Cabal confused with sublibrary when same-named remote package exists

[merijn]

Describe the bug
According to the documentation, the correct way to refer to the (unnamed) main library of a package foo is foo:foo, however specifying bounds on foo:foo when a local library named foo exists, gets interpreted as a bound on the local library. As a result it's impossible to have a package with local library foo:conduit that depends on conduit or conduit:conduit, as any bounds get interpreted as bounds on foo:conduit.

To Reproduce
Steps to reproduce the behavior:

Create a package with a local library whose name matches an existing package, and try to add a bound on the existing package.

$ cabal build ...

Expected behavior
Any bounds on conduit:conduit apply to the conduit package/library, not the local library named conduit.

System information

• cabal version 3.10

Additional context
Add any other context about the problem here.

[phadej]

@merijn which cabal-version you are using. How foo:foo is treated has changed in cabal-version: 3.4 exactly to avoid name shadowing issues.

From https://cabal.readthedocs.io/en/stable/file-format-changelog.html#cabal-version-3-4

Dependencies to sublibraries must be specified explicitly, even for current package. For example: build-depends: mypackage:internal-lib This way you can have an internal library with the same name as some external dependency.

Please provide a reproducer.

[merijn]

Originally this was cabal-version: 3.0 with cabal-install 3.10. I was just trying cabal-version: 3.4, but the linked change requiring that internal packages are fully qualified does not seem to properly work either, i.e. both using conduit or conduit:conduit as dependency still gets interpreted as a dependency on the internal library.

This is rather "in progress" code, so I will try to shrink it to a reproducer.

[merijn]

I created a reproducer here: https://github.com/merijn/cabal-repro

You can try as-is, or trying changing the conduit dependency to conduit:conduit, either will result in the same error:

Error: cabal: The package has an impossible version range for a dependency on
an internal library: conduit ^>=1.3.5. This version range does not include the
current package, and must be removed as the current package's library will
always be used.

If I understand the docs linked by phadej, only repro:conduit should ever refer to the local component in 3.4 and later.

[merijn]

This seems to imply that the 3.4 change never actually worked as intended, behaviour is identical to cabal-version 3.0, only the error has slightly changed.

[Kleidukos]

I can reproduce this on the 3.6 provided by ghcup and 3.11

[phadej]

I cannot reproduce.

[polinukli] /codetmp/cabal-repro master % cabal --version
cabal-install version 3.6.2.0
compiled using version 3.6.2.0 of the Cabal library 
[polinukli] /codetmp/cabal-repro master % cabal build --dry-run
Build profile: -w ghc-8.6.5 -O1
In order, the following would be built (use -v for more details):
 - conduit-1.3.5 (lib) (requires build)
 - repro-0.1.0 (lib) (first run)
 - repro-0.1.0 (lib:conduit) (first run)

cabal-plan prints

% cabal-plan
using '/codetmp/cabal-repro' as project root

Tree
~~~~

repro-0.1.0
 └─ base-4.12.0.0
     ├─ ghc-prim-0.5.3
     │   └─ rts-1.0
     ├─ integer-gmp-1.0.2.0
     │   └─ ghc-prim-0.5.3 ┄┄
     └─ rts-1.0 ┄┄
repro-0.1.0
 [repro-0.1.0:lib:"conduit"]
 ├─ base-4.12.0.0 ┄┄
 └─ conduit-1.3.5

as expected.

Cannot reproduce with cabal-install-3.10.1.0 either:

[polinukli] /codetmp/cabal-repro master % ~/.ghcup/bin/cabal-3.10.1.0 --version      
cabal-install version 3.10.1.0
compiled using version 3.10.1.0 of the Cabal library 
[polinukli] /codetmp/cabal-repro master % ~/.ghcup/bin/cabal-3.10.1.0 build --dry-run
Build profile: -w ghc-8.6.5 -O1
In order, the following would be built (use -v for more details):
 - conduit-1.3.5 (lib) (requires build)
 - repro-0.1.0 (lib) (first run)
 - repro-0.1.0 (lib:conduit) (first run)

[phadej]

I see. When you actually try to build the package, something in Cabal building code fails. I.e. the important bit is

cabal: Failed to build repro-0.1.0. The failure occurred during the configure
step.

cabal-install's solver and Cabal-syntax parser work correctly. It's latter stages which seem to fail, i.e. actually executing the build plan.

---

EDIT: compare with cabal-version: 3.0 behavior:

% cabal build --dry-run 
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] next goal: repro (user goal)
[__0] rejecting: repro-0.1.0 (conflict: repro==0.1.0, repro => repro (lib
conduit)^>=1.3.5)
[__0] fail (backjumping, conflict set: repro)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: repro

There the solving fails as name shadowing occurs.