Fix utf-8 bom positions.

parso/python/prefix.py

@@ -3,6 +3,8 @@ from codecs import BOM_UTF8
 
 from parso.python.tokenize import group
 
+unicode_bom = BOM_UTF8.decode('utf-8')
+
 
 class PrefixPart(object):
     def __init__(self, leaf, typ, value, spacing='', start_pos=None):
@@ -17,6 +19,9 @@ class PrefixPart(object):
     def end_pos(self):
         if self.value.endswith('\n'):
             return self.start_pos[0] + 1, 0
+        if self.value == unicode_bom:
+            # The bom doesn't have a length at the start of a Python file.
+            return self.start_pos
         return self.start_pos[0], self.start_pos[1] + len(self.value)
 
     def create_spacing_part(self):
@@ -35,8 +40,6 @@ class PrefixPart(object):
         )
 
 
-unicode_bom = BOM_UTF8.decode('utf-8')
-
 _comment = r'#[^\n\r\f]*'
 _backslash = r'\\\r?\n'
 _newline = r'\r?\n'
@@ -66,6 +69,7 @@ def split_prefix(leaf, start_pos):
     line, column = start_pos
     start = 0
     value = spacing = ''
+    bom = False
     while start != len(leaf.prefix):
         match =_regex.match(leaf.prefix, start)
         spacing = match.group(1)
@@ -75,8 +79,10 @@ def split_prefix(leaf, start_pos):
         type_ = _types[value[0]]
         yield PrefixPart(
             leaf, type_, value, spacing,
-            start_pos=(line, column + start + len(spacing))
+            start_pos=(line, column + start - int(bom) + len(spacing))
         )
+        if type_ == 'bom':
+            bom = True
 
         start = match.end(0)
         if value.endswith('\n'):