Fix #2213: Fix errors positioned after EOF

Author: nicolasstucki

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

@@ -107,6 +107,7 @@ case class SourceFile(file: AbstractFile, content: Array[Char]) extends interfac
    */
   def offsetToLine(offset: Int): Int = {
     lastLine = Util.bestFit(lineIndices, lineIndices.length, offset, lastLine)
+    if (offset >= length) lastLine -= 1 // compensate for the sentinel
     lastLine
   }
 
@@ -129,7 +130,7 @@ case class SourceFile(file: AbstractFile, content: Array[Char]) extends interfac
     var idx = startOfLine(offset)
     var col = 0
     while (idx != offset) {
-      col += (if (content(idx) == '\t') (tabInc - col) % tabInc else 1)
+      col += (if (idx < length && content(idx) == '\t') (tabInc - col) % tabInc else 1)
       idx += 1
     }
     col

tests/repl/i2213.check

@@ -0,0 +1,8 @@
+scala> def x
+-- [E018] Syntax Error: <console>:1:5 ------------------------------------------
+1 |def x
+  |     ^
+  |     missing return type
+
+longer explanation available when compiling with `-explain`
+scala> :quit