Fix widest type calculation to never return references.

Cython/Compiler/PyrexTypes.py

@@ -4145,6 +4145,10 @@ def merge_template_deductions(a, b):
 def widest_numeric_type(type1, type2):
     """Given two numeric types, return the narrowest type encompassing both of them.
     """
+    if type1.is_reference:
+        type1 = type1.ref_base_type
+    if type2.is_reference:
+        type2 = type2.ref_base_type
     if type1 == type2:
         widest_type = type1
     elif type1.is_complex or type2.is_complex: