[GHC] #15425: GHC does not warn when two conflicting packages with same name are given via a dependent package and -package-db

[GHC]

#15425: GHC does not warn when two conflicting packages with same name are given
via a dependent package and -package-db
-------------------------------------+-------------------------------------
           Reporter:  nh2            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Build System   |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 So I'm compiling a Setup.hs of `mypackage`, directly with `ghc` and
 without use of `Cabal`.

 It has a custom build type and imports `Distribution.Extra.Doctest` from
 `cabal-doctest`.

 For compiling `mypackage`'s Setup.hs I give a `-package-db` that contains
 a custom `Cabal` package.

 But when I compiled `doctest`, I did not give that custom `Cabal` package.

 I compile with

 {{{
 ghc -package-db=/tmp/nix-build-hsyslog-5.0.1.drv-0/setup-package.conf.d
 -package Cabal -j4 -threaded --make -o Setup -odir $TMPDIR -hidir $TMPDIR
 $i -fforce-recomp -keep-tmp-files -v5
 }}}

 The given package db looks like this:

 {{{
 % ls -lah /tmp/nix-build-hsyslog-5.0.1.drv-0/setup-package.conf.d
 total 56K
 drwxr-xr-x  2 niklas niklas 4.0K Jul 20 03:14 ./
 drwxr-xr-x 12 niklas niklas 4.0K Jul 20 03:49 ../
 -r--r--r--  1 niklas niklas  11K Jul 20 02:30
 Cabal-2.2.0.1-302vnEvwKgd3FEKLIV6HbI.conf
 -r--r--r--  1 niklas niklas 2.3K Jul 20 02:30 cabal-
 doctest-1.0.6-E57gwmEN5fa2uY0bq8bb44.conf
 -rw-rw-r--  1 niklas niklas  31K Jul 20 03:14 package.cache
 -rw-r--r--  1 niklas niklas    0 Jul 20 02:30 package.cache.lock
 }}}

 Full [https://pastebin.com/raw/HpVLbe4A ghc-pkg dump -f /tmp/nix-build-
 hsyslog-5.0.1.drv-0/setup-package.conf.d output]

 This results in the linker flags:

 {{{
 -L/nix/store/hfi080vzqqzfip6bd6x4cxc2jgj56xn3-ghc-8.4.3/lib/ghc-8.4.3/Cabal-2.2.0.1
 -L/nix/store/6x9r3kqn8h961ilaw2nj474jrfyg28lp-
 Cabal-2.2.0.1/ghc-8.4.3/Cabal-2.2.0.1
 ...
 -lHSCabal-2.2.0.1
 -lHSCabal-2.2.0.1-302vnEvwKgd3FEKLIV6HbI
 }}}

 When this happens, GHC will always prefer the `Cabal` version from the
 global package db, and not the one I have given with `-package`.

 This results in my compiled `Setup` file to not use the Cabal version I
 desire.

 I would expect GHC to warn me when two Cabal versions are at play, or
 entirely refuse to compile, instead of picking the one that I have NOT
 explicitly specified.

 See full `-v5` ghc output here: https://pastebin.com/raw/jGbLPjxF

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15425>
GHC <http://www.haskell.org/ghc/>
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