Allow ContextBoundParamNames to be unmangled.

Also, fix the unmangling of UniqueExtNames, which seemingly never worked.

Author: odersky

compiler/src/dotty/tools/dotc/core/NameKinds.scala

@@ -182,13 +182,13 @@ object NameKinds {
       case DerivedName(underlying, info: this.NumberedInfo) => Some((underlying, info.num))
       case _ => None
     }
-    protected def skipSeparatorAndNum(name: SimpleName, separator: String): Int = {
+    protected def skipSeparatorAndNum(name: SimpleName, separator: String): Int =
       var i = name.length
-      while (i > 0 && name(i - 1).isDigit) i -= 1
-      if (i > separator.length && i < name.length &&
-          name.slice(i - separator.length, i).toString == separator) i
+      while i > 0 && name(i - 1).isDigit do i -= 1
+      if i >= separator.length && i < name.length
+          && name.slice(i - separator.length, i).toString == separator
+      then i
       else -1
-    }
 
     numberedNameKinds(tag) = this: @unchecked
   }
@@ -240,6 +240,16 @@ object NameKinds {
     }
   }
 
+  /** Unique names that can be unmangled */
+  class UniqueNameKindWithUnmangle(separator: String) extends UniqueNameKind(separator):
+    override def unmangle(name: SimpleName): TermName =
+      val i = skipSeparatorAndNum(name, separator)
+      if i > 0 then
+        val index = name.drop(i).toString.toInt
+        val original = name.take(i - separator.length).asTermName
+        apply(original, index)
+      else name
+
   /** Names of the form `prefix . name` */
   val QualifiedName: QualifiedNameKind = new QualifiedNameKind(QUALIFIED, ".")
 
@@ -288,7 +298,7 @@ object NameKinds {
    *
    *  The "evidence$" prefix is a convention copied from Scala 2.
    */
-  val ContextBoundParamName: UniqueNameKind    = new UniqueNameKind("evidence$")
+  val ContextBoundParamName: UniqueNameKind = new UniqueNameKindWithUnmangle("evidence$")
 
   /** The name of an inferred contextual function parameter:
    *
@@ -323,20 +333,7 @@ object NameKinds {
   val InlineBinderName: UniqueNameKind       = new UniqueNameKind("$proxy")
   val MacroNames: UniqueNameKind             = new UniqueNameKind("$macro$")
 
-  /** A kind of unique extension methods; Unlike other unique names, these can be
-   *  unmangled.
-   */
-  val UniqueExtMethName: UniqueNameKind = new UniqueNameKind("$extension") {
-    override def unmangle(name: SimpleName): TermName = {
-      val i = skipSeparatorAndNum(name, separator)
-      if (i > 0) {
-        val index = name.drop(i).toString.toInt
-        val original = name.take(i - separator.length).asTermName
-        apply(original, index)
-      }
-      else name
-    }
-  }
+  val UniqueExtMethName: UniqueNameKind = new UniqueNameKindWithUnmangle("$extension")
 
   /** Kinds of unique names generated by the pattern matcher */
   val PatMatStdBinderName: UniqueNameKind    = new UniqueNameKind("x")