Merge pull request #1170 from dotty-staging/fix/hash-delta

Fix incorrect hashing leading to cache pollution

Author: odersky

src/dotty/tools/dotc/core/Hashable.scala

@@ -92,7 +92,9 @@ trait Hashable {
   protected final def doHash(x1: Int, x2: Int): Int =
     finishHash(hashing.mix(hashing.mix(hashSeed, x1), x2), 1)
 
-  protected final def addDelta(hc: Int, delta: Int) = avoidNotCached(hc + delta)
+  protected final def addDelta(elemHash: Int, delta: Int) =
+    if (elemHash == NotCached) NotCached
+    else avoidNotCached(elemHash + delta)
 
   private def avoidNotCached(h: Int) = if (h == NotCached) NotCachedAlt else h
 }

src/dotty/tools/dotc/core/Types.scala

@@ -2479,7 +2479,7 @@ object Types {
     def copyBoundType(bt: BT) = new MethodParamImpl(bt, paramNum)
 
     // need to customize hashCode and equals to prevent infinite recursion for dep meth types.
-    override def computeHash = addDelta(System.identityHashCode(binder), paramNum)
+    override def computeHash = addDelta(binder.identityHash, paramNum)
     override def equals(that: Any) = that match {
       case that: MethodParam =>
         (this.binder eq that.binder) && this.paramNum == that.paramNum