Report an error if toplevel denotations come from multiple files

Report an error if toplevel denotations the same name with come
from multiple files that have the same modification date.

The error was reported in some runs for toplevel/stale when run locally
since the files for A_1.scala and B_2.scala were generated too fast so they
ended up with the same modification date. B_2.scala has now been beefed
up to give the compiler more work to do, so that the race is avoided.

Author: odersky

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

@@ -2147,17 +2147,30 @@ object SymDenotations {
             case d => d
       }
 
-      def dropStale(d: DenotUnion): PreDenotation =
-        val compiledNow = d.filterWithPredicate(d =>
+      def dropStale(multi: DenotUnion): PreDenotation =
+        val compiledNow = multi.filterWithPredicate(d =>
           d.symbol.isDefinedInCurrentRun || d.symbol.associatedFile == null
             // if a symbol does not have an associated file, assume it is defined
             // in the current run anyway. This is true for packages, and also can happen for pickling and
             // from-tasty tests that generate a fresh symbol and then re-use it in the next run.
           )
         if compiledNow.exists then compiledNow
         else
-          val youngest = d.aggregate(_.symbol.associatedFile.lastModified, _ max _)
-          d.filterWithPredicate(_.symbol.associatedFile.lastModified == youngest)
+          val assocFiles = multi.aggregate(d => Set(d.symbol.associatedFile), _ union _)
+          if assocFiles.size == 1 then
+            multi // they are all overloaded variants from the same file
+          else
+            // pick the variant(s) from the youngest class file
+            val lastModDate = assocFiles.map(_.lastModified).max
+            val youngest = assocFiles.filter(_.lastModified == lastModDate)
+            if youngest.size > 1 then
+              throw TypeError(em"""Toplevel definition $name is defined in
+                                  |  ${youngest.head}
+                                  |and also in
+                                  |  ${youngest.tail.head}
+                                  |One of these files should be removed from the classpath.""")
+            multi.filterWithPredicate(_.symbol.associatedFile == youngest.head)
+      end dropStale
 
       if (symbol `eq` defn.ScalaPackageClass) {
         val denots = super.computeNPMembersNamed(name)

tests/run/toplevel-stale/A_1.scala

@@ -1 +1 @@
-def foo(x: Int): String = "old"
+def foo234(x: Int): String = "old"

tests/run/toplevel-stale/B_2.scala

@@ -1 +1,38 @@
-def foo(x: Long): String = "new"
+def foo234(x: Long): String = "new"
+
+// Giving the compiler something to do so that we won't get
+// the same modification date as A_1.scala
+import math.Ordering
+
+val y1 = {
+  val x1 = summon[Ordering[Int]]
+  val x2 = summon[Ordering[(Int, Int)]]
+  val x3 = summon[Ordering[(Int, Int, Int)]]
+  val x4 = summon[Ordering[(Int, Int, Int, Int)]]
+  val x5 = summon[Ordering[(Int, Int, Int, Int, Int)]]
+  val x6 = summon[Ordering[(Int, Int, Int, Int, Int, Int)]]
+  val x7 = summon[Ordering[(Int, Int, Int, Int, Int, Int, Int)]]
+  val x8 = summon[Ordering[(Int, Int, Int, Int, Int, Int, Int, Int)]]
+}
+
+val y2 = {
+  val x1 = summon[Ordering[Int]]
+  val x2 = summon[Ordering[(Int, Int)]]
+  val x3 = summon[Ordering[(Int, Int, Int)]]
+  val x4 = summon[Ordering[(Int, Int, Int, Int)]]
+  val x5 = summon[Ordering[(Int, Int, Int, Int, Int)]]
+  val x6 = summon[Ordering[(Int, Int, Int, Int, Int, Int)]]
+  val x7 = summon[Ordering[(Int, Int, Int, Int, Int, Int, Int)]]
+  val x8 = summon[Ordering[(Int, Int, Int, Int, Int, Int, Int, Int)]]
+}
+
+val y3 = {
+  val x1 = summon[Ordering[Int]]
+  val x2 = summon[Ordering[(Int, Int)]]
+  val x3 = summon[Ordering[(Int, Int, Int)]]
+  val x4 = summon[Ordering[(Int, Int, Int, Int)]]
+  val x5 = summon[Ordering[(Int, Int, Int, Int, Int)]]
+  val x6 = summon[Ordering[(Int, Int, Int, Int, Int, Int)]]
+  val x7 = summon[Ordering[(Int, Int, Int, Int, Int, Int, Int)]]
+  val x8 = summon[Ordering[(Int, Int, Int, Int, Int, Int, Int, Int)]]
+}

tests/run/toplevel-stale/Test_3.scala

@@ -1,2 +1,2 @@
 @main def Test() =
-  assert(foo(1) == "new")
+  assert(foo234(1) == "new")