changed dependency generation

"ghc -M" for ghc-7.8.x changed in that an error occurs:

 You must specify at least one -dep-suffix 

By chance (#8746 (closed)) I found that

ghc -M -dep-suffix "" B.hs

works, but only for the new ghc-7.8.x. For module B importing module A the following dependencies are generated:

# DO NOT DELETE: Beginning of Haskell dependencies
A.o : A.hs
B.o : B.hs
B.o : A.hi
# DO NOT DELETE: End of Haskell dependencies

The same command used with ghc-7.6.x produces wrong dependencies:

# DO NOT DELETE: Beginning of Haskell dependencies
A.o A._o : A.hs
B.o B._o : B.hs
B.o : A.hi
B._o : A._hi
# DO NOT DELETE: End of Haskell dependencies

Obviously, only for dynamic linking a dep-suffix should be needed. However for ghc-7.6. this must be "dyn" and for ghc-7.8.x this must "dyn_" plus the empty suffix!

ghc-7.6.3 -M -dep-suffix dyn B.hs
ghc-7.8.2 -M -dep-suffix dyn_ -dep-suffix "" B.hs

Both calls produce:

# DO NOT DELETE: Beginning of Haskell dependencies
A.o A.dyn_o : A.hs
B.o B.dyn_o : B.hs
B.o : A.hi
B.dyn_o : A.dyn_hi
# DO NOT DELETE: End of Haskell dependencies

The changed behavior is not documented in http://www.haskell.org/ghc/docs/7.8.2/html/users_guide/separate-compilation.html#makefile-dependencies

The (accidental?) change should also be documented in the release notes!

Trac metadata

Trac field	Value
Version	7.8.2
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
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