Fix validity period of derived SingleDenotations

When running:

    val f: ( => Int) => Int = i => i ; f(1)

twice in the REPL, the second time crashed with a ClassCastException.

The difference is that in the second run, the denotation for `f.apply` is
created from the SymDenotation for `Function1#apply` which already exists and is
known to be valid in every phase, but that doesn't mean that the derived
denotation for `f.apply` has the same validity: unlike the SymDenotation it
needs to be transformed (in particular to run the `ElimByName` transformer).

It turns out that there were multiple places in the compiler where we created a
new denotation with a validity based on the existing one when this was not
legitimate. I've gone through all these places and replaced them by
`currentStablePeriod`.

Fixes #18756.

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

@@ -580,7 +580,7 @@ object Denotations {
 
   /** A non-overloaded denotation */
   abstract class SingleDenotation(symbol: Symbol, initInfo: Type, isType: Boolean) extends Denotation(symbol, initInfo, isType) {
-    protected def newLikeThis(symbol: Symbol, info: Type, pre: Type, isRefinedMethod: Boolean): SingleDenotation
+    protected def newLikeThis(symbol: Symbol, info: Type, pre: Type, isRefinedMethod: Boolean)(using Context): SingleDenotation
 
     final def name(using Context): Name = symbol.name
 
@@ -1104,7 +1104,9 @@ object Denotations {
            && (derivedInfo eq info) && !needsPrefix then
           this
         else
-          derivedSingleDenotation(symbol, derivedInfo, pre)
+          val d = derivedSingleDenotation(symbol, derivedInfo, pre)
+          d.validFor = currentStablePeriod
+          d
       end derived
 
       // Tt could happen that we see the symbol with prefix `this` as a member a different class
@@ -1162,11 +1164,11 @@ object Denotations {
     prefix: Type) extends NonSymSingleDenotation(symbol, initInfo, prefix) {
     validFor = initValidFor
     override def hasUniqueSym: Boolean = true
-    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean): SingleDenotation =
+    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean)(using Context): SingleDenotation =
       if isRefinedMethod then
-        new JointRefDenotation(s, i, validFor, pre, isRefinedMethod)
+        new JointRefDenotation(s, i, currentStablePeriod, pre, isRefinedMethod)
       else
-        new UniqueRefDenotation(s, i, validFor, pre)
+        new UniqueRefDenotation(s, i, currentStablePeriod, pre)
   }
 
   class JointRefDenotation(
@@ -1177,15 +1179,15 @@ object Denotations {
     override val isRefinedMethod: Boolean) extends NonSymSingleDenotation(symbol, initInfo, prefix) {
     validFor = initValidFor
     override def hasUniqueSym: Boolean = false
-    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean): SingleDenotation =
-      new JointRefDenotation(s, i, validFor, pre, isRefinedMethod)
+    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean)(using Context): SingleDenotation =
+      new JointRefDenotation(s, i, currentStablePeriod, pre, isRefinedMethod)
   }
 
   class ErrorDenotation(using Context) extends NonSymSingleDenotation(NoSymbol, NoType, NoType) {
     override def exists: Boolean = false
     override def hasUniqueSym: Boolean = false
     validFor = Period.allInRun(ctx.runId)
-    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean): SingleDenotation =
+    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean)(using Context): SingleDenotation =
       this
   }
 

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

@@ -1624,11 +1624,11 @@ object SymDenotations {
 
     // ----- copies and transforms  ----------------------------------------
 
-    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean): SingleDenotation =
+    protected def newLikeThis(s: Symbol, i: Type, pre: Type, isRefinedMethod: Boolean)(using Context): SingleDenotation =
       if isRefinedMethod then
-        new JointRefDenotation(s, i, validFor, pre, isRefinedMethod)
+        new JointRefDenotation(s, i, currentStablePeriod, pre, isRefinedMethod)
       else
-        new UniqueRefDenotation(s, i, validFor, pre)
+        new UniqueRefDenotation(s, i, currentStablePeriod, pre)
 
     /** Copy this denotation, overriding selective fields */
     final def copySymDenotation(

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

@@ -905,7 +905,7 @@ object Types extends TypeUtils {
       def goSuper(tp: SuperType) = go(tp.underlying) match {
         case d: JointRefDenotation =>
           typr.println(i"redirecting super.$name from $tp to ${d.symbol.showLocated}")
-          new UniqueRefDenotation(d.symbol, tp.memberInfo(d.symbol), d.validFor, pre)
+          new UniqueRefDenotation(d.symbol, tp.memberInfo(d.symbol), currentStablePeriod, pre)
         case d => d
       }
 

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

@@ -13,6 +13,7 @@ import core.Types.*
 import core.Names.*
 import core.StdNames.*
 import core.NameOps.*
+import core.Periods.currentStablePeriod
 import core.NameKinds.{AdaptedClosureName, BodyRetainerName, DirectMethName}
 import core.Scopes.newScopeWith
 import core.Decorators.*
@@ -132,7 +133,7 @@ class Erasure extends Phase with DenotTransformer {
       }
     case ref: JointRefDenotation =>
       new UniqueRefDenotation(
-        ref.symbol, transformInfo(ref.symbol, ref.symbol.info), ref.validFor, ref.prefix)
+        ref.symbol, transformInfo(ref.symbol, ref.symbol.info), currentStablePeriod, ref.prefix)
     case _ =>
       ref.derivedSingleDenotation(ref.symbol, transformInfo(ref.symbol, ref.symbol.info))
   }

compiler/test-resources/repl/i18756

@@ -0,0 +1,17 @@
+scala> object A { val f: ( => Int) => Int = i => i ; f(1) }
+// defined object A
+
+scala> A.f(1)
+val res0: Int = 1
+
+scala> A.f(1)
+val res1: Int = 1
+
+scala> object B { val f: ( => Int) => Int = i => i ; f(1) }
+// defined object B
+
+scala> B.f(1)
+val res2: Int = 1
+
+scala> B.f(1)
+val res3: Int = 1