to(Seq) doesn't evaluate collection when called on a Seq

Author: Martijn Hoekstra

compat/src/main/scala-2.11_2.12/scala/collection/compat/CompatImpl.scala

@@ -16,6 +16,40 @@ import scala.collection.generic.CanBuildFrom
 import scala.collection.mutable.Builder
 import scala.collection.{immutable => i, mutable => m}
 
+/* builder optimized for a single ++= call, which returns identity on result if possible
+ * and defers to the underlying builder if not.
+ */
+private final class IdentityPreservingSeqBuilder[A, C <: Seq[A]](that: Builder[A, C])
+    extends Builder[A, Seq[A]] {
+  var collection: Seq[A] = null
+  var ruined = false
+
+  final override def ++=(elems: TraversableOnce[A]): this.type =
+      if(!ruined && collection == null && elems.isInstanceOf[Seq[_]]) {
+        collection = elems.asInstanceOf[Seq[A]]
+        this
+      }
+      else {
+        ruined = true
+        if (collection != null) that ++= collection
+        that ++= elems
+        collection = null
+        this
+      }
+
+  final def +=(elem: A): this.type = {
+    collection = null
+    ruined = true
+    that += elem
+    this
+  }
+  final def clear(): Unit = {
+    collection = null
+    if (ruined) that.clear()
+  }
+  final def result(): Seq[A] = if(ruined) that.result() else if (collection eq null) Nil else collection
+}
+
 private[compat] object CompatImpl {
   def simpleCBF[A, C](f: => Builder[A, C]): CanBuildFrom[Any, A, C] = new CanBuildFrom[Any, A, C] {
     def apply(from: Any): Builder[A, C] = apply()

compat/src/main/scala-2.11_2.12/scala/collection/compat/PackageShared.scala

@@ -44,6 +44,8 @@ private[compat] trait PackageShared {
     def newBuilder: m.Builder[A, C] = factory()
   }
 
+  implicit def SeqCBF[A](seq: Seq.type): CanBuildFrom[Any, A, Seq[A]] = simpleCBF(new IdentityPreservingSeqBuilder[A, Seq[A]](Seq.newBuilder[A]))
+
   implicit def genericCompanionToCBF[A, CC[X] <: GenTraversable[X]](
       fact: GenericCompanion[CC]): CanBuildFrom[Any, A, CC[A]] =
     simpleCBF(fact.newBuilder[A])

compat/src/test/scala/test/scala/collection/CollectionTest.scala

@@ -40,6 +40,10 @@ class CollectionTest {
     //val mT: Map[Int, String] = m
     assertEquals(Map(1 -> "a", 2 -> "b"), m)
     assertTrue(m.isInstanceOf[Map[_, _]])
+
+    // Stream.to(Seq) doesn't evaluate the stream
+    val strm = 1 #:: {throw new Exception("not lazy")} #:: Stream.empty[Int]
+    val strmsq: Seq[Int] = strm.to(Seq)
   }
 
   @Test