Avoid const types in inference.

This fixes #1798.

Cython/Compiler/PyrexTypes.py

@@ -4338,6 +4338,10 @@ def widest_numeric_type(type1, type2):
         type1 = type1.ref_base_type
     if type2.is_reference:
         type2 = type2.ref_base_type
+    if type1.is_const:
+        type1 = type1.const_base_type
+    if type2.is_const:
+        type2 = type2.const_base_type
     if type1 == type2:
         widest_type = type1
     elif type1.is_complex or type2.is_complex:

tests/run/cdivision_CEP_516.pyx

@@ -191,3 +191,14 @@ def py_div_long(long a, long b):
     OverflowError: ...
     """
     return a / b
+
+def c_div_const_test(a, b):
+    """
+    >>> c_div_const_test(5, 3)
+    1
+    """
+    return c_div_const(a, b)
+
+cdef long c_div_const(const long a, int b):
+    cdef long c = a / b
+    return c