Reject references to self in super constructor calls

Fixes #11028
Fixes #12557

Author: odersky

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -121,6 +121,7 @@ class Compiler {
          new ElimStaticThis,         // Replace `this` references to static objects by global identifiers
          new CountOuterAccesses) ::  // Identify outer accessors that can be dropped
     List(new DropOuterAccessors,     // Drop unused outer accessors
+         new CheckNoSuperThis,       // Check that supercalls don't contain references to `this`
          new Flatten,                // Lift all inner classes to package scope
          new RenameLifted,           // Renames lifted classes to local numbering scheme
          new TransformWildcards,     // Replace wildcards with default values

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

@@ -0,0 +1,51 @@
+package dotty.tools.dotc
+package transform
+
+import core.*
+import MegaPhase.MiniPhase
+import Contexts.*, Types.*, Symbols.*, SymDenotations.*, Flags.*
+import ast.*
+import Trees.*
+import Decorators.*
+
+import annotation.threadUnsafe
+
+object CheckNoSuperThis:
+  val name: String = "checkNoSuperThis"
+
+/** Checks that super and this calls do not pass `this` as (part of) an argument. */
+class CheckNoSuperThis extends MiniPhase:
+  thisPhase =>
+  import tpd._
+
+  override def phaseName: String = CheckNoSuperThis.name
+
+  override def runsAfterGroupsOf: Set[String] = Set(Constructors.name)
+
+  override def transformDefDef(mdef: DefDef)(using Context): DefDef =
+    if mdef.symbol.isClassConstructor then
+      mdef.rhs match
+        case Block(stats, _) => splitAtSuper(stats) match
+          case (Apply(_, superArgs) :: _, _) =>
+            val cls = mdef.symbol.owner
+            def fail(t: Tree) =
+              report.error(em"super constructor cannot be passed a self reference $t unless parameter is declared by-name", t.srcPos)
+            for arg <- superArgs do
+              arg.foreachSubTree {
+                case t: This if t.symbol == cls =>
+                  fail(t)
+                case t: RefTree => t.tpe match
+                  case tpe @ TermRef(prefix, _)
+                  if prefix == cls.thisType && !tpe.symbol.is(JavaStatic) => fail(t)
+                  case tpe @ TermRef(prefix, _)
+                  if cls.is(Module) && prefix.termSymbol == cls.sourceModule && !tpe.symbol.is(JavaStatic) => fail(t)
+                  case tpe @ TermRef(prefix, _)
+                  if cls.is(Module) && tpe.symbol == cls.sourceModule => fail(t)
+                  case _ =>
+                case _ =>
+              }
+          case _ =>
+        case _ =>
+    mdef
+
+end CheckNoSuperThis

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

@@ -991,18 +991,6 @@ trait Checking {
         report.error(i"""$called is already implemented by super${caller.superClass},
                    |its constructor cannot be called again""", call.srcPos)
 
-      if (caller.is(Module)) {
-        val traverser = new TreeTraverser {
-          def traverse(tree: Tree)(using Context) = tree match {
-            case tree: RefTree if tree.isTerm && (tree.tpe.classSymbol eq caller) =>
-              report.error("super constructor cannot be passed a self reference", tree.srcPos)
-            case _ =>
-              traverseChildren(tree)
-          }
-        }
-        traverser.traverse(call)
-      }
-
       // Check that constructor call is of the form _.<init>(args1)...(argsN).
       // This guards against calls resulting from inserted implicits or applies.
       def checkLegalConstructorCall(tree: Tree, encl: Tree, kind: String): Unit = tree match {

tests/neg/i11028.scala

@@ -0,0 +1,8 @@
+import scala.reflect.ClassTag
+
+@main def run = println(Foo)
+
+abstract class Bar[T](implicit val thisClassTag: ClassTag[T])
+
+class Foo
+object Foo extends Bar[Foo] // error

tests/neg/i11028a.scala

@@ -0,0 +1,6 @@
+class Foo(implicit val foo: Foo)
+
+object Test extends App {
+  implicit object Bar extends Foo  // error
+  Bar.foo
+}

tests/neg/i11045.scala

@@ -0,0 +1,5 @@
+abstract class Foo(x: Any)
+class Boom(var x: Unit, y: Unit) extends Foo((x: Int) => x) // error: super constructor cannot be passed a self reference
+@main def Test =
+  Boom((), ())
+

tests/neg/i12557.scala

@@ -0,0 +1,14 @@
+package example
+
+abstract class X[P <: Product](using val m: scala.deriving.Mirror.ProductOf[P]) {
+  def unapply(p: P): m.MirroredElemTypes = ???
+}
+
+case class A(a: Int)
+object A extends X[A]  // error
+
+object Main {
+  def main(args: Array[String]): Unit = {
+    A.unapply(A(2))
+  }
+}