1 files changed, 34 insertions, 10 deletions

diff --git a/test/pending/pos/t1756.scala b/test/pending/pos/t1756.scala
index c22d5509e1..4f7202114c 100644
--- a/test/pending/pos/t1756.scala
+++ b/test/pending/pos/t1756.scala
@@ -1,3 +1,24 @@
+
+/**
+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
@@ -13,18 +34,21 @@ class Poly[C <: Ring[C]](val c: C) extends Ring[Poly[C]] {
   def *(that: Poly[C]) = new Poly(this.c*that.c)
 }
 
-implicit def coef2poly[C <: Ring[C]](c: C) = new Poly(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)
+  val a = new A
+  val x = new Poly(new A)
 
-x+a // works
-a+x // works
+  println(x+a) // works
+  println(a+x) // works
 
-val y = new Poly(new Poly(new A))
+  val y = new Poly(new Poly(new A))
 
-x+y*x // works
-x*y+x // works
-y*x+x // works
+  println(x+y*x) // works
+  println(x*y+x) // works
+  println(y*x+x) // works
 
-x+x*y
+  println(x+x*y) // failed before
+}