fix(cpp1): contract check invalid alternative

[JohelEGP]

Resolves #433.

I have some simplifications in store.
They might come in further commits,
because according to #433 (comment),
they might not work.

Testing summary.

100% tests passed, 0 tests failed out of 508

Total Test time (real) = 111.30 sec

Acknowledgements.

• Tested thanks to https://github.com/modern-cmake/cppfront.
• The commit message uses Conventional Commits 1.0.0.

[JohelEGP]

You can see the generated code executing at: https://cpp2.godbolt.org/z/8Kvdhc6aG.

[JohelEGP]

Feel free to merge this until someone resolves #492 to move the logic to compile-time.

[JohelEGP]

This is the diff between gcc-10 and gcc-13 under test-results:

# test-results]$
for f in `ls gcc-10/* gcc-13/* | sed 's/gcc-[[:digit:]]\+\///' | sort --unique`; do diff gcc-10/$f gcc-13/$f; done

1,2c1,2
< g++-10 (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0
< Copyright (C) 2020 Free Software Foundation, Inc.
---
> c++ (GCC) 13.1.1 20230429
> Copyright (C) 2023 Free Software Foundation, Inc.
diff: gcc-10/pure2-bugfix-for-runtime-invalid-alternative.cpp.execution: No such file or directory
diff: gcc-10/pure2-bugfix-for-runtime-invalid-alternative.cpp.output: No such file or directory
diff: gcc-13/pure2-cpp1-multitoken-fundamental-types.cpp.execution: No such file or directory
diff: gcc-13/pure2-inspect-values.cpp.execution: No such file or directory
diff: gcc-13/run-tests-gcc-10.sh: No such file or directory

pure2-cpp1-multitoken-fundamental-types is now an -error test.
pure2-inspect-values is now mixed-inspect-values.

For convenience, I think a result of a newer compiler versions should only be in the repository if it is different from the previous compiler version.

[JohelEGP]

Marking as draft to prevent merging
as this should be superseded by #529.

[JohelEGP]

Feel free to merge this until someone resolves #492 to move the logic to compile-time.

[...] this should be superseded by #529.