cabal test fails (but cabal run doctests works)

[martijnbastiaan]

Workaround

cabal build
cabal test

or

cabal run doctests

Issue

Rather annoyingly, if doctests is part of a testsuite it (sometimes) fails to properly run:

$ cabal test
Build profile: -w ghc-8.10.7 -O1
In order, the following will be built (use -v for more details):
 - example-0.0.0 (test:doctests) (ephemeral targets)
Preprocessing test suite 'doctests' for example-0.0.0..
Building test suite 'doctests' for example-0.0.0..
Running 1 test suites...
Test suite doctests: RUNNING...
Could not import module: Example. This can be caused by a number of issues: 

 1. A module found by GHC contained tests, but was not in 'exposed-modules'.

 2. For Cabal users: Cabal did not generate a GHC environment file. Either:
   * Run with '--write-ghc-environment-files=always'
   * Add 'write-ghc-environment-files: always' to your cabal.project

 3. The testsuite executable does not have a dependency on your project library. Please add it to the 'build-depends' section of the testsuite executable.

See the example project at https://github.com/martijnbastiaan/doctest-parallel/blob/main/example/README.md for more information.
Examples: 2  Tried: 0  Errors: 1  Unexpected output: 0

Test suite doctests: FAIL
Test suite logged to:
/home/martijn/code/doctest-parallel/example/dist-newstyle/build/x86_64-linux/ghc-8.10.7/example-0.0.0/t/doctests/test/example-0.0.0-doctests.log
0 of 1 test suites (0 of 1 test cases) passed.
cabal: Tests failed for test:doctests from example-0.0.0.

Running it with run works just fine though:

$ cabal run doctests
Build profile: -w ghc-8.10.7 -O1
In order, the following will be built (use -v for more details):
 - example-0.0.0 (test:doctests) (additional components to build)
Preprocessing test suite 'doctests' for example-0.0.0..
Building test suite 'doctests' for example-0.0.0..
Examples: 2  Tried: 2  Errors: 0  Unexpected output: 0

This seems to be the same issue observed by #9. My guess is that environment files get written out "too late". My evidence is that this works:

cabal test # fails
cabal test # works

or:

cabal run doctests # works
cabal test # now also works

or:

cabal build
cabal test

[martijnbastiaan]

@lehins This is probably what you saw in #9

[martijnbastiaan]

Reported as bug: haskell/cabal#7889