen -Idist/dist-sandbox-bc1ce4eb/build/autogen -Idist/dist-sandbox-bc1ce4eb/build -optP-include -optPdist/dist-sandbox-bc1ce4eb/build/autogen/cabal_macros.h -odir dist/dist-sandbox-bc1ce4eb/build -hidir dist/dist-sandbox-bc1ce4eb/build -stubdir dist/dist-sandbox-bc1ce4eb/build -package-id base-4.3.1.0-bafbc7ad22c91044397c91929f8c61bc -package-id blaze-builder-0.3.3.4-63e06b46852eda2af4821b3cb63d4c9e -package-id bytestring-0.9.1.10-6aa1efbfa95d1689fc03d61e7c4b27c4 -package-id containers-0.4.0.0-b4885363abca642443ccd842502a3b7e -package-id mtl-2.1.3.1-54a4612e930eedd80daabaae26eda825 -package-id text-0.11.3.1-caa9c2d4ee65888a50fc1fdb2bf5afaa -O -XHaskell2010 Text.XML.Generator
[1 of 1] Compiling Text.XML.Generator ( src/Text/XML/Generator.hs, dist/dist-sandbox-bc1ce4eb/build/Text/XML/Generator.o )

src/Text/XML/Generator.hs:319:10:
    Illegal instance declaration for `AddChildren TextContent'
      (All instance types must be of the form (T t1 ... tn)
       where T is not a synonym.
       Use -XTypeSynonymInstances if you want to disable this.)
    In the instance declaration for `AddChildren TextContent'

src/Text/XML/Generator.hs:322:10:
    Illegal instance declaration for `AddChildren String'
      (All instance types must be of the form (T t1 ... tn)
       where T is not a synonym.
       Use -XTypeSynonymInstances if you want to disable this.)
    In the instance declaration for `AddChildren String'
xcabal: Error: some packages failed to install:
xmlgen-0.6.0.0 failed during the building phase. The exception was:
ExitFailure 1