Fix index out of bounds

Author: adpi2

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

@@ -653,22 +653,20 @@ object Parsers {
        * to ensure an indent region is properly closed by outdentation */
       var maximumIndent: Option[IndentWidth] = None
 
-    def testChar(idx: Int, p: Char => Boolean): Boolean = {
+    def testChar(idx: Int, p: Char => Boolean): Boolean =
       val txt = source.content
-      idx < txt.length && p(txt(idx))
-    }
+      idx > -1 && idx < txt.length && p(txt(idx))
 
-    def testChar(idx: Int, c: Char): Boolean = {
+    def testChar(idx: Int, c: Char): Boolean =
       val txt = source.content
-      idx < txt.length && txt(idx) == c
-    }
+      idx > -1 && idx < txt.length && txt(idx) == c
 
     def testChars(from: Int, str: String): Boolean =
       str.isEmpty ||
       testChar(from, str.head) && testChars(from + 1, str.tail)
 
     def skipBlanks(idx: Int, step: Int = 1): Int =
-      if (testChar(idx, c => c == ' ' || c == '\t' || c == Chars.CR)) skipBlanks(idx + step, step)
+      if testChar(idx, c => c == ' ' || c == '\t' || c == Chars.CR) then skipBlanks(idx + step, step)
       else idx
 
     /** Parse indentation region `body` and rewrite it to be in braces instead */