Fix #8762: Make REPL work with indented code

This is great even if we do not use optional braces. For instance, now you can write:
```scala
scala> if c == 1 then
  "yes"
else
  "no"
```
Previously, it would have stopped you after the "yes".

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1288,7 +1288,7 @@ object Parsers {
       if in.token == COLONEOL then
         in.nextToken()
         if in.token != INDENT then
-          syntaxError(i"indented definitions expected")
+          syntaxErrorOrIncomplete(i"indented definitions expected")
       else
         newLineOptWhenFollowedBy(LBRACE)
 

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -581,7 +581,7 @@ object Scanners {
     def observeColonEOL(): Unit =
       if token == COLON then
         lookahead()
-        val atEOL = isAfterLineEnd
+        val atEOL = isAfterLineEnd || token == EOF
         reset()
         if atEOL then token = COLONEOL
 
@@ -618,6 +618,12 @@ object Scanners {
       this.copyFrom(prev)
     }
 
+    def closeIndented() = currentRegion match
+      case r: Indented if !r.isOutermost =>
+        insert(OUTDENT, offset)
+        currentRegion = r.outer
+      case _ =>
+
     /** - Join CASE + CLASS => CASECLASS, CASE + OBJECT => CASEOBJECT, SEMI + ELSE => ELSE, COLON + <EOL> => COLONEOL
      *  - Insert missing OUTDENTs at EOF
      */
@@ -658,13 +664,10 @@ object Scanners {
                 reset()
         case COLON =>
           if colonSyntax then observeColonEOL()
-        case EOF | RBRACE | RPAREN | RBRACKET =>
-          currentRegion match {
-            case r: Indented if !r.isOutermost =>
-              insert(OUTDENT, offset)
-              currentRegion = r.outer
-            case _ =>
-          }
+        case RBRACE | RPAREN | RBRACKET =>
+          closeIndented()
+        case EOF if !source.maybeIncomplete =>
+          closeIndented()
         case _ =>
       }
     }

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -48,6 +48,10 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
     myContent
   }
 
+  private var _maybeInComplete: Boolean = false
+  
+  def maybeIncomplete: Boolean = _maybeInComplete
+
   def this(file: AbstractFile, codec: Codec) = this(file, new String(file.toByteArray, codec.charSet).toCharArray)
 
   /** Tab increment; can be overridden */
@@ -200,7 +204,10 @@ object SourceFile {
 
   def fromId(id: Int): SourceFile = sourceOfChunk(id >> ChunkSizeLog)
 
-  def virtual(name: String, content: String) = new SourceFile(new VirtualFile(name, content.getBytes), scala.io.Codec.UTF8)
+  def virtual(name: String, content: String, maybeIncomplete: Boolean = false) =
+    val src = new SourceFile(new VirtualFile(name, content.getBytes), scala.io.Codec.UTF8)
+    src._maybeInComplete = maybeIncomplete
+    src
 
   private final val ChunkSizeLog = 10
   private final val ChunkSize = 1 << ChunkSizeLog

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

@@ -147,7 +147,7 @@ object ParseResult {
   }
 
   def apply(sourceCode: String)(implicit state: State): ParseResult =
-    apply(SourceFile.virtual(str.REPL_SESSION_LINE + (state.objectIndex + 1), sourceCode))
+    apply(SourceFile.virtual(str.REPL_SESSION_LINE + (state.objectIndex + 1), sourceCode, maybeIncomplete = true))
 
   /** Check if the input is incomplete.
    *
@@ -159,7 +159,7 @@ object ParseResult {
       case CommandExtract(_) | "" => false
       case _ => {
         val reporter = newStoreReporter
-        val source   = SourceFile.virtual("<incomplete-handler>", sourceCode)
+        val source   = SourceFile.virtual("<incomplete-handler>", sourceCode, maybeIncomplete = true)
         val unit     = CompilationUnit(source, mustExist = false)
         val localCtx = ctx.fresh
                           .setCompilationUnit(unit)

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

@@ -176,7 +176,7 @@ class ReplDriver(settings: Array[String],
     compiler
       .typeCheck(expr, errorsAllowed = true)
       .map { tree =>
-        val file = SourceFile.virtual("<completions>", expr)
+        val file = SourceFile.virtual("<completions>", expr, maybeIncomplete = true)
         val unit = CompilationUnit(file)(state.context)
         unit.tpdTree = tree
         implicit val ctx = state.context.fresh.setCompilationUnit(unit)