From 5bd8ea0edffe7b725e5fa665a82a5795d5dafe8f Mon Sep 17 00:00:00 2001
From: Brian McKenna <brian@simpleenergy.com>
Date: Sun, 7 Sep 2014 12:33:52 -0600
Subject: SI-6806 Add an @implicitAmbiguous annotation

Example usage:

    trait =!=[C, D]

    implicit def neq[E, F] : E =!= F = null

    @annotation.implicitAmbiguous("Could not prove ${J} =!= ${J}")
    implicit def neqAmbig1[G, H, J] : J =!= J = null
    implicit def neqAmbig2[I] : I =!= I = null

    implicitly[Int =!= Int]

Which gives the following error:

    implicit-ambiguous.scala:9: error: Could not prove Int =!= Int
      implicitly[Int =!= Int]
                ^

Better than what was previously given:

    implicit-ambiguous.scala:9: error: ambiguous implicit values:
     both method neqAmbig1 in object Test of type [G, H, J]=> Main.$anon.Test.=!=[J,J]
     and method neqAmbig2 in object Test of type [I]=> Main.$anon.Test.=!=[I,I]
     match expected type Main.$anon.Test.=!=[Int,Int]
      implicitly[Int =!= Int]
                ^
---
 .../scala/annotation/implicitAmbiguous.scala       | 34 ++++++++++++++++++++++
 1 file changed, 34 insertions(+)
 create mode 100644 src/library/scala/annotation/implicitAmbiguous.scala

(limited to 'src/library')

diff --git a/src/library/scala/annotation/implicitAmbiguous.scala b/src/library/scala/annotation/implicitAmbiguous.scala
new file mode 100644
index 0000000000..46eab9ae8f
--- /dev/null
+++ b/src/library/scala/annotation/implicitAmbiguous.scala
@@ -0,0 +1,34 @@
+package scala.annotation
+
+import scala.annotation.meta._
+
+/**
+  * To customize the error message that's emitted when an implicit of type
+  * C[T1,..., TN] is found more than once, annotate the class C
+  * with @implicitAmbiguous. Assuming C has type parameters X1,..., XN, the
+  * error message will be the result of replacing all occurrences of ${Xi} in
+  * the string msg with the string representation of the corresponding type
+  * argument Ti. *
+  *
+  * If more than one @implicitAmbiguous annotation is collected, the compiler is
+  * free to pick any of them to display.
+  *
+  * Nice errors can direct users to fix imports or even tell them why code
+  * intentionally doesn't compile.
+  *
+  * {{{
+  * trait =!=[C, D]
+  *
+  * implicit def neq[E, F] : E =!= F = null
+  *
+  * @annotation.implicitAmbiguous("Could not prove ${J} =!= ${J}")
+  * implicit def neqAmbig1[G, H, J] : J =!= J = null
+  * implicit def neqAmbig2[I] : I =!= I = null
+  *
+  * implicitly[Int =!= Int]
+  * }}}
+  *
+  * @author Brian McKenna
+  * @since 2.12.0
+  */
+final class implicitAmbiguous(msg: String) extends scala.annotation.StaticAnnotation {}
-- 
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