doc/bugs/fingertree___62____61___0.1.2_causes_build_to_fail_on_reducers.mdwn

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+### Please describe the problem.
+git-annex's dependencies fail to install unless fingertree is constrained to < 0.1.2.0
+
+Note this has already been fixed upstream in reducers https://github.com/ekmett/reducers/commit/f18a111d66c343bd472f914baaa948191f8ecf55
+
+However, there hasn't been a new release yet of reducers so it still affects git-annex currently.
+
+### What steps will reproduce the problem?
+cabal install --jobs=8 --max-backjumps=100000 --only-dependencies --flags=s3\ webapp
+
+
+### What version of git-annex are you using? On what operating system?
+6.20171003 on macOS
+
+### Please provide any additional information below.
+
+The error is
+[[!format sh """
+Failed to install reducers-3.12.1
+Build log ( /private/tmp/git-annex-20171003-61223-1v5jyd4/git-annex-6.20171003/.cabal-sandbox/logs/ghc-8.2.1/reducers-3.12.1-5Of5cjMdWsgHjbctJQaaa4.log ):
+cabal: Entering directory '/tmp/cabal-tmp-62861/reducers-3.12.1'
+Configuring reducers-3.12.1...
+clang: warning: -Wl,-headerpad_max_install_names: 'linker' input unused
+clang: warning: argument unused during compilation: '-L/usr/local/opt/gettext/lib'
+clang: warning: argument unused during compilation: '-L/usr/local/opt/libffi/lib'
+clang: warning: argument unused during compilation: '-L/usr/local/opt/icu4c/lib'
+clang: warning: argument unused during compilation: '-L/usr/local/lib'
+clang: warning: argument unused during compilation: '-L/System/Library/Frameworks/OpenGL.framework/Versions/Current/Libraries'
+Preprocessing library for reducers-3.12.1..
+Building library for reducers-3.12.1..
+[ 1 of 14] Compiling Data.Semigroup.Instances ( src/Data/Semigroup/Instances.hs, dist/dist-sandbox-c97b5ef4/build/Data/Semigroup/Instances.o )
+
+src/Data/Semigroup/Instances.hs:11:10: error:
+    Duplicate instance declarations:
+      instance Measured v a => Semigroup (FingerTree v a)
+        -- Defined at src/Data/Semigroup/Instances.hs:11:10
+      instance [safe] Measured v a => Semigroup (FingerTree v a)
+        -- Defined in ‘Data.FingerTree’
+   |
+11 | instance Measured v a => Semigroup (FingerTree v a) where
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+cabal: Leaving directory '/tmp/cabal-tmp-62861/reducers-3.12.1'
+"""]]
+
+Full log is here: https://gist.github.com/ilovezfs/544d546785addbe2c5ef41656fa7eda0
+
+### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
+
+Yes :)