Drop special case around Function1

Now only Scala2 mode treats Function1's as implicit conversions. Instead we introduce
a new subclass ImplicitConverter of Function1, instances of which are turned into
implicit conversions.

1 files changed, 20 insertions, 0 deletions

diff --git a/library/src/dotty/DottyPredef.scala b/library/src/dotty/DottyPredef.scala
index 12040e0f3..e78fa9239 100644
--- a/library/src/dotty/DottyPredef.scala
+++ b/library/src/dotty/DottyPredef.scala
@@ -36,4 +36,24 @@ object DottyPredef {
   implicit def eqNumFloat : Eq[Number, Float]  = Eq
   implicit def eqDoubleNum: Eq[Double, Number] = Eq
   implicit def eqNumDouble: Eq[Number, Double] = Eq
+
+  /** A class for implicit values that can serve as implicit conversions
+   *  The implicit resolution algorithm will act as if there existed
+   *  the additional implicit definition:
+   *
+   *    def $implicitConversion[T, U](x: T)(c: ImplicitConverter[T, U]): U = c(x)
+   *
+   *  However, the presence of this definition would slow down implicit search since
+   *  its outermost type matches any pair of types. Therefore, implicit search
+   *  contains a special case in `Implicits#discardForView` which emulates the
+   *  conversion in a more efficient way.
+   *
+   *  Note that this is a SAM class - function literals are automatically converted
+   *  to `ImplicitConverter` values.
+   *
+   *  Also note that in bootstrapped dotty, `Predef.<:<` should inherit from
+   *  `ImplicitConverter`. This would cut the number of special cases in
+   *  `discardForView` from two to one.
+   */
+  abstract class ImplicitConverter[-T, +U] extends Function1[T, U]
 }