Make illegal super acessor type an error

Author: odersky

compiler/src/dotty/tools/dotc/transform/ResolveSuper.scala

@@ -103,15 +103,19 @@ object ResolveSuper {
       val other = bcs.head.info.nonPrivateDecl(memberName)
       if (ctx.settings.Ydebug.value)
         ctx.log(i"rebindsuper ${bcs.head} $other deferred = ${other.symbol.is(Deferred)}")
-      val otherMember = other.matchingDenotation(base.thisType, base.thisType.memberInfo(acc))
-
-      // Having a matching denotation is not enough: it should also be a subtype
-      // of the superaccessor's type, see i5433.scala for an example where this matters
-      if (otherMember.asSeenFrom(base.typeRef).info <:< acc.asSeenFrom(base.typeRef).info)
-        sym = otherMember.symbol
-
+      sym = other.matchingDenotation(base.thisType, base.thisType.memberInfo(acc)).symbol
       bcs = bcs.tail
     }
+    // Having a matching denotation is not enough: it should also be a subtype
+    // of the superaccessor's type, see i5433.scala for an example where this matters
+    if (!(sym.asSeenFrom(base.typeRef).info <:< acc.asSeenFrom(base.typeRef).info))
+      ctx.error(em"""illegal mixin super call target:
+                    |
+                    |  ${sym.showDcl}  in ${sym.owner}
+                    |
+                    |does not conform to expected target type
+                    |
+                    |  ${acc.info}""", base.pos)
     assert(sym.exists)
     sym
   }

tests/run/i5433.scala renamed to tests/neg/i5433.scala

@@ -14,8 +14,7 @@ trait C extends A[Y] {
   def superFoo: Y = super.foo // C will have an abstract `def C$$super$foo: Y` because of this call
 }
 
-class Fail extends B with C
-  // Should generate `def C$$super$foo: Y = super[A].foo` and not `= super[B].foo`
+class Fail extends B with C  // error: illegal mixin super call target
 
 object Test {
   def main(args: Array[String]): Unit = {