also optimise basestring.join() since we now infer this type in a couple of places

Cython/Compiler/Builtin.py

@@ -276,7 +276,10 @@ builtin_types_table = [
                                     BuiltinAttribute('imag', 'cval.imag', field_type = PyrexTypes.c_double_type),
                                     ]),
 
-    ("basestring",   "PyBaseString_Type",      []),
+    ("basestring", "PyBaseString_Type", [
+                                    BuiltinMethod("join",  "TO",   "T", "__Pyx_PyBaseString_Join",
+                                                  utility_code=UtilityCode.load("StringJoin", "StringTools.c")),
+                                    ]),
     ("bytearray", "PyByteArray_Type", []),
     ("bytes",   "PyBytes_Type",    [BuiltinMethod("__contains__",  "TO",   "b", "PySequence_Contains"),
                                     BuiltinMethod("join",  "TO",   "O", "__Pyx_PyBytes_Join",

Cython/Utility/StringTools.c

@@ -654,8 +654,10 @@ static CYTHON_INLINE char __Pyx_PyBytes_GetItemInt(PyObject* bytes, Py_ssize_t i
 
 #if PY_MAJOR_VERSION < 3
 #define __Pyx_PyString_Join __Pyx_PyBytes_Join
+#define __Pyx_PyBaseString_Join(s, v) (PyUnicode_CheckExact(s) ? PyUnicode_Join(s, v) : __Pyx_PyBytes_Join(s, v))
 #else
 #define __Pyx_PyString_Join PyUnicode_Join
+#define __Pyx_PyBaseString_Join PyUnicode_Join
 #endif
 
 #if CYTHON_COMPILING_IN_CPYTHON

tests/run/builtin_basestring.pyx

 
+cimport cython
+
 import sys
 IS_PY3 = sys.version_info[0] >= 3
 
@@ -84,3 +86,23 @@ def basestring_typed_argument(basestring obj):
     TypeError: ...got S...
     """
     return obj
+
+
+@cython.test_assert_path_exists(
+    "//SimpleCallNode",
+    "//SimpleCallNode//NoneCheckNode",
+    "//SimpleCallNode//AttributeNode[@is_py_attr = false]")
+def basestring_join(basestring s, *values):
+    """
+    >>> print(basestring_join(ustring, 'a', 'b', 'c'))
+    aabcdefbabcdefc
+    >>> print(basestring_join(sstring, 'a', 'b', 'c'))
+    aabcdefbabcdefc
+    >>> if IS_PY3: print('abcdefabcdefabcdef')
+    ... else: print(basestring_join(bstring, bstring, bstring).decode('utf8'))
+    abcdefabcdefabcdef
+    >>> basestring_join(None, 'a', 'b', 'c')
+    Traceback (most recent call last):
+    AttributeError: 'NoneType' object has no attribute 'join'
+    """
+    return s.join(values)