SI-3346 implicit parameters can now guide implicit view inference

This simple patch makes it possible for implicit views to benefit from
fundep-guided type inference, eliminating a nasty special case in
implicit inference.

Here are the changes that I had to apply to the tests (they exposed
quite a number of interesting questions that I was happy to answer):

1) neg/t5845.scala now works, so I moved it to pos. That easily makes sense,
because the test was just a canary to track previous state of things.

2) neg/divergent_implicit.scala, neg/t5578.scala and neg/t7519.scala
changed their messages to less informative ones, because inapplicable
implicit views now get disqualified early and therefore don't display
their error messages to the user. This is an unfortunate but necessary
byproduct of this change, and one can argue that the behavior is now
completely consistent with implicit vals (that also don't explain why
this or that implicit val got disqualified, but rather display a generic
implicit value not found message).

3) scaladoc/implicits-chaining.scala and scaladoc/implicits-base.scala.
Immediate culling of apriori inapplicable implicit views messes things up
for Scaladoc, because it's interested in potentially applicable views,
having infrastructure to track various constraints (type bounds, requirements
for implicit parameters, etc) that guide applicability of views and then
present it to the user. Therefore, when scaladoc is detected, implicit search
reverts to the old behavior.

4) We still cannot have Jason's workaround to type constructor inference
mentioned in comments to SI-3346, because it's not really about implicit
parameters of implicit views, but about type inference flowing from the
implicit parameter list to a preceding parameter list in order to affect
inference of an implicit view. This is something that's still too ambitious.

1 files changed, 61 insertions, 0 deletions

diff --git a/test/files/neg/t3346c.scala b/test/files/neg/t3346c.scala
new file mode 100644
index 0000000000..a5ac166b2d
--- /dev/null
+++ b/test/files/neg/t3346c.scala
@@ -0,0 +1,61 @@
+object Test extends App {
+  //
+  // An attempt to workaround SI-2712, foiled by SI-3346
+  //
+  trait TC[M[_]]
+
+  type EitherInt[A] = Either[Int, A]
+
+  implicit object EitherTC extends TC[EitherInt]
+
+  def foo[M[_]: TC, A](ma: M[A]) = ()
+
+  val eii: Either[Int, String] = Right("")
+
+  foo[EitherInt, String](eii)
+
+  // This one needs SI-2712 Higher Order Unification
+  //foo(eii) // not inferred
+
+  // A workaround is to provide a set of implicit conversions that take values
+  // based on type constructors of various shapes, and search for the
+  // type class instances.
+  //
+  // This is the approach taken by scalaz7.
+
+  trait TCValue[M[_], A] {
+    implicit def self: M[A]
+    def M: TC[M]
+
+    // instead of `foo(eii)`, we'll try `eii.foo`
+    def foo[M[_], A] = ()
+  }
+
+
+  implicit def ToTCValue[M[_], A](ma: M[A])(implicit M0: TC[M]) = new TCValue[M, A] {
+    implicit val M = M0
+    val self = ma
+  }
+  implicit def ToTCValueBin1[M[_, _], A, B](ma: M[A, B])(implicit M0: TC[({type λ[α]=M[A, α]})#λ]): TCValue[({type λ[α] = M[A, α]})#λ, B] = new TCValue[({type λ[α]=M[A, α]})#λ, B] {
+    implicit val M = M0
+    val self = ma
+  }
+  implicit def ToTCValueBin2[M[_, _], A, B](ma: M[A, B])(implicit M0: TC[({type λ[α]=M[α, B]})#λ]): TCValue[({type λ[α]=M[α, B]})#λ, A] = new TCValue[({type λ[α]=M[α, B]})#λ, A] {
+    implicit val M = M0
+    val self = ma
+  }
+
+
+  ToTCValueBin1(eii).foo
+
+  // as expected, could not find implicit parameter
+  // ToTCValueBin2(eii).bar
+
+  // error: implicit conversions are not applicable because they are ambiguous, both method ToTCValueBin1 ... and  method ToTCValueBin2
+  //        annoying!!
+  //        https://issues.scala-lang.org/browse/SI-3346
+  //
+  // Works if we remove ToTCValueBin2
+  //
+  eii.bar
+}