Allow multidenotations with same signature

Instead of issuing a merge error, allow multi-denotations with alternatives that have the same
signature.

This does not work yet for unpickling.

Author: odersky

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

@@ -407,7 +407,7 @@ object Denotations {
       }
 
       /** Try to merge single-denotations. */
-      def mergeSingleDenot(denot1: SingleDenotation, denot2: SingleDenotation): SingleDenotation = {
+      def mergeSingleDenot(denot1: SingleDenotation, denot2: SingleDenotation): Denotation = {
         val info1 = denot1.info
         val info2 = denot2.info
         val sym1 = denot1.symbol
@@ -465,9 +465,10 @@ object Denotations {
           preferSym(sym1, sym2) &&
           info1.overrides(info2, sym1.matchNullaryLoosely || sym2.matchNullaryLoosely, checkClassInfo = false)
 
-        def handleDoubleDef =
+        def handleDoubleDef: Denotation =
           if (preferSym(sym1, sym2)) denot1
           else if (preferSym(sym2, sym1)) denot2
+          else if sym1.exists then MultiDenotation(denot1, denot2)
           else doubleDefError(denot1, denot2, pre)
 
         if (sym2Accessible && prefer(sym2, sym1, info2, info1)) denot2

compiler/test/dotc/pos-from-tasty.blacklist

@@ -13,3 +13,5 @@ i7087.scala
 # Nullability
 nullable.scala
 notNull.scala
+
+i9050.scala

compiler/test/dotc/pos-test-pickling.blacklist

@@ -43,3 +43,5 @@ i7757.scala
 
 # splice type tag dealiased in this reference
 i8651b.scala
+
+i9050.scala

tests/neg/i1240b.scala

@@ -9,4 +9,4 @@ abstract class A[X] extends T[X] {
 trait U[X] extends T[X] {
   abstract override def foo(x: X): X = super.foo(x)
 }
-object Test extends A[String] with U[String] // error: accidental override // error: merge error
+object Test extends A[String] with U[String] // error: accidental override

tests/pos/i9050.scala

@@ -0,0 +1,6 @@
+
+object Foo {
+  val foo = scala.collection.mutable.ArrayBuffer.empty[Seq[Double]]
+  val bar = Seq.empty[Double]
+  foo.append(bar)
+}