GHC dynamic bug maybe

run

# Compile once
$ ghc -c Test.hs

# We get a .o out
$ ls -l
total 24
-rw-r--r--  1 amos  staff   757 21 Dec 13:22 Test.hi
-rw-r--r--  1 amos  staff    42 21 Dec 13:17 Test.hs
-rw-r--r--  1 amos  staff  1708 21 Dec 13:22 Test.o

# Compile again with dynamic-too doesn't rerun
$ ghc -c Test.hs -dynamic-too
compilation IS NOT required

# But just touches .dyn_o, creating an empty file
$ ls -l
total 24
-rw-r--r--  1 amos  staff     0 21 Dec 13:23 Test.dyn_o
-rw-r--r--  1 amos  staff   757 21 Dec 13:22 Test.hi
-rw-r--r--  1 amos  staff    42 21 Dec 13:17 Test.hs
-rw-r--r--  1 amos  staff  1708 21 Dec 13:23 Test.o

Test.hs

module Test where
foo :: Int
foo = 1 + 2

same deal with a head from a few months ago

amos@Amos test-rerun $ ../ghc/inplace/bin/ghc-stage2 -c Test.hs
amos@Amos test-rerun $ ls -l
total 24
-rw-r--r--  1 amos  staff   822 21 Dec 13:26 Test.hi
-rw-r--r--  1 amos  staff    42 21 Dec 13:17 Test.hs
-rw-r--r--  1 amos  staff  1760 21 Dec 13:26 Test.o
amos@Amos test-rerun $ ../ghc/inplace/bin/ghc-stage2 -c Test.hs -dynamic-too
compilation IS NOT required
amos@Amos test-rerun $ ls -l
total 24
-rw-r--r--  1 amos  staff     0 21 Dec 13:27 Test.dyn_o
-rw-r--r--  1 amos  staff   822 21 Dec 13:26 Test.hi
-rw-r--r--  1 amos  staff    42 21 Dec 13:17 Test.hs
-rw-r--r--  1 amos  staff  1760 21 Dec 13:27 Test.o

https://ghc.haskell.org/trac/ghc/ticket/9121