Passing hs-boot files as ghc arguments rarely works

Given a setup with A.hs-boot, A.hs and B.hs which contains import {-# source #-} A the current behaviour is:

In make mode:

ghc A.hs-boot produces A.hi-boot and A.o (instead of A.o-boot)

ghc A.hs-boot B.hs compiles as A.hs-boot as above and then throws

attempting to use module ‘main:A’ (A.hs-boot-boot) which is not loaded

ghc A.hs-boot B.hs A.hs throws

module ‘main:A’ is defined in multiple files: A.hs-boot A.hs

ghc B.hs A.hs works fine

In one-shot mode:

ghc -c A.hs-boot works: it produces A.hi-boot and A.o-boot

ghc -c A.hs-boot B.hs throws

Bad interface file: A.hi-boot-boot
        A.hi-boot-boot: withBinaryFile: does not exist (No such file or directory)

ghc -c A.hs-boot B.hs A.hs throws the same

ghc -c B.hs A.hs throws

 Could not find module ‘A’

So in make mode passing A.hs-boot never works. In one-shot mode passing A.hs-boot is required to get everything to compile, but you cannot also pass A.hs or another file that source-imports A. It would be nice if it all just works, especially without mentioning hs-boot-boot in messages!

@torsten.schmits diagnosed the issue as follows:

In several locations, modules are added to the Finder without considering whether they are boot interfaces for all involved components.
This results in corrupt ModLocation values that have boot suffixes only on some paths, or a .hs-boot-boot suffix on some.
Additionally, it depends on the order of targets whether a lookup will return the ModLocation for the boot file or the source file.

and proposed this fix: !13237 (closed)