closes 2462. better implicit error messages.

@implicitNotFound(msg="Custom error message that may refer to type parameters ${T} and ${U}")
trait Constraint[T, U]

whenever an implicit argument of type Constraint[A, B] cannot be found,
the custom error message will be used, where the type arguments are
interpolated in the obvious way

note: if the msg in the annotation references non-existing type params,
a warning is emitted

the patch also cleans up annotation argument retrieval (moved it to
AnnotationInfo from Symbol)

review by odersky

1 files changed, 12 insertions, 0 deletions

diff --git a/test/files/neg/t2462b.scala b/test/files/neg/t2462b.scala
new file mode 100644
index 0000000000..7a1389cc8e
--- /dev/null
+++ b/test/files/neg/t2462b.scala
@@ -0,0 +1,12 @@
+package test
+
+import scala.annotation.implicitNotFound
+
+@implicitNotFound(msg = "Cannot construct a collection of type ${Too} with elements of type ${Elem} based on a collection of type ${From}.")
+trait Meh[-From, +To]
+
+@implicitNotFound(msg = "Cannot construct a collection of type ${To} ${Elem}.")
+trait Meh2[-From, +To]
+
+class thankyoupartest { def x = 42 }
+class testmustfail extends thankyoupartest { def x = 43 }