Fixed #2444

1 files changed, 54 insertions, 0 deletions

diff --git a/test/files/pos/t1756.scala b/test/files/pos/t1756.scala
new file mode 100755
index 0000000000..4f7202114c
--- /dev/null
+++ b/test/files/pos/t1756.scala
@@ -0,0 +1,54 @@
+
+/**
+This is a tricky issue which has to do with the fact that too much conflicting
+type information is propagated into a single implicit search, where the intended
+solution applies two implicit searches.
+
+Roughly, in x + x * y, the first x is first typed as Poly[A]. That
+means the x * y is then typed as Poly[A]. Then the second x is typed
+as Poly[A], then y is typed as Poly[Poly[A]]. The application x * y
+fails, so the coef2poly implicit conversion is applied to x. That
+means we look for an implicit conversion from type Poly[A] to type
+?{val *(x$1: ?>: Poly[Poly[A]] <: Any): Poly[A]}. Note that the result
+type Poly[A] is propagated into the implicit search. Poly[A] comes as
+expected type from x+, because the lhs x is still typed as a Poly[A].
+This means that the argument of the implicit conversion is typechecked
+with expected type A with Poly[A]. And no solution is found.
+
+To solve this, I added a fallback scheme similar to implicit arguents:
+When an implicit view that adds a method matching given arguments and result
+type fails, try again without the result type.
+*/
+trait Ring[T <: Ring[T]] {
+  def +(that: T): T
+  def *(that: T): T
+}
+
+class A extends Ring[A] {
+  def +(that: A) = new A
+  def *(that: A) = new A
+}
+
+class Poly[C <: Ring[C]](val c: C) extends Ring[Poly[C]] {
+  def +(that: Poly[C]) = new Poly(this.c+that.c)
+  def *(that: Poly[C]) = new Poly(this.c*that.c)
+}
+
+object Test extends Application {
+
+  implicit def coef2poly[C <: Ring[C]](c: C): Poly[C] = new Poly(c)
+
+  val a = new A
+  val x = new Poly(new A)
+
+  println(x+a) // works
+  println(a+x) // works
+
+  val y = new Poly(new Poly(new A))
+
+  println(x+y*x) // works
+  println(x*y+x) // works
+  println(y*x+x) // works
+
+  println(x+x*y) // failed before
+}