How does Maven determine if it builds a snapshot version?

I have a project pom that specifies:

    <groupId>net.example.myspace</groupId>
    <artifactId>myproject</artifactId>
    <version>0.9.0-SNAPSHOT</version>
    <packaging>pom</packaging>

and I have specified a repository to deploy to via altSnapshotDeploymentRepository. This used to work in my other projects, but this time, the Maven build fails when trying to deploy the build artifacts, complaining:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-deploy-plugin:2.7:deploy 
(default-deploy) on project myproject: 
Deployment failed: repository element was not specified in the POM inside
distributionManagement element or in -DaltDeploymentRepository=id::layout::url parameter 
-> [Help 1]

Adding altDeploymentRepository helped, but the docs say that snapshots are deployed to altSnapshotDeploymentRepository, while altDeploymentRepository is only used if the artifact is neither recognized as a snapshot nor as a release. So the question is, why doesn't Maven recognize a snapshot build here?

I've always assumed that the -SNAPSHOT suffix is how Maven determines a snapshot version, but it doesn't seem to work this way here? Or perhaps I'm overlooking something?

Solution

The error is due to the version of maven-deploy-plugin used. As per the plugin documentation, <altReleaseDeploymentRepository> and <altSnapshotDeploymentRepository> parameters are present only from version 2.8. As per the error message above, the plugin version is 2.7.

You can upgrade maven-deploy-plugin to fix the issue.