Fix scala#5895: REPL autocompletion crushes in certain cases

The crashes happen due to the fact that in certain cases,
certain trees get mispositioned during code completion. E.g.
this happens with `opaque type T = Int` or `object Foo { type T = Int`.
The tree spans are set incorrectly because repl doesn't set the
sources of these trees correctly. Precisely, when typechecking
on code completion, a virtual source is used. However, when
parsing for autocompletion, a line source is used which is
created by the parsing logic. This commit makes sure that REPL
is consistent in its source choices when computing code completions.

Author: anatoliykmetyuk

compiler/src/dotty/tools/repl/ParseResult.scala

@@ -116,7 +116,8 @@ object ParseResult {
   }
 
   /** Extract a `ParseResult` from the string `sourceCode` */
-  def apply(sourceCode: String)(implicit state: State): ParseResult =
+  def apply(sourceCodeOrFile: Either[String, SourceFile])(implicit state: State): ParseResult = {
+    val sourceCode = sourceCodeOrFile.map(_.content().mkString).merge
     sourceCode match {
       case "" => Newline
       case CommandExtract(cmd, arg) => cmd match {
@@ -132,7 +133,7 @@ object ParseResult {
       case _ =>
         implicit val ctx: Context = state.context
 
-        val source = SourceFile.virtual(str.REPL_SESSION_LINE + (state.objectIndex + 1), sourceCode)
+        val source = sourceCodeOrFile.toOption.getOrElse(SourceFile.virtual(str.REPL_SESSION_LINE + (state.objectIndex + 1), sourceCode))
 
         val reporter = newStoreReporter
         val stats = parseStats(sourceCode)(state.context.fresh.setReporter(reporter).withSource(source))
@@ -145,6 +146,10 @@ object ParseResult {
         else
           Parsed(source, stats)
     }
+  }
+
+  def apply(sourceFile: SourceFile)(implicit state: State): ParseResult = apply(Right(sourceFile))
+  def apply(sourceCode: String)(implicit state: State): ParseResult = apply(Left(sourceCode))
 
   /** Check if the input is incomplete
    *

compiler/src/dotty/tools/repl/ReplCompiler.scala

@@ -241,7 +241,7 @@ class ReplCompiler extends Compiler {
         PackageDef(Ident(nme.EMPTY_PACKAGE), List(wrapper))
       }
 
-      ParseResult(expr)(state) match {
+      ParseResult(sourceFile)(state) match {
         case Parsed(_, trees) =>
           wrap(trees).result
         case SyntaxErrors(_, reported, trees) =>

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

@@ -124,4 +124,8 @@ class TabcompleteTests extends ReplTest {
     val comp = tabComplete("???.")
     assertEquals(Nil, comp)
   }
+
+  @Test def moduleCompletion = fromInitialState { implicit s =>
+    assertEquals(List("Predef"), tabComplete("object Foo { type T = Pre"))
+  }
 }