Option for testing for double bindings

A double binding is if a named type gets assigned two denotations in the same period. This is
a side-effect and also a race condition, so it's very bad. I am trying to eliminate all causes of this. But one cause which will likely remain are double defitions in a prgram, if a user writes

    class C {
      val x: Int
      val x: Int
    }

Then it's really hard to avoid setting two meanings of C.this.x! That's why the testing
against double bindings is enabled by a -YnoDoubleBindings option. The understanding is that
-YnoDoubleBindings should be set only if there are no double def errors anticipated. Otherwise
the program might fail with an assertion error before the double def error is reported.

Author: odersky

src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -163,6 +163,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val Ytyperdebug = BooleanSetting("-Ytyper-debug", "Trace all type assignments.")
   val Ypatmatdebug = BooleanSetting("-Ypatmat-debug", "Trace pattern matching translation.")
   val Yexplainlowlevel = BooleanSetting("-Yexplainlowlevel", "When explaining type errors, show types at a lower level.")
+  val YnoDoubleBindings = BooleanSetting("-YnoDoubleBindings", "Assert no namedtype is bound twice (should be enabled only if program is error-free).")
 
   val optimise = BooleanSetting("-optimise", "Generates faster bytecode by applying optimisations to the program") withAbbreviation "-optimize"
 

src/dotty/tools/dotc/core/Types.scala

@@ -1078,8 +1078,10 @@ object Types {
         this
       }
 
-    private[dotc] final def setDenot(denot: Denotation): Unit = {
-      if (Config.checkTermRefs) checkSymAssign(denot.symbol)
+    private[dotc] final def setDenot(denot: Denotation)(implicit ctx: Context): Unit = {
+      if (Config.checkTermRefs)
+        if (ctx.settings.YnoDoubleBindings.value)
+          checkSymAssign(denot.symbol)
       lastDenotation = denot
       lastSymbol = denot.symbol
     }
@@ -1092,8 +1094,14 @@ object Types {
         this
       }
 
-    private[dotc] final def setSym(sym: Symbol): Unit = {
-      if (Config.checkTermRefs) checkSymAssign(sym)
+    private[dotc] final def setSym(sym: Symbol)(implicit ctx: Context): Unit = {
+      if (Config.checkTermRefs)
+        if (ctx.settings.YnoDoubleBindings.value)
+          checkSymAssign(sym)
+      uncheckedSetSym(sym)
+    }
+
+    private[dotc] final def uncheckedSetSym(sym: Symbol): Unit = {
       lastDenotation = null
       lastSymbol = sym
       checkedPeriod = Nowhere
@@ -1238,7 +1246,7 @@ object Types {
   trait WithNonMemberSym extends NamedType {
     def fixedSym: Symbol
     assert(fixedSym ne NoSymbol)
-    setSym(fixedSym)
+    uncheckedSetSym(fixedSym)
 
     override def withDenot(denot: Denotation)(implicit ctx: Context): ThisType = {
       assert(denot.symbol eq fixedSym)

test/dotc/tests.scala

@@ -14,7 +14,7 @@ class tests extends CompilerTest {
 //        "-Yshow-suppressed-errors",
         "-pagewidth", "160"
     )
-  val twice = List("#runs", "2")
+  val twice = List("#runs", "2", "-YnoDoubleBindings")
   val doErase = List("-Ystop-before:terminal")
 
   val posDir = "./tests/pos/"
@@ -55,7 +55,7 @@ class tests extends CompilerTest {
   @Test def neg_typedapply() = compileFile(negDir, "typedapply", xerrors = 4)
   @Test def neg_typedidents() = compileFile(negDir, "typedIdents", xerrors = 2)
   @Test def neg_assignments() = compileFile(negDir, "assignments", xerrors = 3)
-  @Test def neg_typers() = compileFile(negDir, "typers", xerrors = 10)
+  @Test def neg_typers() = compileFile(negDir, "typers", xerrors = 13)
   @Test def neg_privates() = compileFile(negDir, "privates", xerrors = 2)
   @Test def neg_rootImports = compileFile(negDir, "rootImplicits", xerrors = 2)
   @Test def neg_templateParents() = compileFile(negDir, "templateParents", xerrors = 3)