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

With value classes it is possible that a bridge is needed because the semi erased
types of a member and its overridden method differ, yet the fully erased types
of the member and the bridge are the same. With this commit we flag this situation
as an error. Before it generated a ClassCastException at runtime instead.

Author: odersky

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
+  }
+}