Alternative fix for #10044

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -24,13 +24,8 @@ class Compiler {
    *  all refs to it would become outdated - they could not be dereferenced in the
    *  new phase.
    *
-   *  After erasure, signature changing denot-transformers are OK because erasure
-   *  will make sure that only term refs with fixed SymDenotations survive beyond it. This
-   *  is possible because:
-   *
-   *   - splitter has run, so every ident or select refers to a unique symbol
-   *   - after erasure, asSeenFrom is the identity, so every reference has a
-   *     plain SymDenotation, as opposed to a UniqueRefDenotation.
+   *  After erasure, signature changing denot-transformers are OK because signatures
+   *  are never recomputed later than erasure.
    */
   def phases: List[List[Phase]] =
     frontendPhases ::: picklerPhases ::: transformPhases ::: backendPhases

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

@@ -1079,17 +1079,13 @@ object Types {
      *  @param checkClassInfo if true we check that ClassInfos are within bounds of abstract types
      */
     final def overrides(that: Type, relaxedCheck: Boolean, matchLoosely: => Boolean, checkClassInfo: Boolean = true)(using Context): Boolean = {
-      def widenNullary(tp: Type) = tp match {
-        case tp @ MethodType(Nil) => tp.resultType
-        case _ => tp
-      }
       val overrideCtx = if relaxedCheck then ctx.relaxedOverrideContext else ctx
       inContext(overrideCtx) {
         !checkClassInfo && this.isInstanceOf[ClassInfo]
         || (this.widenExpr frozen_<:< that.widenExpr)
         || matchLoosely && {
-            val this1 = widenNullary(this)
-            val that1 = widenNullary(that)
+            val this1 = this.widenNullaryMethod
+            val that1 = that.widenNullaryMethod
             ((this1 `ne` this) || (that1 `ne` that)) && this1.overrides(that1, relaxedCheck, false, checkClassInfo)
           }
       }
@@ -1326,6 +1322,11 @@ object Types {
         this
     }
 
+    /** If this is a nullary method type, its result type */
+    def widenNullaryMethod(using Context): Type = this match
+      case tp @ MethodType(Nil) => tp.resType
+      case _ => this
+
     /** The singleton types that must or may be in this type. @see Atoms.
      *  Overridden and cached in OrType.
      */
@@ -2561,7 +2562,7 @@ object Types {
      *  A test case is neg/opaque-self-encoding.scala.
      */
     final def withDenot(denot: Denotation)(using Context): ThisType =
-      if (denot.exists) {
+      if denot.exists then
         val adapted = withSym(denot.symbol)
         val result =
           if (adapted.eq(this)
@@ -2570,9 +2571,25 @@ object Types {
               || adapted.info.eq(denot.info))
             adapted
           else this
-        result.setDenot(denot)
+        val lastDenot = result.lastDenotation
+        denot match
+          case denot: SymDenotation
+          if denot.validFor.firstPhaseId < ctx.phase.id
+            && lastDenot != null
+            && lastDenot.validFor.lastPhaseId > denot.validFor.firstPhaseId
+            && !lastDenot.isInstanceOf[SymDenotation] =>
+            // In this case the new SymDenotation might be valid for all phases, which means
+            // we would not recompute the denotation when travelling to an earlier phase, maybe
+            // in the next run. We fix that problem by creating a UniqueRefDenotation instead.
+            core.println(i"overwrite ${result.toString} / ${result.lastDenotation}, ${result.lastDenotation.getClass} with $denot at ${ctx.phaseId}")
+            result.setDenot(
+              UniqueRefDenotation(
+                denot.symbol, denot.info,
+                Period(ctx.runId, ctx.phaseId, denot.validFor.lastPhaseId),
+                this.prefix))
+          case _ =>
+            result.setDenot(denot)
         result.asInstanceOf[ThisType]
-      }
       else // don't assign NoDenotation, we might need to recover later. Test case is pos/avoid.scala.
         this
 

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

@@ -189,7 +189,9 @@ object GenericSignatures {
             fullNameInSig(tp.typeSymbol)
             builder.append(';')
           case _ =>
-            boxedSig(tp)
+            boxedSig(tp.widenDealias.widenNullaryMethod)
+              // `tp` might be a singleton type referring to a getter.
+              // Hence the widenNullaryMethod.
         }
 
       if (pre.exists) {

compiler/test-resources/repl/i10044

@@ -0,0 +1,12 @@
+scala> object Foo { opaque type Bar = Int; object Bar { extension (b: Bar) def flip: Bar = -b; def apply(x: Int): Bar = x }}
+// defined object Foo
+scala> val a = Foo.Bar(42)
+val a: Foo.Bar = 42
+scala> val b = a.flip
+val b: Foo.Bar = -42
+scala> val c = b.flip
+val c: Foo.Bar = 42
+scala> val d = c.flip
+val d: Foo.Bar = -42
+scala> val e = d.flip
+val e: Foo.Bar = 42