Avoid matches against NoSymbol

Author: odersky

compiler/src/dotty/tools/backend/jvm/BCodeBodyBuilder.scala

@@ -1213,10 +1213,11 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
     def genLoadModule(tree: Tree): BType = {
       val module = (
         if (!tree.symbol.is(PackageClass)) tree.symbol
-        else tree.symbol.info.member(nme.PACKAGE).symbol match {
-          case NoSymbol => abort(s"SI-5604: Cannot use package as value: $tree")
-          case s        => abort(s"SI-5604: found package class where package object expected: $tree")
-        }
+        else
+          val s = tree.symbol.info.member(nme.PACKAGE).symbol
+          if s.exists
+          then abort(s"SI-5604: found package class where package object expected: $tree")
+          else abort(s"SI-5604: Cannot use package as value: $tree")
       )
       lineNumber(tree)
       genLoadModule(module)

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

@@ -428,9 +428,8 @@ object Comments {
      *  @param vble  The variable for which a definition is searched
      *  @param site  The class for which doc comments are generated
      */
-    def lookupVariable(vble: String, site: Symbol)(using Context): Option[String] = site match {
-      case NoSymbol => None
-      case _        =>
+    def lookupVariable(vble: String, site: Symbol)(using Context): Option[String] =
+      if site.exists then
         val searchList =
           if (site.flags.is(Flags.Module)) site :: site.info.baseClasses
           else site.info.baseClasses
@@ -439,7 +438,7 @@ object Comments {
           case Some(str) if str startsWith "$" => lookupVariable(str.tail, site)
           case res                             => res orElse lookupVariable(vble, site.owner)
         }
-    }
+      else None
 
     /** The position of the raw doc comment of symbol `sym`, or NoPosition if missing
      *  If a symbol does not have a doc comment but some overridden version of it does,

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

@@ -249,9 +249,9 @@ sealed trait GadtConstraint (
   // ---- Private ----------------------------------------------------------
 
   private def externalize(tp: Type, theMap: TypeMap | Null = null)(using Context): Type = tp match
-    case param: TypeParamRef => reverseMapping(param) match
-      case sym: Symbol => sym.typeRef
-      case null        => param
+    case param: TypeParamRef =>
+      val sym = reverseMapping(param)
+      if sym != null then sym.typeRef else param
     case tp: TypeAlias       => tp.derivedAlias(externalize(tp.alias, theMap))
     case tp                  => (if theMap == null then ExternalizeMap() else theMap).mapOver(tp)
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -712,9 +712,9 @@ extends SyntaxMsg(WrongNumberOfTypeArgsID) {
       .mkString("[", ", ", "]")
     val actualArgString = actual.map(_.show).mkString("[", ", ", "]")
     val prettyName =
-      try fntpe.termSymbol match
-        case NoSymbol => fntpe.show
-        case symbol   => symbol.showFullName
+      try
+        val symbol = fntpe.termSymbol
+        if symbol.exists then symbol.showFullName else fntpe.show
       catch case NonFatal(ex) => fntpe.show
     i"""|$msgPrefix type arguments for $prettyName$expectedArgString
         |expected: $expectedArgString