[dbrattli]

Why

Nested tuple matches can reuse an outer match's Python match_value binding, causing union-field access to read from the inner scrutinee and raise AttributeError. The same collision affects captured fields in closures and user bindings named match_value.

Fixes #5026.

How

Convert generated identifiers to Python naming before checking and reserving unique names. Add three regression tests covering nested matches, closures, and user bindings, including fallback branches and repeated closure calls.

Validation

• All three regressions fail with the original compiler and pass with the fix.
• ./build.sh test python --force-fable-library --type-check: 2,395 .NET tests and 2,485 Python tests passed with a freshly generated runtime.
• Pyright diagnostics match unchanged main: 34 existing errors and 45 warnings, with no added errors and no errors in the regression file.
• Release compiler build, compiler-source Fantomas check, and git diff --check passed.

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded