Annotations on abstract vals should emit a warning before they silently disappear

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Annotation on a val is by default added to the underlying field of a val.

If a val in question is abstract, there's no underlying field, so the annotation goes to waste. Writing @( @scala.annotation.meta.getter) would attach the annotation to the getter as desired, but it's not obvious.

Therefore I propose we emit a warning when an annotation is going to silently disappear.

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Imported From: https://issues.scala-lang.org/browse/SI-6375?orig=1
Reporter: @xeno-by

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@xeno-by said:
Related: #6325

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@paulp said:
scala/scala#1803

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@paulp said:
dc1b7d5

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Guillaume Massé (masgui) said (edited on Jun 12, 2015 7:31:25 PM UTC):
The parser wont accept annotation with paramers:

cat A.scala

import scala.annotation.meta.param
import scala.annotation.StaticAnnotation
class Foo(a: Int) extends StaticAnnotation
case class Anotated(@(Foo(1) @param) p1: Int)

/nix/store/22dsrlsqp33kqarh4cb9d7dis4kwgvma-scala-2.10.4/bin/scalac A.scala

A.scala:4: error: ')' expected but '(' found.
case class Anotated(@(Foo(1) @param) p1: Int)
                         ^
A.scala:4: error: identifier expected but ')' found.
case class Anotated(@(Foo(1) @param) p1: Int)
                                   ^
A.scala:4: error: ':' expected but eof found.
case class Anotated(@(Foo(1) @param) p1: Int)
                                             ^
three errors found

/nix/store/8ws1mx6fvgf3hijc49356vmm2q4zxzcj-scala-2.11.6/bin/scalac A.scala

A.scala:4: error: ')' expected but '(' found.
case class Anotated(@(Foo(1) @param) p1: Int)
                         ^
A.scala:4: error: identifier expected but ')' found.
case class Anotated(@(Foo(1) @param) p1: Int)
                                   ^
A.scala:4: error: ':' expected but eof found.
case class Anotated(@(Foo(1) @param) p1: Int)
                                             ^
three errors found

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@lrytz said:
scala/scala#5141 drops the warnings (commit 4b657bbe384bd364035060d3702d321b488e9263). Commit message: "Drop warning on potentially discarded annotations, I don't think this warrants a warning".

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@paulp said:
Is the theory that people often annotate members with the intention of that annotation being silently discarded?

Scoreboard: Implementor convenience N+1, User 0