Merge pull request #3889 from som-snytt/issue/6476-alt

SI-6476 Improve error on escapement 

1 files changed, 7 insertions, 2 deletions

diff --git a/src/library/scala/StringContext.scala b/src/library/scala/StringContext.scala
index fe69c6fbf8..2d2601c6fb 100644
--- a/src/library/scala/StringContext.scala
+++ b/src/library/scala/StringContext.scala
@@ -173,8 +173,13 @@ object StringContext {
    *  @param  str   The offending string
    *  @param  idx   The index of the offending backslash character in `str`.
    */
-  class InvalidEscapeException(str: String, @deprecatedName('idx) val index: Int)
-    extends IllegalArgumentException("invalid escape character at index "+index+" in \""+str+"\"")
+  class InvalidEscapeException(str: String, @deprecatedName('idx) val index: Int) extends IllegalArgumentException(
+    s"""invalid escape ${
+      require(index >= 0 && index < str.length)
+      val ok = """[\b, \t, \n, \f, \r, \\, \", \']"""
+      if (index == str.length - 1) "at terminal" else s"'\\${str(index + 1)}' not one of $ok at"
+    } index $index in "$str". Use \\\\ for literal \\."""
+  )
 
   /** Expands standard Scala escape sequences in a string.
    *  Escape sequences are: