Make str unsafe for type inference. See trac #553.

Cython/Compiler/TypeInference.py

@@ -314,8 +314,14 @@ def safe_spanning_type(types, might_overflow):
     if result_type.is_reference:
         result_type = result_type.ref_base_type
     if result_type.is_pyobject:
-        # any specific Python type is always safe to infer
-        return result_type
+        # In theory, any specific Python type is always safe to
+        # infer. However, inferring str can cause some existing code
+        # to break, since we are also now much more strict about
+        # coercion from str to char *. See trac #553.
+        if result_type.name == 'str':
+            return py_object_type
+        else:
+            return result_type
     elif result_type is PyrexTypes.c_double_type:
         # Python's float type is just a C double, so it's safe to use
         # the C type instead

tests/run/type_inference.pyx

@@ -362,6 +362,11 @@ def safe_only():
     res = ~d
     assert typeof(d) == "long", typeof(d)
 
+    # we special-case inference to type str, see
+    # trac # 
+    s = "abc"
+    assert typeof(s) == "Python object", typeof(s)
+
     # potentially overflowing arithmetic
     e = 1
     e += 1