Add integration test for #4105

test/integration/tests/4105-test-coverage-of-internal-lib/Main.hs

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+import Control.Monad (unless)
+import Data.List (isInfixOf, isPrefixOf)
+import StackTest
+
+main :: IO ()
+main = do
+  stack ["clean"]
+  stack ["build"]
+  res <- getCoverageLines . snd <$> stackStderr ["test", "--coverage"]
+  case res of
+    _:exprs:_ -> unless ("2/2" `isInfixOf` exprs) testFail
+    _ -> testFail
+  where
+    testFail = fail "Stack didn't generate coverage from both libraries"
+
+getCoverageLines :: String -> [String]
+getCoverageLines = dropWhile (not . isCoverageHeader) . lines
+  where
+    isCoverageHeader = isPrefixOf "Generating coverage report for "

test/integration/tests/4105-test-coverage-of-internal-lib/files/files.cabal

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+name:           files
+version:        0.1.0.0
+build-type:     Simple
+cabal-version:  >= 2.0
+
+library
+  exposed-modules: Src
+  hs-source-dirs: src
+  build-depends: base
+  default-language: Haskell2010
+
+library sublib
+  exposed-modules: B
+  hs-source-dirs: src-sublib
+  build-depends: base
+  default-language: Haskell2010
+
+test-suite test
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs: test
+  build-depends: base, files, sublib
+  default-language: Haskell2010

test/integration/tests/4105-test-coverage-of-internal-lib/files/src-sublib/B.hs

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+module B where
+
+-- | A function of the internal library
+funInternal :: Int -> Int
+funInternal = pred

test/integration/tests/4105-test-coverage-of-internal-lib/files/src/Src.hs

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+module Src where
+
+-- | A function of the main library
+funMainLib :: Int -> Int
+funMainLib = succ

test/integration/tests/4105-test-coverage-of-internal-lib/files/stack.yaml

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+resolver: lts-11.11

test/integration/tests/4105-test-coverage-of-internal-lib/files/test/Main.hs

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+import Control.Monad (when)
+
+import Src
+import B
+
+main = when (funMainLib 41 /= funInternal 43) $ error "test failed"