Fix string additions when used in certain ways

jedi/api/file_name.py

@@ -7,27 +7,32 @@ from jedi.evaluate.helpers import get_str_or_none
 
 
 def file_name_completions(evaluator, module_context, start_leaf, string, like_name):
+    # First we want to find out what can actually be changed as a name.
     base_name = os.path.basename(string)
-    like_name = base_name + like_name
-    string = os.path.dirname(string)
+    like_name_length = len(base_name + like_name)
 
     string = _get_string_additions(module_context, start_leaf) + string
+    # Here we use basename again, because if strings are added like
+    # `'foo' + 'bar`, it should complete to `foobar/`.
+    must_start_with = os.path.basename(string) + like_name
+    string = os.path.dirname(string)
+
     base_path = os.path.join(evaluator.project._path, string)
     try:
         listed = os.listdir(base_path)
     except FileNotFoundError:
         return
     for name in listed:
-        if name.startswith(like_name):
+        if name.startswith(must_start_with):
             path_for_name = os.path.join(base_path, name)
             if os.path.isdir(path_for_name):
                 name += os.path.sep
 
             yield classes.Completion(
                 evaluator,
-                FileName(evaluator, name),
+                FileName(evaluator, name[len(must_start_with) - like_name_length:]),
                 stack=None,
-                like_name_length=len(like_name),
+                like_name_length=like_name_length
             )
 
 
@@ -55,8 +60,6 @@ def _get_string_additions(module_context, start_leaf):
         if child_node != '+':
             break
         was_addition = True
-        child_node
-    module_context
     return string