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.

1 files changed, 13 insertions, 5 deletions

diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 7561abb10..1dba728bb 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/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)