Don't make selectors _i retractable

If we allow user-defined selectors _1, _2, and so on,
we break the link with pattern matching.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -20,13 +20,19 @@ object desugar {
   /** Info of a variable in a pattern: The named tree and its type */
   private type VarInfo = (NameTree, Tree)
 
-  /** Names of methods that are added unconditionally to case classes */
-  def isDesugaredCaseClassMethodName(name: Name)(implicit ctx: Context): Boolean = name match {
+  /** Is `name` the name of a method that can be invalidated as a compiler-generated
+   *  case class method that clashes with a user-defined method?
+   */
+  def isRetractableCaseClassMethodName(name: Name)(implicit ctx: Context): Boolean = name match {
     case nme.apply | nme.unapply | nme.copy => true
     case DefaultGetterName(nme.copy, _) => true
-    case _ => name.isSelectorName
+    case _ => false
   }
 
+  /** Is `name` the name of a method that is added unconditionally to case classes? */
+  def isDesugaredCaseClassMethodName(name: Name)(implicit ctx: Context): Boolean =
+    isRetractableCaseClassMethodName(name) || name.isSelectorName
+
 // ----- DerivedTypeTrees -----------------------------------
 
   class SetterParamTree extends DerivedTypeTree {

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

@@ -780,7 +780,7 @@ class Namer { typer: Typer =>
         }
       val isClashingSynthetic =
         denot.is(Synthetic) &&
-        desugar.isDesugaredCaseClassMethodName(denot.name) &&
+        desugar.isRetractableCaseClassMethodName(denot.name) &&
         isCaseClass(denot.owner) &&
         denot.owner.info.decls.lookupAll(denot.name).exists(alt =>
           alt != denot.symbol && alt.info.matchesLoosely(denot.info))

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

@@ -1415,7 +1415,7 @@ class Typer extends Namer
 
   def typedDefDef(ddef: untpd.DefDef, sym: Symbol)(implicit ctx: Context): Tree = track("typedDefDef") {
     if (!sym.info.exists) { // it's a discarded synthetic case class method, drop it
-      assert(sym.is(Synthetic) && desugar.isDesugaredCaseClassMethodName(sym.name))
+      assert(sym.is(Synthetic) && desugar.isRetractableCaseClassMethodName(sym.name))
       sym.owner.info.decls.openForMutations.unlink(sym)
       return EmptyTree
     }

tests/pos/i4564.scala

@@ -14,7 +14,6 @@ object ClashNoSig { // ok
   }
 }
 case class ClashNoSig private (x: Int) {
-  def _1: Int = x
   def copy(y: Int) = ClashNoSig(y)
 }