Fix validID & expand tests

Make sure that we generate valid ID (according to matches(ID, s)) so
that we properly test the new validID implementation.

And that's what led to the bug fix. :)

internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala

@@ -317,7 +317,7 @@ object DefaultParsers extends Parsers with ParserMain {
   /** Returns `true` if `s` parses successfully according to [[ID]].*/
   def validID(s: String): Boolean = {
     // Handwritten version of `matches(ID, s)` because validID turned up in profiling.
-    def isIdChar(c: Char): Boolean = Character.isLetterOrDigit(c) || (c == '_')
+    def isIdChar(c: Char): Boolean = Character.isLetterOrDigit(c) || (c == '-') || (c == '_')
     @tailrec def isRestIdChar(cur: Int, s: String, length: Int): Boolean =
       if (cur < length)
         isIdChar(s.charAt(cur)) && isRestIdChar(cur + 1, s, length)

internal/util-complete/src/test/scala/DefaultParsersSpec.scala

@@ -8,10 +8,21 @@
 package sbt.internal.util
 package complete
 
-import org.scalacheck._, Prop._
+import org.scalacheck._, Gen._, Prop._
 
 object DefaultParsersSpec extends Properties("DefaultParsers") {
-  import DefaultParsers._
+  import DefaultParsers.{ ID, isIDChar, matches, validID }
 
-  property("validID == matches(ID, s)") = forAll((s: String) => validID(s) == matches(ID, s))
+  property("∀ s ∈ String: validID(s) == matches(ID, s)") = forAll(
+    (s: String) => validID(s) == matches(ID, s))
+
+  property("∀ s ∈ genID: matches(ID, s)") = forAll(genID)(s => matches(ID, s))
+  property("∀ s ∈ genID: validID(s)") = forAll(genID)(s => validID(s))
+
+  private val chars: Seq[Char] = Char.MinValue to Char.MaxValue
+  private val genID: Gen[String] =
+    for {
+      c <- oneOf(chars filter (_.isLetter))
+      cs <- listOf(oneOf(chars filter isIDChar))
+    } yield (c :: cs).mkString
 }