Weird import cycle when using plugins

Here's a demo:

$ ghc Plugin.hs
[1 of 1] Compiling Plugin           ( Plugin.hs, Plugin.o )
$ ghc Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...


$ rm Main.hi Main.o
$ ghc Main.hs -fplugin=Plugin
Module imports form a cycle:
  module ‘Plugin’ (./Plugin.hs) imports itself


$ cat Plugin.hs
module Plugin (plugin) where

plugin = undefined

$ cat Main.hs
module Main (main) where

main :: IO ()
main = return ()

Tried with 8.0.1, HEAD.

I don't know why installing plugins using Cabal works but this doesn't. I can't add plugin tests because of this.

I can try fixing this myself if someone points me to the right direction -- I have no idea which part of the compiler could be doing this.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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