Fix indent calculation with tabs when computing prefixes

Closes psf#262

blib2to3/pgen2/driver.py

@@ -134,7 +134,7 @@ def _partially_consume_prefix(self, prefix, column):
             elif char == ' ':
                 current_column += 1
             elif char == '\t':
-                current_column += 4
+                current_column += 1
             elif char == '\n':
                 # unexpected empty line
                 current_column = 0

tests/test_black.py

@@ -501,6 +501,19 @@ def test_bracket_match(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, line_length=ll)
 
+    def test_comment_indentation(self) -> None:
+        contents_tab = "if 1:\n\tif 2:\n\t\tpass\n\t# comment\n\tpass\n"
+        contents_spc = "if 1:\n    if 2:\n        pass\n    # comment\n    pass\n"
+
+        self.assertFormatEqual(fs(contents_spc), contents_spc)
+        self.assertFormatEqual(fs(contents_tab), contents_spc)
+
+        contents_tab = "if 1:\n\tif 2:\n\t\tpass\n\t\t# comment\n\tpass\n"
+        contents_spc = "if 1:\n    if 2:\n        pass\n        # comment\n    pass\n"
+
+        self.assertFormatEqual(fs(contents_tab), contents_spc)
+        self.assertFormatEqual(fs(contents_spc), contents_spc)
+
     def test_report_verbose(self) -> None:
         report = black.Report(verbose=True)
         out_lines = []