Guard against a few invalid list comprehension cases

Some valid-looking list comprehensions, such as (genexpr x []) can crash
Python 2.7. The AST we generate from these cannot be expressed in
Python, but were valid in Hy.

Added two guards to guard against this, so we raise an error instead of
crashing Python.

Closes #572, #591 and #634.

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

hy/compiler.py

@@ -1473,9 +1473,16 @@ class HyASTCompiler(object):
         # (list-comp expr (target iter) cond?)
         expr.pop(0)
         expression = expr.pop(0)
+        gen_gen = expr[0]
+
+        if not isinstance(gen_gen, HyList):
+            raise HyTypeError(gen_gen, "Generator expression must be a list.")
 
         gen_res, gen = self._compile_generator_iterables(expr)
 
+        if len(gen) == 0:
+            raise HyTypeError(gen_gen, "Generator expression cannot be empty.")
+
         compiled_expression = self.compile(expression)
         ret = compiled_expression + gen_res
         ret += ast.ListComp(

tests/compilers/test_ast.py

@@ -519,3 +519,11 @@ def test_attribute_access():
 def test_cons_correct():
     """Ensure cons gets compiled correctly"""
     can_compile("(cons a b)")
+
+
+def test_invalid_list_comprehension():
+    """Ensure that invalid list comprehensions do not break the compiler"""
+    cant_compile("(genexpr x [])")
+    cant_compile("(genexpr [x [1 2 3 4]] x)")
+    cant_compile("(list-comp None [])")
+    cant_compile("(list-comp [x [1 2 3]] x)")