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

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 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

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ezyang, nh2
Operating system
Architecture