Add a test to verify an early call to iter() on the outermost for

expression.

Lib/test/test_genexps.py

@@ -91,6 +91,15 @@ Verify early binding for the outermost for-expression
     >>> list(g)
     [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
 
+Verify that the outermost for-expression makes an immediate check
+for iterability
+
+    >>> (i for i in 6)
+    Traceback (most recent call last):
+      File "<pyshell#4>", line 1, in -toplevel-
+        (i for i in 6)
+    TypeError: iteration over non-sequence
+
 Verify late binding for the outermost if-expression
 
     >>> include = (2,4,6,8)