Fix: Avoid ast.Name for True/False/None in mark expressions (debug build crash)

[rowan-stein]

User Request

Pytest crashes the interpreter on Python debug builds for 3.8+ when compiling mark expressions that contain certain identifiers.

Short reproducer:

>>> Expression.compile("False")
python: Python/compile.c:3559: compiler_nameop: Assertion `!_PyUnicode_EqualToASCIIString(name, "None") && !_PyUnicode_EqualToASCIIString(name, "True") && !_PyUnicode_EqualToASCIIString(name, "False")' failed.
[1]    29440 abort (core dumped)  python

Related upstream: bpo-40870 — https://bugs.python.org/issue40870

Researcher Specification

Root cause: mark expression parsing builds an AST using ast.Name nodes for identifiers, including the special names "True", "False", and "None". On CPython debug builds (3.8+), compiling such an AST triggers an assertion in compiler_nameop because these are meant to be literal constants, not names.

Proposed fix:

• Add _ident_to_name(name: str) -> ast.expr in src/_pytest/compat.py that:
  • For "True"/"False"/"None": returns a literal constant node — ast.Constant on Python >= 3.8; ast.NameConstant on Python < 3.8.
  • For any other identifier: returns ast.Name(name, ast.Load()).
• Update src/_pytest/mark/expression.py to:
  • Use _ident_to_name("False") as the default for an empty expression (instead of ast.NameConstant(False)).
  • Use _ident_to_name(ident.value) wherever identifiers are turned into AST nodes (instead of ast.Name(...)).

Compatibility notes:

• Aligns literals with Python semantics across versions (3.4–3.7 use ast.NameConstant; 3.8+ use ast.Constant).
• Slight behavior change: exact identifiers "True", "False", and "None" now behave as literals and are not resolved through the matcher. Lowercase variants remain identifiers.

Observed Failure / Stack Trace

• As above, CPython debug build (3.8+) assertion failure in compiler_nameop when compiling an expression containing "False" (applies similarly to "True"/"None").

Reproduction Steps

1. Use a CPython debug build (3.8+): python-dbg or equivalent.
2. Run:

from _pytest.mark.expression import Expression
Expression.compile("False")

3. Prior behavior: crashes with an assertion as shown above.
4. Expected behavior after fix: compiles cleanly; evaluating should yield False.

Testing Plan

• Add tests asserting that "True"/"False"/"None" are treated as literals and not looked up in the matcher.
• Ensure lowercase variants (e.g., "true") remain identifiers.
• Optional: version-aware structural checks to confirm AST node types (ast.Constant vs ast.NameConstant).

Links

• CPython: https://bugs.python.org/issue40870