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This is a valid issue as the subversion plugin does not allow to configure its setting with Jcasc

the origin issue description is completely different, so now it is a new issue, I've unassigned me because I am no longer the maintainer of the plugin, and keep the issue open in case someone wants to take it.

Jenkins startup log warnings from JCasC:

WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class jenkins.scm.impl.subversion.SubversionSCMSource#owner: type is abstract but not Describable.

=> Extracted/moved the problem from (very generic issue) https://github.com/jenkinsci/configuration-as-code-plugin/issues/536

After adding some more JCasC configurations (in comparison to before/above), I am now getting (only) the following warning during Jenkins startup (twice):

WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class jenkins.scm.impl.subversion.SubversionSCMSource#owner: type is abstract but not Describable.

I am pretty sure the JCasC context is the configuration of a "global library": this is the interesting parts of the "jenkins.yaml" file:

jenkins:
  ...

  securityRealm:
    ldap:
      ...

credentials:
  system:
    domainCredentials:
    - credentials:
      ...
      - usernamePassword:
          scope: GLOBAL
          id: jenkins-ldap
          username: ***
          password: ***
          description: "Jenkins LDAP user and password credentials"
      ...

unclassified:
  ...
  globalLibraries:
    libraries:
    - name: "acme-shared-library"
      defaultVersion: "trunk"
      allowVersionOverride: true
      includeInChangesets: false
      retriever:
        modernSCM:
          scm:
            subversion:
              remoteBase: "https://<SVN PATH>/"
              credentialsId: jenkins-ldap
  ...

These are all Jenkins logs relevant for this warning or the "global library" configuration (please mind but ignore the duplicate logging or are these maybe even temporary duplicate configuration object instantiations: this is another issue => #899):

...
2019-06-11 10:06:43.572+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class org.jenkinsci.plugins.workflow.libs.LibraryConfiguration.name = acme-shared-library
2019-06-11 10:06:43.576+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class jenkins.scm.impl.subversion.SubversionSCMSource.remoteBase = https://<SVN PATH>/
2019-06-11 10:06:43.577+0000 [id=35]    WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class jenkins.scm.impl.subversion.SubversionSCMSource#owner: type is abstract but not Describable.
2019-06-11 10:06:43.578+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting jenkins.scm.impl.subversion.SubversionSCMSource{id='4b43e974-7f07-4a35-9646-da9361bd0127'}.credentialsId = jenkins-ldap
2019-06-11 10:06:43.578+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class org.jenkinsci.plugins.workflow.libs.SCMSourceRetriever.scm = {subversion={}}
2019-06-11 10:06:43.579+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class org.jenkinsci.plugins.workflow.libs.LibraryConfiguration.retriever = {modernSCM={}}
2019-06-11 10:06:43.579+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting org.jenkinsci.plugins.workflow.libs.LibraryConfiguration@40045ff7.allowVersionOverride = true
2019-06-11 10:06:43.580+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting org.jenkinsci.plugins.workflow.libs.LibraryConfiguration@40045ff7.defaultVersion = trunk
2019-06-11 10:06:43.580+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting org.jenkinsci.plugins.workflow.libs.LibraryConfiguration@40045ff7.includeInChangesets = false
...
2019-06-11 10:06:45.472+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class org.jenkinsci.plugins.workflow.libs.LibraryConfiguration.name = acme-shared-library
2019-06-11 10:06:45.474+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class jenkins.scm.impl.subversion.SubversionSCMSource.remoteBase = https://<SVN PATH>/
2019-06-11 10:06:45.474+0000 [id=35]    WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class jenkins.scm.impl.subversion.SubversionSCMSource#owner: type is abstract but not Describable.
2019-06-11 10:06:45.475+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting jenkins.scm.impl.subversion.SubversionSCMSource{id='32242226-c66e-4032-9067-6e5bd9ee2db9'}.credentialsId = jenkins-ldap
2019-06-11 10:06:45.475+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class org.jenkinsci.plugins.workflow.libs.SCMSourceRetriever.scm = {subversion={}}
2019-06-11 10:06:45.475+0000 [id=35]    INFO    i.j.p.c.i.c.DataBoundConfigurator#tryConstructor: Setting class org.jenkinsci.plugins.workflow.libs.LibraryConfiguration.retriever = {modernSCM={}}
2019-06-11 10:06:45.476+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting org.jenkinsci.plugins.workflow.libs.LibraryConfiguration@1a7cac19.allowVersionOverride = true
2019-06-11 10:06:45.476+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting org.jenkinsci.plugins.workflow.libs.LibraryConfiguration@1a7cac19.defaultVersion = trunk
2019-06-11 10:06:45.476+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting org.jenkinsci.plugins.workflow.libs.LibraryConfiguration@1a7cac19.includeInChangesets = false
2019-06-11 10:06:45.477+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting org.jenkinsci.plugins.workflow.libs.GlobalLibraries@1a129020.libraries = [org.jenkinsci.plugins.workflow.libs.LibraryConfiguration@1a7cac19]
2019-06-11 10:06:45.506+0000 [id=35]    INFO    i.jenkins.plugins.casc.Attribute#setValue: Setting jenkins.model.GlobalConfigurationCategory$Unclassified@42736d5a.globalLibraries = org.jenkinsci.plugins.workflow.libs.GlobalLibraries@1a129020
...