Allow conversion of std::complex to/from Python objects.

Cython/Compiler/PyrexTypes.py

@@ -3231,6 +3231,7 @@ builtin_cpp_conversions = {
     "std::unordered_set": 1,
     "std::map":           2,
     "std::unordered_map": 2,
+    "std::complex":       1,
 }
 
 class CppClassType(CType):

Cython/Utility/CppConvert.pyx

@@ -227,3 +227,35 @@ cdef object {{cname}}(const map[X,Y]& s):
         o[X_to_py(key_value.first)] = Y_to_py(key_value.second)
         cython.operator.preincrement(iter)
     return o
+
+
+#################### complex.from_py ####################
+
+{{template_type_declarations}}
+
+cdef extern from *:
+    cdef cppclass std_complex "std::complex" [T]:
+        std_complex()
+        std_complex(T, T) except +
+
+@cname("{{cname}}")
+cdef std_complex[X] {{cname}}(object o) except *:
+    cdef double complex z = o
+    return std_complex[X](<X>z.real, <X>z.imag)
+
+
+#################### complex.to_py ####################
+
+{{template_type_declarations}}
+
+cdef extern from *:
+    cdef cppclass std_complex "std::complex" [T]:
+        X real()
+        X imag()
+
+@cname("{{cname}}")
+cdef object {{cname}}(const std_complex[X]& z):
+    cdef double complex tmp
+    tmp.real = <double>z.real()
+    tmp.imag = <double>z.imag()
+    return tmp

tests/run/complex_numbers_cpp.pyx

+# tag: cpp
+
+from libcpp.complex cimport complex as complex_class
+
+def double_complex(complex_class[double] a):
+    """
+    >>> double_complex(1 + 2j)
+    (1+2j)
+    >>> double_complex(1.5 + 2.5j)
+    (1.5+2.5j)
+    """
+    return a
+
+def double_int(complex_class[int] a):
+    """
+    >>> double_int(1 + 2j)
+    (1+2j)
+    >>> double_int(1.5 + 2.5j)
+    (1+2j)
+    """
+    return a