Avoid crasher when first token of a program is in error

This used to give a crash in SourcePositiom, promoted by feeding
its calculations with a negative offset.

Author: odersky

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

@@ -192,7 +192,7 @@ object Parsers {
           case _ =>
             if (mustStartStat &&
                 in.isAfterLineEnd() &&
-                isLeqIndented(in.offset, lastStatOffset))
+                isLeqIndented(in.offset, lastStatOffset max 0))
               return
         }
         in.nextToken()

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

@@ -103,7 +103,10 @@ case class SourceFile(file: AbstractFile, content: Array[Char]) {
     lastLine
   }
 
-  def startOfLine(offset: Int): Int = lineToOffset(offsetToLine(offset))
+  def startOfLine(offset: Int): Int = {
+    require(offset >= 0)
+    lineToOffset(offsetToLine(offset))
+  }
 
   def nextLine(offset: Int): Int =
     lineToOffset(offsetToLine(offset) + 1 min lineIndices.length - 1)

test/dotc/tests.scala

@@ -140,6 +140,7 @@ class tests extends CompilerTest {
   @Test def neg_shadowedImplicits = compileFile(negDir, "arrayclone-new", xerrors = 2)
   @Test def neg_traitParamsTyper = compileFile(negDir, "traitParamsTyper", xerrors = 5)
   @Test def neg_traitParamsMixin = compileFile(negDir, "traitParamsMixin", xerrors = 2)
+  @Test def neg_firstError = compileFile(negDir, "firstError", xerrors = 3)
 
   @Test def run_all = runFiles(runDir)
 

tests/neg/firstError.scala

@@ -0,0 +1,4 @@
+.
+
+\u890u3084eu
+