fix negative-index warning on empty slices

Cython/Compiler/ExprNodes.py

@@ -106,10 +106,10 @@ def check_negative_indices(*nodes):
     Used to find (potential) bugs inside of "wraparound=False" sections.
     """
     for node in nodes:
-        if not isinstance(node.constant_result, (int, float, long)):
-            continue
-        if node.constant_result >= 0:
+        if (node is None
+                or not isinstance(node.constant_result, (int, float, long))):
             continue
+        if node.constant_result < 0:
             warning(node.pos,
                     "the result of using negative indices inside of "
                     "code sections marked as 'wraparound=False' is "

tests/errors/wraparound_warnings.pyx

@@ -7,13 +7,18 @@ s = "abc"
 l = [1, 2, 3]
 
 def normal_wraparound(int i, bytes B not None, list L not None):
+    a = s[:]
     a = s[1:2]
     a = s[-2:-1]
     a = "abc"[-2:-1]
     a = "abc"[-2:i]
     a = B[-2:-1]
+    a = B[:-1]
+    a = B[-2:]
 
     b = l[1:2]
+    b = l[:2]
+    b = l[1:]
     b = l[-2:-1]
     b = [1, 2, 3][-2:-1]
     b = [1, 2, 3][-2:i]
@@ -21,27 +26,35 @@ def normal_wraparound(int i, bytes B not None, list L not None):
 
 @cython.wraparound(False)
 def no_wraparound(int i, bytes B not None, list L not None):
+    a = s[:]
     a = s[1:2]
     a = s[-2:-1]
-    a = "abc"[-2:-1]
+    a = "abc"[-2:-1]  # evaluated at compile time
     a = "abc"[-2:i]
+    a = B[:]
     a = B[-2:-1]
+    a = B[:-1]
+    a = B[-2:]
 
     b = l[1:2]
+    b = l[:2]
+    b = l[1:]
     b = l[-2:-1]
     b = [1, 2, 3][-2:i]
     b = L[-2:-1]
 
 
 _ERRORS = """
-25:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
-25:14: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
-27:15: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
-28:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
-28:14: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
 31:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
 31:14: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
-32:19: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
-33:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
-33:14: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+33:15: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+35:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+35:14: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+36:12: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+37:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+42:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+42:14: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+43:19: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+44:11: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
+44:14: the result of using negative indices inside of code sections marked as 'wraparound=False' is undefined
 """