bpo-29602: fix signed zero handling in complex constructor. (#203)

* Fix incorrect handling of signed zeros for complex-related classes.

* Add Misc/NEWS entry.

Author: mdickinson

Lib/test/test_complex.py

@@ -387,6 +387,29 @@ def __complex__(self):
         self.assertAlmostEqual(complex(complex1(1j)), 2j)
         self.assertRaises(TypeError, complex, complex2(1j))
 
+    @support.requires_IEEE_754
+    def test_constructor_special_numbers(self):
+        class complex2(complex):
+            pass
+        for x in 0.0, -0.0, INF, -INF, NAN:
+            for y in 0.0, -0.0, INF, -INF, NAN:
+                with self.subTest(x=x, y=y):
+                    z = complex(x, y)
+                    self.assertFloatsAreIdentical(z.real, x)
+                    self.assertFloatsAreIdentical(z.imag, y)
+                    z = complex2(x, y)
+                    self.assertIs(type(z), complex2)
+                    self.assertFloatsAreIdentical(z.real, x)
+                    self.assertFloatsAreIdentical(z.imag, y)
+                    z = complex(complex2(x, y))
+                    self.assertIs(type(z), complex)
+                    self.assertFloatsAreIdentical(z.real, x)
+                    self.assertFloatsAreIdentical(z.imag, y)
+                    z = complex2(complex(x, y))
+                    self.assertIs(type(z), complex2)
+                    self.assertFloatsAreIdentical(z.real, x)
+                    self.assertFloatsAreIdentical(z.imag, y)
+
     def test_underscores(self):
         # check underscores
         for lit in VALID_UNDERSCORE_LITERALS:

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- bpo-29602: Fix incorrect handling of signed zeros in complex constructor for
+  complex subclasses and for inputs having a __complex__ method. Patch
+  by Serhiy Storchaka.
+
 - bpo-29347: Fixed possibly dereferencing undefined pointers
   when creating weakref objects.
 

Objects/complexobject.c

@@ -1025,11 +1025,11 @@ complex_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
             return NULL;
         }
         cr.real = PyFloat_AsDouble(tmp);
-        cr.imag = 0.0; /* Shut up compiler warning */
+        cr.imag = 0.0;
         Py_DECREF(tmp);
     }
     if (i == NULL) {
-        ci.real = 0.0;
+        ci.real = cr.imag;
     }
     else if (PyComplex_Check(i)) {
         ci = ((PyComplexObject*)i)->cval;
@@ -1051,7 +1051,7 @@ complex_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     if (ci_is_complex) {
         cr.real -= ci.imag;
     }
-    if (cr_is_complex) {
+    if (cr_is_complex && i != NULL) {
         ci.real += cr.imag;
     }
     return complex_subtype_from_doubles(type, cr.real, ci.real);