Add explanation to checkCaseClassInheritanceInvariant error msg (#20141)

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 <[email protected]> Co-authored-by: Natsu Kagami <[email protected]> [Cherry-picked ed9fecc][modified]
scala · Jul 5, 2024 · 68f31cf · 68f31cf
1 parent ff1bc12
commit 68f31cf
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Showing 3 changed files with 36 additions and 9 deletions.
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -811,11 +811,16 @@ 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, from = `3.0`)
+        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.
+              |This is a limitation that enables better GADT constraints in case class patterns""".stripMargin
+        do report.errorOrMigrationWarning(withExplain, clazz.srcPos, from = `3.0`)
+
       checkNoAbstractMembers()
       if (abstractErrors.isEmpty)
         checkNoAbstractDecls(clazz)
@@ -844,7 +849,7 @@ 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)
       }
@@ -867,7 +872,7 @@ 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""")
@@ -966,9 +971,9 @@ object RefChecks {
   end checkNoPrivateOverrides
 
   def checkVolatile(sym: Symbol)(using Context): Unit =
-    if sym.isVolatile && !sym.is(Mutable) then 
+    if sym.isVolatile && !sym.is(Mutable) then
       report.warning(VolatileOnVal(), sym.srcPos)
-  
+
   /** Check that unary method definition do not receive parameters.
    *  They can only receive inferred parameters such as type parameters and implicit parameters.
    */

diff --git a/tests/neg/i18552.check b/tests/neg/i18552.check
@@ -0,0 +1,13 @@
+-- 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.
+  | This is a limitation that enables better GADT constraints in case class patterns
+   ---------------------------------------------------------------------------------------------------------------------
diff --git a/tests/neg/i18552.scala b/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