Improve the error reporting

versionCheck checks that the declared version number (e.g. 1.2.3) conforms to the declared compatibility guarantees (e.g. Compatibility.BinaryAndSourceCompatible).

Previously, when versionCheck failed, it only reported that the declared version was invalid. Now, it shows both the declared version and the compatibility intention. It also mentions the possibility to restrict the compatibility guarantees to fix the failure.

sbt-version-policy/src/main/scala/sbtversionpolicy/Compatibility.scala

@@ -64,8 +64,14 @@ object Compatibility {
    * Validates that the given new `version` matches the claimed `compatibility` level.
    * @return Some validation error, or None if the version is valid.
    */
-  def isValidVersion(compatibility: Compatibility, version: String): Boolean = {
-    val versionNumber = VersionNumber(version)
+  def isValidVersion(compatibility: Compatibility, version: String): Boolean =
+    isValidVersion(compatibility, VersionNumber(version))
+
+  /**
+   * Validates that the given new `version` matches the claimed `compatibility` level.
+   * @return Some validation error, or None if the version is valid.
+   */
+  private[sbtversionpolicy] def isValidVersion(compatibility: Compatibility, versionNumber: VersionNumber): Boolean = {
     val major = versionNumber._1
     val minor = versionNumber._2
     val patch = versionNumber._3

sbt-version-policy/src/main/scala/sbtversionpolicy/SbtVersionPolicySettings.scala

@@ -218,16 +218,24 @@ object SbtVersionPolicySettings {
         versionPolicyIntention.?.value
           .getOrElse(throw new MessageOnlyException("Please set the key versionPolicyIntention to declare the compatibility guarantees of this release"))
       val versionValue = version.value
+      val versionNumber = VersionNumber(versionValue)
       val s = streams.value
       val moduleName = projectID.value.name
 
-      if (Compatibility.isValidVersion(intention, versionValue)) {
+      if (Compatibility.isValidVersion(intention, versionNumber)) {
         s.log.info(s"Module ${moduleName} has a valid version number: $versionValue (versionPolicyIntention is set to 'Compatibility.${intention}')")
       } else {
         val detail =
-          if (intention == Compatibility.None) "You must increment the major version number (or the minor version number, if major version is 0) to publish a binary incompatible release."
-          else "You must increment the minor version number to publish a source incompatible release."
-        throw new MessageOnlyException(s"Module ${moduleName} has an invalid version number: $versionValue. $detail")
+          if (intention == Compatibility.None) {
+            val matchingIntention =
+              if (versionNumber._3.contains(0)) Compatibility.BinaryCompatible else Compatibility.BinaryAndSourceCompatible
+            "If you want to publish a binary incompatible release, you must increment the major version number (or the minor version number, if the major version is 0). " +
+            s"Otherwise, to release version ${versionValue}, you need to restrict the versionPolicyIntention to ${matchingIntention} and run versionPolicyCheck again."
+          } else {
+            "If you want to publish a source incompatible release, you must increment the minor version number. " +
+            s"Otherwise, to release version ${versionValue}, you need to restrict the versionPolicyIntention to ${Compatibility.BinaryAndSourceCompatible} and run versionPolicyCheck again."
+          }
+        throw new MessageOnlyException(s"Module ${moduleName} has a declared version number ${versionValue} that does not conform to its declared versionPolicyIntention of ${intention}. $detail")
       }
     }).value,
     versionPolicyCheck := Def.ifS((versionPolicyCheck / skip).toTask)(Def.task {