"Dependency on unbuildable library" with three sublibraries #6470
Description
For a minimal reproduction of the problem, try using this demo.cabal file (no other files required):
cabal-version: 3.0
name: demo
version: 0
common all
default-language: Haskell2010
hs-source-dirs: .
build-depends: base
library
library a
import: all
visibility: public
build-depends: demo:b
library b
import: all
visibility: public
build-depends: demo:c
library c
import: all
visibility: public
Cabal cannot build this project:
$ cabal --version
cabal-install version 3.0.0.0
compiled using version 3.0.0.0 of the Cabal library
$ cabal build
Error:
Dependency on unbuildable library 'b' from demo
In the stanza 'library a'
In the inplace package 'demo-0'
It seems to require three sublibraries to reproduce this issue - it is important both that demo:a depends on demo:b, and that demo:b depends on demo:c. If we remove either of those build-depends lines, the package does build.
The alternative or perhaps further issue here is that the error message does not explain why the demo:b library is unbuildable. But it seems rather suspicious that the presence of demo:a depending on demo:b somehow causes demo:b to become unbuildable.