2 files changed, 23 insertions, 0 deletions
diff --git a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
index 7440f69e93..d18348f51e 100644
--- a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
@@ -600,6 +600,23 @@ trait TypeDiagnostics {
         )
     }
 
+    // warn about class/method/type-members' type parameters that shadow types already in scope
+    def warnTypeParameterShadow(tparams: List[TypeDef], sym: Symbol): Unit =
+      if (settings.warnTypeParameterShadow && !isPastTyper && !sym.isSynthetic) {
+        def enclClassOrMethodOrTypeMember(c: Context): Context =
+          if (!c.owner.exists || c.owner.isClass || c.owner.isMethod || (c.owner.isType && !c.owner.isParameter)) c
+          else enclClassOrMethodOrTypeMember(c.outer)
+
+        val tt = tparams.filter(_.name != typeNames.WILDCARD).foreach { tp =>
+        // we don't care about type params shadowing other type params in the same declaration
+        enclClassOrMethodOrTypeMember(context).outer.lookupSymbol(tp.name, s => s != tp.symbol && s.hasRawInfo && reallyExists(s)) match {
+          case LookupSucceeded(_, sym2) => context.warning(tp.pos,
+            s"type parameter ${tp.name} defined in $sym shadows $sym2 defined in ${sym2.owner}. You may want to rename your type parameter, or possibly remove it.")
+          case _ =>
+        }
+      }
+    }
+
     /** Report a type error.
      *
      *  @param pos    The position where to report the error
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)