Handle for-from ranges with unsigned targets and near-zero endpoints.

Cython/Compiler/Nodes.py

@@ -6201,12 +6201,24 @@ class ForFromStatNode(LoopNode, StatNode):
             loopvar_name = code.funcstate.allocate_temp(self.target.type, False)
         else:
             loopvar_name = self.loopvar_node.result()
-        code.putln(
-            "for (%s = %s%s; %s %s %s; %s%s) {" % (
-                loopvar_name,
-                self.bound1.result(), offset,
-                loopvar_name, self.relation2, self.bound2.result(),
-                loopvar_name, incop))
+        if self.target.type.is_int and not self.target.type.signed and self.relation2[0] == '>':
+            # Handle the case where the endpoint of an unsigned int iteration
+            # is within step of 0.
+            if not self.step:
+                step = 1
+            code.putln(
+                "for (%s = %s%s + %s; %s %s %s + %s; ) { %s%s;" % (
+                    loopvar_name,
+                    self.bound1.result(), offset, step,
+                    loopvar_name, self.relation2, self.bound2.result(), step,
+                    loopvar_name, incop))
+        else:
+            code.putln(
+                "for (%s = %s%s; %s %s %s; %s%s) {" % (
+                    loopvar_name,
+                    self.bound1.result(), offset,
+                    loopvar_name, self.relation2, self.bound2.result(),
+                    loopvar_name, incop))
         if self.py_loopvar_node:
             self.py_loopvar_node.generate_evaluation_code(code)
             self.target.generate_assignment_code(self.py_loopvar_node, code)

tests/run/reversed_iteration.pyx

@@ -759,3 +759,39 @@ def reversed_bytes_slice_step_only(bytes s):
     for c in reversed(s[::-1]):
         result.append(c)
     return result
+
+
+@cython.test_assert_path_exists('//ForFromStatNode')
+def reversed_unsigned(int a, int b):
+    """
+    >>> reversed_unsigned(0, 5)
+    [4, 3, 2, 1, 0]
+    >>> reversed_unsigned(1, 5)
+    [4, 3, 2, 1]
+    >>> reversed_unsigned(1, 1)
+    []
+    """
+    cdef unsigned int i
+    return [i for i in reversed(range(a, b))]
+
+@cython.test_assert_path_exists('//ForFromStatNode')
+def reversed_unsigned_by_3(int a, int b):
+    """
+    >>> reversed_unsigned_by_3(0, 5)
+    [3, 0]
+    >>> reversed_unsigned_by_3(0, 7)
+    [6, 3, 0]
+    """
+    cdef unsigned int i
+    return [i for i in reversed(range(a, b, 3))]
+
+@cython.test_assert_path_exists('//ForFromStatNode')
+def range_unsigned_by_neg_3(int a, int b):
+    """
+    >>> range_unsigned_by_neg_3(-1, 6)
+    [6, 3, 0]
+    >>> range_unsigned_by_neg_3(0, 7)
+    [7, 4, 1]
+    """
+    cdef unsigned int i
+    return [i for i in range(b, a, -3)]