Fix REPL completion of symbolic identifiers

Recover part of the identifier that preceded the cursor from the
source, rather than from the name in the `Select` node, which might
contains an encoded name that differs in length from the one in
source.

Author: retronym

src/interactive/scala/tools/nsc/interactive/Global.scala

@@ -1223,9 +1223,8 @@ class Global(settings: Settings, _reporter: Reporter, projectName: String = "")
     def typeCompletions(tree: Tree, qual: Tree, nameStart: Int, name: Name): CompletionResult = {
       val qualPos = qual.pos
       val allTypeMembers = typeMembers(qualPos).toList.flatten
-
       val positionDelta: Int = nameStart - pos.start
-      val subName = name.subName(0, -positionDelta)
+      val subName: Name = name.newName(new String(pos.source.content, nameStart, pos.start - nameStart)).encodedName
       CompletionResult.TypeMembers(positionDelta, qual, tree, allTypeMembers, subName)
     }
     focus1 match {

test/junit/scala/tools/nsc/interpreter/CompletionTest.scala

@@ -48,7 +48,14 @@ class CompletionTest {
     checkExact(completer, """class C { asInstanceO""")("asInstanceOf")
 
     // Output is sorted
-    assertEquals(List("prefix_aaa", "prefix_nnn", "prefix_zzz"), completer.complete("""class C { def prefix_nnn = 0; def prefix_zzz = 0; def prefix_aaa = 0; prefix_""").candidates)
+    assertEquals(List("prefix_aaa", "prefix_nnn", "prefix_zzz"), completer.complete( """class C { def prefix_nnn = 0; def prefix_zzz = 0; def prefix_aaa = 0; prefix_""").candidates)
+  }
+
+  @Test
+  def symbolically(): Unit = {
+    val intp = newIMain()
+    val completer = new PresentationCompilerCompleter(intp)
+    checkExact(completer, """class C { def +++(a: Any) = 0; def ---(a: Any) = 0; this.++""")("+++")
   }
 
   @Test