Fix a REPL bad symbolic reference (#19786)

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

@@ -110,17 +110,28 @@ object Symbols extends SymUtils {
     }
 
     private def computeDenot(lastd: SymDenotation)(using Context): SymDenotation = {
+      // Written that way so that it comes in at 32 bytes and is therefore inlineable for
+      // the JIT (reputedly, cutoff is at 35 bytes)
       util.Stats.record("Symbol.computeDenot")
       val now = ctx.period
       checkedPeriod = now
-      if (lastd.validFor contains now) lastd else recomputeDenot(lastd)
+      if lastd.validFor.contains(now) then lastd else recomputeDenot(lastd)
     }
 
     /** Overridden in NoSymbol */
     protected def recomputeDenot(lastd: SymDenotation)(using Context): SymDenotation = {
       util.Stats.record("Symbol.recomputeDenot")
       val newd = lastd.current.asInstanceOf[SymDenotation]
-      lastDenot = newd
+      if newd.exists || lastd.initial.validFor.firstPhaseId <= ctx.phaseId then
+        lastDenot = newd
+      else
+        // We are trying to bring forward a symbol that is defined only at a later phase
+        // (typically, a nested Java class, invisible before erasure).
+        // In that case, keep lastDenot as it was and set the checked period to lastDenot's
+        // previous validity, which means we will try another bring forward when the symbol
+        // is referenced at a later phase. Otherwise we'd get stuck on NoDenotation here.
+        // See #15562 and test i15562b in ReplCompilerTests
+        checkedPeriod = lastd.validFor
       newd
     }
 
@@ -791,7 +802,7 @@ object Symbols extends SymUtils {
       cls: ClassSymbol,
       name: TermName = nme.WILDCARD,
       selfInfo: Type = NoType)(using Context): TermSymbol =
-    newSymbol(cls, name, SelfSymFlags, selfInfo orElse cls.classInfo.selfType, coord = cls.coord)
+    newSymbol(cls, name, SelfSymFlags, selfInfo.orElse(cls.classInfo.selfType), coord = cls.coord)
 
   /** Create new type parameters with given owner, names, and flags.
    *  @param boundsFn  A function that, given type refs to the newly created
@@ -958,7 +969,7 @@ object Symbols extends SymUtils {
    */
   def getPackageClassIfDefined(path: PreName)(using Context): Symbol =
     staticRef(path.toTypeName, isPackage = true, generateStubs = false)
-      .disambiguate(_ is PackageClass).symbol
+      .disambiguate(_.is(PackageClass)).symbol
 
   def requiredModule(path: PreName)(using Context): TermSymbol = {
     val name = path.toTermName

compiler/test/dotty/tools/repl/ReplCompilerTests.scala

@@ -409,6 +409,34 @@ class ReplCompilerTests extends ReplTest:
   @Test def `i13097 expect template after colon` = contextually:
     assert(ParseResult.isIncomplete("class C:"))
 
+  @Test def i15562: Unit = initially {
+    val s1 = run("List(1, 2).filter(_ % 2 == 0).foreach(println)")
+    assertEquals("2", storedOutput().trim)
+    s1
+  } andThen { s1 ?=>
+    val comp = tabComplete("List(1, 2).filter(_ % 2 == 0).fore")
+    assertEquals(List("foreach"), comp.distinct)
+    s1
+  } andThen {
+    val s2 = run("List(1, 2).filter(_ % 2 == 0).foreach(println)")
+    assertEquals("2", storedOutput().trim)
+    s2
+  }
+
+  @Test def i15562b: Unit = initially {
+    val s1 = run("List(1, 2).filter(_ % 2 == 0).foreach(println)")
+    assertEquals("2", storedOutput().trim)
+    s1
+  } andThen { s1 ?=>
+    val comp = tabComplete("val x = false + true; List(1, 2).filter(_ % 2 == 0).fore")
+    assertEquals(List("foreach"), comp.distinct)
+    s1
+  } andThen {
+    val s2 = run("List(1, 2).filter(_ % 2 == 0).foreach(println)")
+    assertEquals("2", storedOutput().trim)
+    s2
+  }
+
 object ReplCompilerTests:
 
   private val pattern = Pattern.compile("\\r[\\n]?|\\n");

compiler/test/dotty/tools/repl/ReplTest.scala

@@ -40,6 +40,10 @@ extends ReplDriver(options, new PrintStream(out, true, StandardCharsets.UTF_8.na
 
   def contextually[A](op: Context ?=> A): A = op(using initialState.context)
 
+  /** Returns the `(<instance completions>, <companion completions>)`*/
+  def tabComplete(src: String)(implicit state: State): List[String] =
+    completions(src.length, src, state).map(_.value).sorted
+
   extension [A](state: State)
     infix def andThen(op: State ?=> A): A = op(using state)
 

compiler/test/dotty/tools/repl/TabcompleteTests.scala

@@ -8,10 +8,6 @@ import org.junit.Test
 /** These tests test input that has proved problematic */
 class TabcompleteTests extends ReplTest {
 
-  /** Returns the `(<instance completions>, <companion completions>)`*/
-  private def tabComplete(src: String)(implicit state: State): List[String] =
-    completions(src.length, src, state).map(_.value).sorted
-
   @Test def tabCompleteList = initially {
     val comp = tabComplete("List.r")
     assertEquals(List("range"), comp.distinct)