ScannerTokens: yet another leading infix fix

- first, match HSpace instead of Whitespace, to exclude LFLF
- second, on double EOL, return No instead of InvalidArg (which is used
  to indicate incorrectly indented infix arguments).

scalameta/parsers/shared/src/main/scala/scala/meta/internal/parsers/ScannerTokens.scala

@@ -969,8 +969,8 @@ final class ScannerTokens(val tokens: Tokens)(implicit dialect: Dialect) {
   private def isLeadingInfixArg(afterOpPos: Int, nextIndent: Int) = {
     // we don't check for pos to be within bounds since we would exit on EOF first
     @tailrec def iter(pos: Int, indent: Int, prevNoNL: Boolean): LeadingInfix = tokens(pos) match {
-      case _: EOL => if (prevNoNL) iter(pos + 1, 0, false) else LeadingInfix.InvalidArg
-      case _: Whitespace => iter(pos + 1, if (prevNoNL) indent else indent + 1, prevNoNL)
+      case _: EOL => if (prevNoNL) iter(pos + 1, 0, false) else LeadingInfix.No
+      case _: HSpace => iter(pos + 1, if (prevNoNL) indent else indent + 1, prevNoNL)
       case c: Comment =>
         val commentIndent = multilineCommentIndent(c)
         iter(pos + 1, if (commentIndent < 0) indent else commentIndent, true)
@@ -981,7 +981,7 @@ final class ScannerTokens(val tokens: Tokens)(implicit dialect: Dialect) {
     }
     tokens(afterOpPos) match {
       case _: EOL => iter(afterOpPos + 1, 0, false)
-      case _: Whitespace => iter(afterOpPos + 1, -1, true)
+      case _: HSpace => iter(afterOpPos + 1, -1, true)
       case _: Comment => LeadingInfix.InvalidArg
       case _ => LeadingInfix.No
     }

tests/shared/src/test/scala/scala/meta/tests/parsers/DefnSuite.scala

@@ -428,11 +428,16 @@ class DefnSuite extends ParseSuite {
          |
          |}
          |""".stripMargin
-    val error =
-      """|<input>:2: error: illegal start of simple expression
-         |  def b: C =
-         |            ^""".stripMargin
-    runTestError[Stat](code, error)
+    val layout =
+      """|new A { def b: C = ??? }
+         |""".stripMargin
+    val tree = Term.NewAnonymous(
+      tpl(
+        List(Init(pname("A"), anon, Nil)),
+        List(Defn.Def(Nil, tname("b"), Nil, Some(pname("C")), tname("???")))
+      )
+    )
+    runTestAssert[Stat](code, layout)(tree)
   }
 
 }