Address reviewers comments

Author: odersky

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -45,7 +45,12 @@ class PatternMatcher extends MiniPhaseTransform {
 object PatternMatcher {
   import ast.tpd._
 
-  final val selfCheck = true // debug option, if on we check that no case gets generated twice
+  final val selfCheck = false // debug option, if on we check that no case gets generated twice
+
+  /** Was symbol generated by pattern matcher? */
+  def isPatmatGenerated(sym: Symbol)(implicit ctx: Context): Boolean =
+    sym.is(Synthetic) &&
+    (sym.name.is(PatMatStdBinderName) || sym.name.is(PatMatCaseName))
 
   /** The pattern matching translator.
    *  Its general structure is a pipeline:
@@ -77,6 +82,7 @@ object PatternMatcher {
     private def newVar(rhs: Tree, flags: FlagSet): TermSymbol =
       ctx.newSymbol(ctx.owner, PatMatStdBinderName.fresh(), Synthetic | Case | flags,
         sanitize(rhs.tpe), coord = rhs.pos)
+        // TODO: Drop Case once we use everywhere else `isPatmatGenerated`.
 
     /** The plan `let x = rhs in body(x)` where `x` is a fresh variable */
     private def letAbstract(rhs: Tree)(body: Symbol => Plan): Plan = {
@@ -93,11 +99,6 @@ object PatternMatcher {
       LabelledPlan(label, body(CallPlan(label, Nil)), Nil)
     }
 
-    /** Was symbol generated by pattern matcher? */
-    private def isPatmatGenerated(sym: Symbol) =
-      sym.is(Synthetic) &&
-      (sym.is(Label) || sym.name.is(PatMatStdBinderName))
-
     /** Test whether a type refers to a pattern-generated variable */
     private val refersToInternal = new TypeAccumulator[Boolean] {
       def apply(x: Boolean, tp: Type) =
@@ -687,7 +688,11 @@ object PatternMatcher {
       val LetPlan(topSym, _) = plan
 
       def toDrop(sym: Symbol) =
-        initializer.contains(sym) && isPatmatGenerated(sym) && refCount(sym) <= 1 && sym != topSym
+        initializer.contains(sym) &&
+        isPatmatGenerated(sym) &&
+        refCount(sym) <= 1 &&
+        sym != topSym &&
+        isPureExpr(initializer(sym))
 
       object Inliner extends PlanTransform {
         override val treeMap = new TreeMap {