HPC: do notation marks () as non-covered

With this Main.hs, the "()" in "pure ()" is marked by HPC as a non-covered expression:

foo :: IO ()
foo = do
   _ <- putStrLn "Unimportant string"
   pure ()

main :: IO ()
main = do
   foo
   putStrLn "Unimportant #2"

Repro:

• ghc -fhpc Main.hs
• ./Main
• hpc markup Main.tix

This is an admittedly small issue, but it can be valuable for a project to aspire to 100% code coverage and false negatives prevent us from getting there.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Code Coverage
Test case
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