Favour p to q in ||| parser if consuming same #chars

shared/src/main/scala/scala/util/parsing/combinator/Parsers.scala

@@ -365,7 +365,6 @@ trait Parsers {
      */
     def | [U >: T](q: => Parser[U]): Parser[U] = append(q).named("|")
 
-    // TODO
     /** A parser combinator for alternative with longest match composition.
      *
      *  `p ||| q` succeeds if `p` succeeds or `q` succeeds.
@@ -382,11 +381,11 @@ trait Parsers {
         val res2 = q(in)
 
         (res1, res2) match {
-          case (s1 @ Success(_, next1), s2 @ Success(_, next2)) => if (next2.pos < next1.pos) s1 else s2
+          case (s1 @ Success(_, next1), s2 @ Success(_, next2)) => if (next2.pos < next1.pos || next2.pos == next1.pos) s1 else s2
           case (s1 @ Success(_, _), _) => s1
           case (_, s2 @ Success(_, _)) => s2
           case (e1 @ Error(_, _), _) => e1
-          case (f1 @ Failure(_, next1), ns2 @ NoSuccess(_, next2)) => if (next2.pos < next1.pos) f1 else ns2
+          case (f1 @ Failure(_, next1), ns2 @ NoSuccess(_, next2)) => if (next2.pos < next1.pos || next2.pos == next1.pos) f1 else ns2
         }
       }
       override def toString = "|||"

shared/src/test/scala/scala/util/parsing/combinator/gh72.scala

@@ -0,0 +1,28 @@
+import scala.util.parsing.combinator.Parsers
+import scala.util.parsing.input.CharSequenceReader
+
+import org.junit.Test
+import org.junit.Assert.assertEquals
+
+class gh72 {
+  class TestParsers extends Parsers {
+    type Elem = Char
+    val left: Parser[String] = 'a' ~ 'b' ~ 'c' ^^^ "left" withFailureMessage "failure on left"
+    val right: Parser[String] = 'a' ~ 'b' ~ 'c' ^^^ "right" withFailureMessage "failure on right"
+    def p: Parser[String] = left ||| right
+  }
+
+  @Test
+  def test(): Unit = {
+    val tstParsers = new TestParsers
+    val s = new CharSequenceReader("abc")
+    assertEquals("[1.4] parsed: left", tstParsers.p(s).toString)
+
+    val t = new CharSequenceReader("def")
+    val expectedFailure = """[1.1] failure: failure on left
+
+def
+^"""
+    assertEquals(expectedFailure, tstParsers.p(t).toString)
+  }
+}