Better diagnostics for clashing classes

We do not allow same-named class members in supertraits
of a mixin composition anymore. This commit gives a
better error message and avoids a crash in RefChecks.

4 files changed, 18 insertions, 7 deletions

diff --git a/src/dotty/tools/dotc/core/Denotations.scala b/src/dotty/tools/dotc/core/Denotations.scala
index b6519e1cd..fcd60ef16 100644
--- a/src/dotty/tools/dotc/core/Denotations.scala
+++ b/src/dotty/tools/dotc/core/Denotations.scala
@@ -324,7 +324,7 @@ object Denotations {
                         info1 // follow Scala2 linearization -
                               // compare with way merge is performed in SymDenotation#computeMembersNamed
                       else
-                        throw new MergeError(s"${ex.getMessage} as members of type ${pre.show}")
+                        throw new MergeError(s"${ex.getMessage} as members of type ${pre.show}", ex.tp1, ex.tp2)
                   }
                 new JointRefDenotation(sym, jointInfo, denot1.validFor & denot2.validFor)
               }
diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index f8ff84ba1..71dede951 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1168,7 +1168,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       case bounds: TypeBounds => i"type bounds $bounds"
       case _ => tp.show
     }
-    throw new MergeError(s"cannot merge ${showType(tp1)} with ${showType(tp2)}")
+    throw new MergeError(s"cannot merge ${showType(tp1)} with ${showType(tp2)}", tp1, tp2)
   }
 
   /** Merge two lists of names. If names in corresponding positions match, keep them,
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 159e776b4..0691d979a 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -3318,7 +3318,7 @@ object Types {
     }
   }
 
-  class MergeError(msg: String) extends TypeError(msg)
+  class MergeError(msg: String, val tp1: Type, val tp2: Type) extends TypeError(msg)
 
   // ----- Debug ---------------------------------------------------------
 
diff --git a/src/dotty/tools/dotc/typer/RefChecks.scala b/src/dotty/tools/dotc/typer/RefChecks.scala
index 3ffbb8079..416a2cb92 100644
--- a/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -15,6 +15,7 @@ import TreeTransforms._
 import util.DotClass
 import scala.util.{Try, Success, Failure}
 import config.{ScalaVersion, NoScalaVersion}
+import Decorators._
 import typer.ErrorReporting._
 import DenotTransformers._
 import ValueClasses.isDerivedValueClass
@@ -340,10 +341,20 @@ object RefChecks {
           }*/
     }
 
-    val opc = new OverridingPairs.Cursor(clazz)
-    while (opc.hasNext) {
-      checkOverride(opc.overriding, opc.overridden)
-      opc.next()
+    try {
+      val opc = new OverridingPairs.Cursor(clazz)
+      while (opc.hasNext) {
+        checkOverride(opc.overriding, opc.overridden)
+        opc.next()
+      }
+    } catch {
+      case ex: MergeError =>
+        val addendum = ex.tp1 match {
+          case tp1: ClassInfo =>
+            "\n(Note that having same-named member classes in types of a mixin composition is no longer allowed)"
+          case _ => ""
+        }
+        ctx.error(ex.getMessage + addendum, clazz.pos)
     }
     printMixinOverrideErrors()