Lint pattern varids which are backquotable

For the `case x =>` where user intended to match
a value `x` in scope.

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -2019,7 +2019,7 @@ self =>
           atPos(p.pos.start, p.pos.start, body.pos.end) {
             val t = Bind(name, body)
             body match {
-              case Ident(nme.WILDCARD) if settings.warnUnusedPatVars => t updateAttachment NoWarnAttachment
+              case Ident(nme.WILDCARD) if settings.warnUnusedPatVars || settings.warnPatternShadow => t updateAttachment NoWarnAttachment
               case _ => t
             }
           }

src/compiler/scala/tools/nsc/settings/Warnings.scala

@@ -192,6 +192,7 @@ trait Warnings {
     val ByNameImplicit         = LintWarning("byname-implicit",           "Block adapted by implicit with by-name parameter.")
     val RecurseWithDefault     = LintWarning("recurse-with-default",      "Recursive call used default argument.")
     val UnitSpecialization     = LintWarning("unit-special",              "Warn for specialization of Unit in parameter position.")
+    val PatternShadow          = LintWarning("pattern-shadow",            "Pattern variable id is also a term in scope.")
 
     def allLintWarnings = values.toSeq.asInstanceOf[Seq[LintWarning]]
   }
@@ -223,6 +224,7 @@ trait Warnings {
   def warnByNameImplicit         = lint contains ByNameImplicit
   def warnRecurseWithDefault     = lint contains RecurseWithDefault
   def unitSpecialization         = lint contains UnitSpecialization
+  def warnPatternShadow          = lint contains PatternShadow
 
   // The Xlint warning group.
   val lint = MultiChoiceSetting(

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -4628,6 +4628,9 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
             tree setSymbol sym setType sym.tpeHK
 
           case Bind(name: TermName, body)  =>
+            if (settings.warnPatternShadow && !tree.hasAttachment[NoWarnAttachment.type] && context.isNameInScope(name))
+              context.warning(tree.pos, s"Name $name is already in scope. Did you intend backquoted `$name`?", WarningCategory.OtherShadowing)
+
             val sym =
               if (tree.symbol != NoSymbol) tree.symbol
               else context.owner.newValue(name, tree.pos)

test/files/neg/t11850.check

@@ -0,0 +1,6 @@
+t11850.scala:9: warning: Name x is already in scope. Did you intend backquoted `x`?
+      case x   => 1    // warn
+           ^
+error: No warnings can be incurred under -Werror.
+1 warning
+1 error

test/files/neg/t11850.scala

@@ -0,0 +1,15 @@
+// scalac: -Werror -Xlint:pattern-shadow
+
+trait T {
+  val x = 42
+
+  def f(i: Int) =
+    i match {
+      case `x` => 0    // presence of this case really obviates warning on the next?
+      case x   => 1    // warn
+    }
+  def g(i: Int) =
+    i match {
+      case x @ _ => 1
+    }
+}