`-Wmissing-home-modules` doesn't properly recognize filepath-qualified modules

The -Wmissing-home-modules feature was implemented in order to be used by Cabal (see also #13129 (closed)), however it turns out that it currently has a minor bug which makes it much less useful for cabal as we can't enable it for executables this way, as Cabal needs to provide the path to the Main module directly.

Consider the following two files:

• src-lib/M1.hs

module M1 where

• src-exe/Main.hs

import M1

main = return ()

And now consider the following inconsistent results:

Expected good behaviour:

ghc-8.2.1 -fforce-recomp -Wmissing-home-modules -isrc-lib -isrc-exe Main

<no location info>: warning: [-Wmissing-home-modules]
    Modules are not listed in command line: M1
[1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
[2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
Linking src-exe/Main ...

ghc-8.2.1 -fforce-recomp -Wmissing-home-modules -isrc-lib -isrc-exe M1 Main
[1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
[2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
Linking src-exe/Main ...

Unexpected bad behaviour:

ghc-8.2.1 -fforce-recomp -Wmissing-home-modules src-lib/M1.hs src-exe/Main.hs 

<no location info>: warning: [-Wmissing-home-modules]
    Modules are not listed in command line: M1 Main
[1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
[2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
Linking src-exe/Main ...

ghc-8.2.1 -fforce-recomp -Wmissing-home-modules -isrc-lib M1 src-exe/Main

<no location info>: warning: [-Wmissing-home-modules]
    Modules are not listed in command line: Main
[1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
[2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
Linking src-exe/Main ...

Trac metadata

Trac field	Value
Version	8.2.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	Yuras
Operating system
Architecture

changed milestone to %8.2.1

changed weight to 5

added Tbug Trac import labels

assigned to @trac-Yuras

The problem here: when file is passed to command line, we don't know yet the module name at the time we generate the warning. And we can't use the file name to guess the module name because executable main module filename could be anything. What if we just ignore "Main" module unconditionally, and don't generate the warning for it? Will it work for cabal?

Replying to [ticket:13727#comment:136991 Yuras]:

What if we just ignore "Main" module unconditionally, and don't generate the warning for it? Will it work for cabal?

Yeah, I think that's a good enough hack for now. Can we ignore the Main module only when we compile an executable?

IMO that makes sense, given that the user can also specify -main-is via ghc-options.

I didn't figured out how to add a test case with subdirectory. hvr, do you know how to do it? Could you please add it?

Trac metadata

Trac field	Value
Differential revisions	- → D3598

Replying to [ticket:13727#comment:137001 Yuras]:

I didn't figured out how to add a test case with subdirectory. hvr, do you know how to do it? Could you please add it?

I've found a way (it feels like a minor hack though), I'll soon commandeer phab:D3598 and add what I came up with...

PS: I've updated phab:D3598 with the testcases

mentioned in commit dac49bdc

Merged to ghc-8.2 as 72eade6d.

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added error messages label

added Pnormal label