Add explanation to checkCaseClassInheritanceInvariant error msg

Closes #18552 which was actually not an error, see:
https://github.com/scala/scala3/blob/73882c5b62b8cd96031ad975f7677949433a9f21/compiler/src/dotty/tools/dotc/typer/RefChecks.scala#L889-L893

Co-authored-by: Anna Herlihy <herlihyap@gmail.com>
Co-authored-by: Natsu Kagami <natsukagami@gmail.com>

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -891,11 +891,14 @@ object RefChecks {
        *  can assume invariant refinement for case classes in `constrainPatternType`.
        */
       def checkCaseClassInheritanceInvariant() =
-        for (caseCls <- clazz.info.baseClasses.tail.find(_.is(Case)))
-          for (baseCls <- caseCls.info.baseClasses.tail)
-            if (baseCls.typeParams.exists(_.paramVarianceSign != 0))
-              for (problem <- variantInheritanceProblems(baseCls, caseCls, "non-variant", "case "))
-                report.errorOrMigrationWarning(problem, clazz.srcPos, MigrationVersion.Scala2to3)
+        for
+          caseCls <- clazz.info.baseClasses.tail.find(_.is(Case))
+          baseCls <- caseCls.info.baseClasses.tail
+          if baseCls.typeParams.exists(_.paramVarianceSign != 0)
+          problem <- variantInheritanceProblems(baseCls, caseCls, i"base $baseCls", "case ")
+          withExplain = problem.appendExplanation:
+            "Refining a basetype of a case class is not allowed, enabling better GADT constraints in case class patterns."
+        do report.errorOrMigrationWarning(withExplain, clazz.srcPos, MigrationVersion.Scala2to3)
       checkNoAbstractMembers()
       if (abstractErrors.isEmpty)
         checkNoAbstractDecls(clazz)
@@ -924,7 +927,7 @@ object RefChecks {
         for {
           cls <- clazz.info.baseClasses.tail
           if cls.paramAccessors.nonEmpty && !mixins.contains(cls)
-          problem <- variantInheritanceProblems(cls, clazz.asClass.superClass, "parameterized", "super")
+          problem <- variantInheritanceProblems(cls, clazz.asClass.superClass, i"parameterized base $cls", "super")
         }
         report.error(problem, clazz.srcPos)
       }
@@ -947,7 +950,7 @@ object RefChecks {
       if (combinedBT =:= thisBT) None // ok
       else
         Some(
-          em"""illegal inheritance: $clazz inherits conflicting instances of $baseStr base $baseCls.
+          em"""illegal inheritance: $clazz inherits conflicting instances of $baseStr.
               |
               |  Direct basetype: $thisBT
               |  Basetype via $middleStr$middle: $combinedBT""")

tests/neg/i18552.check

@@ -0,0 +1,12 @@
+-- Error: tests/neg/i18552.scala:9:6 -----------------------------------------------------------------------------------
+9 |class MB(id:Int) extends MA(id) with M[B] // error
+  |      ^
+  |      illegal inheritance: class MB inherits conflicting instances of base trait M.
+  |
+  |        Direct basetype: M[B]
+  |        Basetype via case class MA: M[A]
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | Refining a basetype of a case class is not allowed, enabling better GADT constraints in case class patterns.
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg/i18552.scala

@@ -0,0 +1,9 @@
+//> using options -explain
+
+trait A
+trait B extends A
+
+trait M[+T]
+
+case class MA(id:Int) extends M[A]
+class MB(id:Int) extends MA(id) with M[B] // error