[clang-tidy] suggest adding `constexpr` when it is possible

[5chmidti]

Add a check to suggest adding constexpr to variables and functions to the extent that each standard version allows. Using constexpr can, e.g., potentially uncover UB in code, as that is not permitted in a constexpr context.

I'm refactoring my prototype that ran successfully on the clang and clang-tools-extra targets (using a more restrictive mode than the standard permits) with working fixits. I need to finish the implementation, add more tests, and evaluate the check against some code-bases before posting the PR though.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Julian Schmidt (5chmidti)

Add a check to suggest adding `constexpr` to variables and functions to the extent that each standard version allows. Using `constexpr` can, e.g., potentially uncover UB in code, as that is not permitted in a `constexpr` context.

I'm refactoring my prototype that ran successfully on the clang and clang-tools-extra targets (using a more restrictive mode than the standard permits) with working fixits. I need to finish the implementation, add more tests, and evaluate the check against some code-bases before posting the PR though.

[schenker]

Hi @5chmidti, since this probably won't make it into llvm 20, would you be willing to sharing the prototype?