SI-5691 lint warning when a type parameter shadows an existing type.

This adds a new lint warning for when a class/method/type-member's
type parameter shadows an existing type: `-Xlint:type-parameter-shadow`.

It excludes type parameters of synthetic methods (the user can't
rename or remove those anyway), otherwise, for example, every case class
triggers the warning.

Also fixes a test that contained wrong java sources (that didn't even
compile...), discovered thanks to the warning.

---
This kind of errors shows up every now and then on the mailing-list, on
stackoverflow, etc. so maybe a warning would be useful.
I was afraid this would yield too many warnings for libraries that are
heavy on type parameters, but no: running this on scalaz and shapeless
HEAD (`v7.1.0-RC1-41-g1cc0a96` and `v2.0.0-M1-225-g78426a0` respectively)
yields 44 warnings. None of them are false positives; they usually come
from:
 - scalaz loving using `A` as type parameter, even several levels deep
   of parametrized classes/methods
 - or calling a type parameter that will hold a map `Map`, or similar,
   thus shadowing an existing type

1 files changed, 3 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/Warnings.scala b/src/compiler/scala/tools/nsc/settings/Warnings.scala
index bec068b56a..4c37633301 100644
--- a/src/compiler/scala/tools/nsc/settings/Warnings.scala
+++ b/src/compiler/scala/tools/nsc/settings/Warnings.scala
@@ -38,6 +38,7 @@ trait Warnings {
     warnMissingInterpolator,
     warnDocDetached,
     warnPrivateShadow,
+    warnTypeParameterShadow,
     warnPolyImplicitOverload,
     warnOptionImplicit,
     warnDelayedInit,
@@ -83,6 +84,8 @@ trait Warnings {
                                 "A ScalaDoc comment appears to be detached from its element.")
   val warnPrivateShadow        = lintflag("private-shadow",
                                 "A private field (or class parameter) shadows a superclass field.")
+  val warnTypeParameterShadow  = lintflag("type-parameter-shadow",
+                                "A local type parameter shadows a type already in scope.")
   val warnPolyImplicitOverload = lintflag("poly-implicit-overload",
                                 "Parameterized overloaded implicit methods are not visible as view bounds")
   val warnOptionImplicit       = lintflag("option-implicit",