When I tried to remove the old for comprehensio...

When I tried to remove the old for comprehension syntax in r24958, I
managed to instead only remove the deprecation warnings. No wonder it
went so smoothly. Sorry to get your hopes up only to execute you, old
syntax. Now the for comprehensions have to get it right:

  for (x <- 1 to 5 ; y = x) yield x+y           // nope
  for (val x <- 1 to 5 ; y = x) yield x+y       // nope
  for (val x <- 1 to 5 ; val y = x) yield x+y   // nope
  for (x <- 1 to 5 ; val y = x) yield x+y       // that's the one

No review.

1 files changed, 15 insertions, 6 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index 68651fc1c9..28cb088f4e 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -1631,15 +1631,24 @@ self =>
      *  }}}
      */
     def generator(enums: ListBuffer[Enumerator], eqOK: Boolean) {
-      val start = in.offset
-      if (in.token == VAL) in.nextToken()
-      val pat = noSeq.pattern1()
+      val start  = in.offset
+      val hasVal = in.token == VAL
+      if (hasVal)
+        in.nextToken()
+
+      val pat   = noSeq.pattern1()
       val point = in.offset
-      val tok = in.token
-      if (tok == EQUALS && eqOK) in.nextToken()
+      val hasEq = in.token == EQUALS
+
+      if (hasVal && !hasEq)
+        syntaxError(in.offset, "val in for comprehension must be followed by assignment")
+      if (!hasVal && hasEq)
+        syntaxError(in.offset, "assignment in for comprehension must be preceded by `val`")
+
+      if (hasEq && eqOK) in.nextToken()
       else accept(LARROW)
       val rhs = expr()
-      enums += makeGenerator(r2p(start, point, in.lastOffset max start), pat, tok == EQUALS, rhs)
+      enums += makeGenerator(r2p(start, point, in.lastOffset max start), pat, hasEq, rhs)
       // why max above? IDE stress tests have shown that lastOffset could be less than start,
       // I guess this happens if instead if a for-expression we sit on a closing paren.
       while (in.token == IF) enums += makeFilter(in.offset, guard())