Normalize extra comparison in markers for output

This guarantees that getting the string representation of a marker is always normalized for extras.

Part of pypa#526

packaging/markers.py

@@ -6,7 +6,7 @@
 import os
 import platform
 import sys
-from typing import Any, Callable, Dict, List, Optional, Tuple, Union
+from typing import Any, Callable, Dict, List, Optional, Tuple, TypeVar, Union, overload
 
 from pyparsing import (  # noqa: N817
     Forward,
@@ -139,11 +139,37 @@ def serialize(self) -> str:
 MARKER = stringStart + MARKER_EXPR + stringEnd
 
 
-def _coerce_parse_result(results: Union[ParseResults, List[Any]]) -> List[Any]:
+_T = TypeVar("_T")
+
+
+@overload
+def _coerce_parse_result(results: ParseResults) -> List[Any]:
+    ...
+
+
+@overload
+def _coerce_parse_result(results: _T) -> _T:
+    ...
+
+
+def _coerce_parse_result(results):
+    """
+    Flatten the parse results into a list of results.
+
+    Also normalize extra values.
+    """
     if isinstance(results, ParseResults):
         return [_coerce_parse_result(i) for i in results]
-    else:
-        return results
+    elif isinstance(results, tuple):
+        lhs, op, rhs = results
+        if isinstance(lhs, Variable) and lhs.value == "extra":
+            normalized_extra = canonicalize_name(rhs.value)
+            rhs = Value(normalized_extra)
+        elif isinstance(rhs, Variable) and rhs.value == "extra":
+            normalized_extra = canonicalize_name(lhs.value)
+            lhs = Value(normalized_extra)
+        results = lhs, op, rhs
+    return results
 
 
 def _format_marker(

tests/test_markers.py

@@ -362,3 +362,12 @@ def test_evaluate_setuptools_legacy_markers(self):
         marker_string = "python_implementation=='Jython'"
         args = [{"platform_python_implementation": "CPython"}]
         assert Marker(marker_string).evaluate(*args) is False
+
+    def test_extra_str_normalization(self):
+        raw_name = "S_P__A_M"
+        normalized_name = "s-p-a-m"
+        lhs = f"{raw_name!r} == extra"
+        rhs = f"extra == {raw_name!r}"
+
+        assert str(Marker(lhs)) == f'"{normalized_name}" == extra'
+        assert str(Marker(rhs)) == f'extra == "{normalized_name}"'