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]
                ^

7 files changed, 44 insertions, 0 deletions

diff --git a/test/files/neg/implicit-ambiguous-2.check b/test/files/neg/implicit-ambiguous-2.check
new file mode 100644
index 0000000000..4a10b0dd65
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous-2.check
@@ -0,0 +1,4 @@
+implicit-ambiguous-2.scala:10: error: Could not prove Int =!= Int
+  implicitly[Int =!= Int]
+            ^
+one error found
diff --git a/test/files/neg/implicit-ambiguous-2.scala b/test/files/neg/implicit-ambiguous-2.scala
new file mode 100644
index 0000000000..563c8c583f
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous-2.scala
@@ -0,0 +1,11 @@
+object Test {
+  trait =!=[C, D]
+
+  implicit def neq[E, F] : E =!= F = null
+
+  implicit def neqAmbig1[G, H, J] : J =!= J = null
+  @annotation.implicitAmbiguous("Could not prove ${I} =!= ${I}")
+  implicit def neqAmbig2[I] : I =!= I = null
+
+  implicitly[Int =!= Int]
+}
diff --git a/test/files/neg/implicit-ambiguous-invalid.check b/test/files/neg/implicit-ambiguous-invalid.check
new file mode 100644
index 0000000000..68b607c4c2
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous-invalid.check
@@ -0,0 +1,7 @@
+implicit-ambiguous-invalid.scala:5: warning: Invalid implicitAmbiguous message for method neqAmbig1 in object Test:
+The type parameter B referenced in the message of the @implicitAmbiguous annotation is not defined by method neqAmbig1.
+  implicit def neqAmbig1[A] : A =!= A = null
+               ^
+error: No warnings can be incurred under -Xfatal-warnings.
+one warning found
+one error found
diff --git a/test/files/neg/implicit-ambiguous-invalid.flags b/test/files/neg/implicit-ambiguous-invalid.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous-invalid.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
diff --git a/test/files/neg/implicit-ambiguous-invalid.scala b/test/files/neg/implicit-ambiguous-invalid.scala
new file mode 100644
index 0000000000..f8f9da655f
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous-invalid.scala
@@ -0,0 +1,6 @@
+object Test {
+  trait =!=[C, D]
+
+  @annotation.implicitAmbiguous("Could not prove ${A} =!= ${B}")
+  implicit def neqAmbig1[A] : A =!= A = null
+}
diff --git a/test/files/neg/implicit-ambiguous.check b/test/files/neg/implicit-ambiguous.check
new file mode 100644
index 0000000000..0b3cebcb6f
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous.check
@@ -0,0 +1,4 @@
+implicit-ambiguous.scala:10: error: Could not prove Int =!= Int
+  implicitly[Int =!= Int]
+            ^
+one error found
diff --git a/test/files/neg/implicit-ambiguous.scala b/test/files/neg/implicit-ambiguous.scala
new file mode 100644
index 0000000000..79b1297915
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous.scala
@@ -0,0 +1,11 @@
+object Test {
+  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]
+}