Fix constant folding of multiplied string literals when they appear in another...

Fix constant folding of multiplied string literals when they appear in another constant expression. In this case, the multiplied value was lost and replaced by the original value.

CHANGES.rst

@@ -67,6 +67,9 @@ Bugs fixed
 * 2-value slicing of typed sequences failed if the start or stop index was None.
   Patch by Christian Gibson.  (Github issue #2508)
 
+* Multiplied string literals lost their factor when they are part of another
+  constant expression (e.g. 'x' * 10 + 'y' => 'xy').
+
 * The directive ``language_level=3`` did not apply to the first token in the
   source file.  (Github issue #2230)
 

Cython/Compiler/Optimize.py

@@ -4247,7 +4247,7 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
                     string_node.bytes_value.encoding)
         else:
             assert False, "unknown string node type: %s" % type(string_node)
-        string_node.value = build_string(
+        string_node.constant_result = string_node.value = build_string(
             string_node.value * multiplier,
             string_node.value.encoding)
         return string_node

tests/run/unicodeliterals.pyx

@@ -85,6 +85,8 @@ __doc__ = br"""
     True
     >>> wide_literal == u'\\U00101234'   # unescaped by Python
     True
+    >>> ustring_in_constant_tuple == ('a', u'abc', u'\\N{SNOWMAN}', u'x' * 3, u'\\N{SNOWMAN}' * 4 + u'O')  or  ustring_in_constant_tuple  # unescaped by Python
+    True
 """
 
 if sys.version_info >= (2,6,5):
@@ -118,3 +120,5 @@ add = u'Søk ik' + u'üÖä' + u'abc'
 null = u'\x00'
 
 wide_literal = u'\U00101234'
+
+ustring_in_constant_tuple = ('a', u'abc', u'\N{SNOWMAN}', u'x' * 3, u'\N{SNOWMAN}' * 4 + u'O')