optimise away conditional expressions (x if cond else y) based on constant condition results

Cython/Compiler/Optimize.py

@@ -3138,6 +3138,15 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
         return ExprNodes.BoolNode(node.pos, value=bool_result,
                                   constant_result=bool_result)
 
+    def visit_CondExprNode(self, node):
+        self._calculate_const(node)
+        if node.test.constant_result is ExprNodes.not_a_constant:
+            return node
+        if node.test.constant_result:
+            return node.true_val
+        else:
+            return node.false_val
+
     def visit_IfStatNode(self, node):
         self.visitchildren(node)
         # eliminate dead code based on constant condition results

tests/run/ifelseexpr_T267.pyx

@@ -36,6 +36,7 @@ def nested(x):
     """
     return 1 if x == 1 else (2 if x == 2 else 3)
 
+@cython.test_fail_if_path_exists('//CondExprNode')
 def const_true():
     """
     >>> const_true()
@@ -43,6 +44,7 @@ def const_true():
     """
     return 1 if 1 == 1 else 2
 
+@cython.test_fail_if_path_exists('//CondExprNode')
 def const_false():
     """
     >>> const_false()