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, 6 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 70f44c4fc6..31a15063bd 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -1749,6 +1749,8 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
              |you want, you must write the annotation class in Java.""".stripMargin)
       }
 
+      warnTypeParameterShadow(tparams1, clazz)
+
       if (!isPastTyper) {
         for (ann <- clazz.getAnnotation(DeprecatedAttr)) {
           val m = companionSymbolOf(clazz, context)
@@ -2151,6 +2153,8 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       val tparams1 = ddef.tparams mapConserve typedTypeDef
       val vparamss1 = ddef.vparamss mapConserve (_ mapConserve typedValDef)
 
+      warnTypeParameterShadow(tparams1, meth)
+
       meth.annotations.map(_.completeInfo())
 
       for (vparams1 <- vparamss1; vparam1 <- vparams1 dropRight 1)
@@ -2227,6 +2231,8 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       val typedMods = typedModifiers(tdef.mods)
       tdef.symbol.annotations.map(_.completeInfo())
 
+      warnTypeParameterShadow(tparams1, tdef.symbol)
+
       // @specialized should not be pickled when compiling with -no-specialize
       if (settings.nospecialization && currentRun.compiles(tdef.symbol)) {
         tdef.symbol.removeAnnotation(definitions.SpecializedClass)