Summary

After a clean install, launching the ghc interpret:

C:\WINDOWS\system32>ghci

GHCi, version 8.6.5: http://www.haskell.org/ghc/  :? for help
ghc.exe: Could not load `libwinpthread-1.dll'. Reason: addDLL: libwinpthread-1.dll or dependencies not loaded. (Win32 error 1392)

ghc.exe: panic! (the 'impossible' happened)
  (GHC version 8.6.5 for x86_64-unknown-mingw32):
        loadArchive "C:\\Program Files\\Haskell Platform\\8.6.5\\mingw\\x86_64-w64-mingw32\\lib\\libpthread.dll.a": failed

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

The archive is there.

Environment

• GHC version used: 8.6.5

Optional:

• Operating System: Windows 10 1903
• System Architecture: 64bit