Don't cast public interface MutableVersionConstraint to internal internal implementations (DefaultMutableVersionConstraint).

[yogurtearl]

I want to implement a custom implementation of the VersionCatalog interface, I may also need to also implement MinimalExternalModuleDependency and VersionConstraint

But this causes error like this:

Caused by: java.lang.ClassCastException: class com.example.MyCustomVersionConstraint cannot be cast to class org.gradle.api.internal.artifacts.dependencies.DefaultMutableVersionConstraint (com.example.MyCustomVersionConstraint is in unnamed module of loader org.gradle.internal.classloader.VisitableURLClassLoader @41a95ba7; org.gradle.api.internal.artifacts.dependencies.DefaultMutableVersionConstraint is in unnamed module of loader org.gradle.initialization.MixInLegacyTypesClassLoader @1f760b47)
        at org.gradle.api.internal.artifacts.dependencies.AbstractExternalModuleDependency.<init>(AbstractExternalModuleDependency.java:54)
        at org.gradle.api.internal.artifacts.dependencies.DefaultExternalModuleDependency.<init>(DefaultExternalModuleDependency.java:35)
        at org.gradle.api.internal.artifacts.dependencies.DefaultExternalModuleDependency_Decorated.<init>(Unknown Source)
        at jdk.internal.reflect.GeneratedConstructorAccessor79.newInstance(Unknown Source)
        at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
        at org.gradle.internal.instantiation.generator.AsmBackedClassGenerator$InvokeConstructorStrategy.newInstance(AsmBackedClassGenerator.java:1913)
        at org.gradle.internal.instantiation.generator.AbstractClassGenerator$GeneratedClassImpl$GeneratedConstructorImpl.newInstance(AbstractClassGenerator.java:506)
        at org.gradle.internal.instantiation.generator.DependencyInjectingInstantiator.doCreate(DependencyInjectingInstantiator.java:64)
        ... 214 more

This is because gradle is casting the public interface (MutableVersionConstraint) to an internal implementation (org.gradle.api.internal.artifacts.dependencies.DefaultMutableVersionConstraint) here:

gradle/subprojects/dependency-management/src/main/java/org/gradle/api/internal/artifacts/dependencies/AbstractExternalModuleDependency.java

Line 54 in c267a7d

this.versionConstraint = (DefaultMutableVersionConstraint) version;

It defeats the purpose of the interface if you assume that it is always an instance of DefaultMutableVersionConstraint (internal API)

If you change that line to:

        this.versionConstraint = new DefaultMutableVersionConstraint(version);

that should fix it.

[jbartok]

Could you please explain to us what your use case is?
MutableVersionConstraint is annotated with @HasInternalProtocol, which means it's not something designed to be implemented by build authors.

[yogurtearl]

@HasInternalProtocol is not visible in the javadocs:
https://docs.gradle.org/7.6-rc-3/javadoc/org/gradle/api/artifacts/MutableVersionConstraint.html

For doing a custom implementation of VersionCatalog, you need to be able to implement this:

Optional<Provider<MinimalExternalModuleDependency>> findLibrary(String alias);

MinimalExternalModuleDependency needs an impl of:

VersionConstraint getVersionConstraint();

If you try to just implement VersionConstraint you hit issue #22718 , which means you need to implement MutableVersionConstraint instead.

If #22718 is fixed, this issue becomes less relevant.

[ljacomet]

Can you explain why you are implementing a custom version catalog?
Is it a missing feature? A migration from an internal alternative? Something else?

[yogurtearl]

Mostly working around issues like these:

• VersionCatalogExtension not added to the gradle-kotlin-dsl-accessors during generatePrecompiledScriptPluginAccessors #22468
• Make generated type-safe version catalogs accessors accessible from precompiled script plugins #15383
• buildSrc/test loads LibrariesForLibs twice from 2 different classloaders. #21637

But implementing public interfaces is also useful for making Fake objects for use in tests.

You can avoid these types of issues if you wrap the interfaces with internal impls instead of casting them to internal impls, to avoid these type of issues.

As noted above, wrapping instead of casting solves this issue:

this.versionConstraint = new DefaultMutableVersionConstraint(version);