Don't use _ as a binder for abstract type arguments in patterns

Use a freshly generated WildCardParam name instead. This is necessary so that we can
pickle a reference to that symbol.

WildcardParam names no longer leak into error messages, unless -Yprint-debug is set.

Fixes #12169

Author: odersky

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -257,7 +257,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
     Text(lam.paramNames.lazyZip(lam.paramInfos).map(paramText), ", ")
   }
 
-  protected def ParamRefNameString(name: Name): String = name.toString
+  protected def ParamRefNameString(name: Name): String = nameString(name)
 
   protected def ParamRefNameString(param: ParamRef): String =
     ParamRefNameString(param.binder.paramNames(param.paramNum))

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -20,6 +20,7 @@ import typer.ProtoTypes._
 import Trees._
 import TypeApplications._
 import Decorators._
+import NameKinds.WildcardParamName
 import util.Chars.isOperatorPart
 import transform.TypeUtils._
 import transform.SymUtils._
@@ -76,8 +77,8 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
   }
 
   override def nameString(name: Name): String =
-    if ctx.settings.YdebugNames.value
-    then name.debugString
+    if ctx.settings.YdebugNames.value then name.debugString
+    else if name.isTypeName && name.is(WildcardParamName) && !printDebug then "_"
     else super.nameString(name)
 
   override protected def simpleNameString(sym: Symbol): String =
@@ -963,9 +964,6 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
   def optText[T >: Untyped](tree: List[Tree[T]])(encl: Text => Text): Text =
     if (tree.exists(!_.isEmpty)) encl(blockText(tree)) else ""
 
-  override protected def ParamRefNameString(name: Name): String =
-    name.toString
-
   override protected def treatAsTypeParam(sym: Symbol): Boolean = sym.is(TypeParam)
 
   override protected def treatAsTypeArg(sym: Symbol): Boolean =

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1560,9 +1560,9 @@ class Typer extends Namer
       super.transform(trt.withType(stripTypeVars(trt.tpe))) match {
         case b: Bind =>
           val sym = b.symbol
-          if (sym.name != tpnme.WILDCARD)
-            if (ctx.scope.lookup(b.name) == NoSymbol) ctx.enter(sym)
-            else report.error(new DuplicateBind(b, cdef), b.srcPos)
+          assert(sym.name != tpnme.WILDCARD)
+          if ctx.scope.lookup(b.name) == NoSymbol then ctx.enter(sym)
+          else report.error(new DuplicateBind(b, cdef), b.srcPos)
           if (!ctx.isAfterTyper) {
             val bounds = ctx.gadt.fullBounds(sym)
             if (bounds != null) sym.info = bounds
@@ -1969,8 +1969,9 @@ class Typer extends Namer
       //val ptt = if (lo.isEmpty && hi.isEmpty) pt else
       if (ctx.isAfterTyper) tree1
       else {
-        val wildcardSym = newPatternBoundSymbol(tpnme.WILDCARD, tree1.tpe & pt, tree.span)
-        untpd.Bind(tpnme.WILDCARD, tree1).withType(wildcardSym.typeRef)
+        val boundName = WildcardParamName.fresh().toTypeName
+        val wildcardSym = newPatternBoundSymbol(boundName, tree1.tpe & pt, tree.span)
+        untpd.Bind(boundName, tree1).withType(wildcardSym.typeRef)
       }
     else tree1
   }

tests/neg-custom-args/kind-projector.check

@@ -9,4 +9,4 @@
 -- Error: tests/neg-custom-args/kind-projector.scala:6:22 --------------------------------------------------------------
 6 |class Bar2 extends Foo[*] // error
   |                      ^
-  |                      Type argument _$4 does not have the same kind as its bound [_$1]
+  |                      Type argument _ does not have the same kind as its bound [_$1]

tests/neg/i4986c.check

@@ -61,4 +61,4 @@
 -- Error: tests/neg/i4986c.scala:62:19 ---------------------------------------------------------------------------------
 62 |  i.m[Option[Long]] // error
    |                   ^
-   |                   String; List; [A, _$6] =>> List[Option[?]]; Int; Option[Long]; 
+   |                   String; List; [A, _] =>> List[Option[?]]; Int; Option[Long]; 

tests/pos/i12169.scala

@@ -0,0 +1,13 @@
+class Property[T]
+
+class VObject {
+  def properties() = {
+    List.empty[Property[?]].collect {
+      case p: Property[?] => List(p)
+    }
+  }
+}
+
+class Event extends VObject {
+  override def properties() = ???
+}