Disallow mixins where super calls bind to vals (#16908)

Author: dwijnand

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

@@ -119,6 +119,9 @@ object ResolveSuper {
           report.error(IllegalSuperAccessor(base, memberName, targetName, acc, accTp, other.symbol, otherTp), base.srcPos)
       bcs = bcs.tail
     }
+    if sym.is(Accessor) then
+      report.error(
+        em"parent ${acc.owner} has a super call which binds to the value ${sym.showFullName}. Super calls can only target methods.", base)
     sym.orElse {
       val originalName = acc.name.asTermName.originalOfSuperAccessorName
       report.error(em"Member method ${originalName.debugString} of mixin ${acc.owner} is missing a concrete super implementation in $base.", base.srcPos)

tests/neg/t12715.scala

@@ -0,0 +1,17 @@
+trait A              { def f: String }
+trait B extends A    { def f = "B" }
+trait C extends A    { override val f = "C" }
+trait D extends C    { override val f = "D" }
+trait E extends A, B { def d = super.f }
+final class O1 extends B, C, D, E // error: parent trait E has a super call which binds to the value D.f. Super calls can only target methods.
+final class O2 extends B, C, E, D // error: parent trait E has a super call which binds to the value C.f. Super calls can only target methods.
+final class O3 extends B, E, C, D
+
+object Main:
+  def main(args: Array[String]): Unit =
+    println(O1().f) // D
+    println(O2().f) // D
+    println(O3().f) // D
+    println(O3().d) // B
+    O1().d // was: NoSuchMethodError: 'java.lang.String D.f$(D)'
+    O2().d // was: NoSuchMethodError: 'java.lang.String C.f$(C)'

tests/neg/t12715b.scala

@@ -0,0 +1,16 @@
+trait B:
+  def f: Float = 1.0f
+
+class A(override val f: Float) extends B
+
+trait C extends B:
+  abstract override val f = super.f + 100.0f
+
+trait D extends B:
+  abstract override val f = super.f + 1000.0f
+
+class ACD10 extends A(10.0f) with C with D // error: parent trait D has a super call to method B.f, which binds to the value C.f. Super calls can only target methods.
+
+object Test:
+  def main(args: Array[String]): Unit =
+    new ACD10 // was: NoSuchMethodError: 'float C.f$(C)'