core.macros: Fix a for corner case

In case for doesn't get a body, raise the appropriate, descriptive error
instead of an IndexOutOfBounds one. Also updated the failing test case.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

hy/core/macros.hy

@@ -98,6 +98,8 @@
     (for* [y bar]
       baz))"
   (setv body (list body))
+  (if (empty? body)
+    (macro-error None "`for' requires a body to evaluate"))
   (setv lst (get body -1))
   (setv belse (if (and (isinstance lst HyExpression) (= (get lst 0) "else"))
                 [(body.pop)]

tests/compilers/test_ast.py

@@ -484,7 +484,7 @@ def test_for_compile_error():
         assert(False)
 
     try:
-        can_compile("(fn [] (for [x]))")
+        can_compile("(fn [] (for [x] x))")
     except HyTypeError as e:
         assert(e.message == "`for' requires an even number of args.")
     else:
@@ -497,6 +497,13 @@ def test_for_compile_error():
     else:
         assert(False)
 
+    try:
+        can_compile("(fn [] (for [x xx] (else 1)))")
+    except HyTypeError as e:
+        assert(e.message == "`for' requires a body to evaluate")
+    else:
+        assert(False)
+
 
 def test_attribute_access():
     """Ensure attribute access compiles correctly"""