Fix #1905: Detect case where bridge would clash with the member

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -170,6 +170,12 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       case tp: TypeRef =>
         if (tp.symbol.isAnonymousClass && !ctx.settings.uniqid.value)
           return toText(tp.info)
+        if (tp.symbol.is(Param))
+          tp.prefix match {
+            case pre: ThisType if pre.cls == tp.symbol.owner =>
+              return nameString(tp.symbol)
+            case _ =>
+          }
       case ExprType(result) =>
         return "=> " ~ toText(result)
       case ErasedValueType(tycon, underlying) =>
@@ -501,7 +507,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
             typeDefText(tparamsTxt, toText(rhs))
           case LambdaTypeTree(tparams, body) =>
             recur(body, tparamsText(tparams))
-          case rhs: TypeTree if rhs.tpe.isInstanceOf[TypeBounds] =>
+          case rhs: TypeTree if rhs.typeOpt.isInstanceOf[TypeBounds] =>
             typeDefText(tparamsTxt, toText(rhs))
           case rhs =>
             typeDefText(tparamsTxt, optText(rhs)(" = " ~ _))

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

@@ -16,6 +16,7 @@ class Bridges(root: ClassSymbol)(implicit ctx: Context) {
 
   assert(ctx.phase == ctx.erasurePhase.next)
   private val preErasureCtx = ctx.withPhase(ctx.erasurePhase)
+  private val elimErasedCtx = ctx.withPhase(ctx.elimErasedValueTypePhase.next)
 
   private class BridgesCursor(implicit ctx: Context) extends OverridingPairs.Cursor(root) {
 
@@ -35,31 +36,47 @@ class Bridges(root: ClassSymbol)(implicit ctx: Context) {
   private val bridgesScope = newScope
   private val bridgeTarget = newMutableSymbolMap[Symbol]
 
+  def bridgePosFor(member: Symbol) =
+    if (member.owner == root && member.pos.exists) member.pos else root.pos
+
   /** Add a bridge between `member` and `other`, where `member` overrides `other`
    *  before erasure, if the following conditions are satisfied.
    *
-   *   - `member` and other have different signatures
-   *   - there is not yet a bridge with the same name and signature in `root`
+   *   - `member` and `other` have different signatures
+   *   - there is not yet a bridge with the same name and signature in `root`.
    *
    *  The bridge has the erased info of `other` and forwards to `member`.
+   *  Additionally, if `member` and `other` do have the same signature,
+   *  but not the same type after erasure and before elimErasedValueTypes
+   *  issue an error: A bridge would be needed yet it would clash with the member itself.
+   *  See neg/i1905.scala
    */
   private def addBridgeIfNeeded(member: Symbol, other: Symbol) = {
     def bridgeExists =
       bridgesScope.lookupAll(member.name).exists(bridge =>
         bridgeTarget(bridge) == member && bridge.signature == other.signature)
-    if (!(member.signature == other.signature || bridgeExists))
+    def info(sym: Symbol)(implicit ctx: Context) = sym.info
+    def desc(sym: Symbol)= i"$sym${info(sym)(preErasureCtx)} in ${sym.owner}"
+    if (member.signature == other.signature) {
+      if (!member.info.matches(other.info))
+        ctx.error(em"""bridge generated for member ${desc(member)}
+                      |which overrides ${desc(other)}
+                      |clashes with definition of the member itself; both have erased type ${info(member)(elimErasedCtx)}."""",
+                  bridgePosFor(member))
+    }
+    else if (!bridgeExists)
       addBridge(member, other)
   }
 
   /** Generate bridge between `member` and `other`
    */
   private def addBridge(member: Symbol, other: Symbol) = {
-    val bridgePos = if (member.owner == root && member.pos.exists) member.pos else root.pos
     val bridge = other.copy(
       owner = root,
       flags = (member.flags | Method | Bridge | Artifact) &~
         (Accessor | ParamAccessor | CaseAccessor | Deferred | Lazy | Module),
-      coord = bridgePos).enteredAfter(ctx.erasurePhase.asInstanceOf[DenotTransformer]).asTerm
+      coord = bridgePosFor(member))
+      .enteredAfter(ctx.erasurePhase.asInstanceOf[DenotTransformer]).asTerm
 
     ctx.debuglog(
       i"""generating bridge from ${other.showLocated}: ${other.info}

tests/neg/i1905.scala

@@ -0,0 +1,17 @@
+class Arr[T](private val underlying: scala.Array[T]) extends AnyVal
+
+abstract class SeqMonoTransforms[+A, +Repr] {
+  protected[this] def fromIterableWithSameElemType(): Repr
+  def getFIWSET: Repr = fromIterableWithSameElemType()
+}
+
+class ArrOps[A](val xs: Arr[A]) extends SeqMonoTransforms[A, Arr[A]] {
+  def fromIterableWithSameElemType(): Arr[A] = xs // error: bridge clashes with member
+}
+
+object Test {
+  def main(args: Array[String]) = {
+    val t = new ArrOps(new Arr(Array(1, 2, 3)))
+    val t2 = t.getFIWSET
+  }
+}