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

4 files changed, 33 insertions, 0 deletions

diff --git a/test/files/neg/t2462a.check b/test/files/neg/t2462a.check
new file mode 100644
index 0000000000..b609ec519f
--- /dev/null
+++ b/test/files/neg/t2462a.check
@@ -0,0 +1,4 @@
+implicitNotFound.scala:2: error: Cannot construct a collection of type List[String] with elements of type Int based on a collection of type List[String].
+  List(1,2,3).map[Int, List[String]](x => 1)
+                                    ^
+one error found
diff --git a/test/files/neg/t2462a.scala b/test/files/neg/t2462a.scala
new file mode 100644
index 0000000000..2d523b4dd8
--- /dev/null
+++ b/test/files/neg/t2462a.scala
@@ -0,0 +1,3 @@
+object Test {
+  List(1,2,3).map[Int, List[String]](x => 1)
+}
\ No newline at end of file
diff --git a/test/files/neg/t2462b.check b/test/files/neg/t2462b.check
new file mode 100644
index 0000000000..ac9563878c
--- /dev/null
+++ b/test/files/neg/t2462b.check
@@ -0,0 +1,14 @@
+implicitNotFound_syntax.scala:6: warning: Invalid implicitNotFound message for trait Meh in package test:
+The type parameters Too, Elem referenced in the message of the @implicitNotFound annotation are not defined by trait Meh.
+trait Meh[-From, +To]
+      ^
+implicitNotFound_syntax.scala:9: warning: Invalid implicitNotFound message for trait Meh2 in package test:
+The type parameter Elem referenced in the message of the @implicitNotFound annotation is not defined by trait Meh2.
+trait Meh2[-From, +To]
+      ^
+implicitNotFound_syntax.scala:12: error: overriding method x in class thankyoupartest of type => Int;
+ method x needs `override' modifier
+class testmustfail extends thankyoupartest { def x = 43 }
+                                                 ^
+two warnings found
+one error found
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 }