Only use PyUnicode_Concat on unicode object operations (GH-3433)

Cython/Compiler/ExprNodes.py

@@ -11390,19 +11390,24 @@ class AddNode(NumBinopNode):
                 self, type1, type2)
 
     def py_operation_function(self, code):
-        is_unicode_concat = False
-        if isinstance(self.operand1, FormattedValueNode) or isinstance(self.operand2, FormattedValueNode):
-            is_unicode_concat = True
-        else:
         type1, type2 = self.operand1.type, self.operand2.type
+
         if type1 is unicode_type or type2 is unicode_type:
-                is_unicode_concat = type1.is_builtin_type and type2.is_builtin_type
+            if type1 in (unicode_type, str_type) and type2 in (unicode_type, str_type):
+                is_unicode_concat = True
+            elif isinstance(self.operand1, FormattedValueNode) or isinstance(self.operand2, FormattedValueNode):
+                # Assume that even if we don't know the second type, it's going to be a string.
+                is_unicode_concat = True
+            else:
+                # Operation depends on the second type.
+                is_unicode_concat = False
 
             if is_unicode_concat:
                 if self.operand1.may_be_none() or self.operand2.may_be_none():
                     return '__Pyx_PyUnicode_ConcatSafe'
                 else:
                     return '__Pyx_PyUnicode_Concat'
+
         return super(AddNode, self).py_operation_function(code)
 
 

tests/run/test_unicode_string_tests.pxi

@@ -993,6 +993,14 @@ class CommonTest(BaseTest):
         self.checkequal('\u019b\u1d00\u1d86\u0221\u1fb7',
                         '\u019b\u1d00\u1d86\u0221\u1fb7', 'capitalize')
 
+    def test_list_concat(self):
+        # https://github.com/cython/cython/issues/3426
+        y = []
+        y += 'ab'
+        self.assertEqual('a', y[0])
+        self.assertEqual('b', y[1])
+        self.assertEqual(['a', 'b'], y)
+
 
 class MixinStrUnicodeUserStringTest:
     # additional tests that only work for