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limitations.md

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Limitations

Assertions in regular functions

Since Bugspray allows constexpr evaluation of most test cases, it needs to implement all its functionality with the C++ subset available in constexpr functions. In particular this means that it cannot make use of mutable global state. Therefore, all information about test results has to be passed in and out using arguments and return values. This, unfortunately, means that the assertion macros cannot be used in any odd function, since they assume to be placed in a function that has certain parameters. This is the reason why asserting functions were introduced.

Stringification of built-in types

In C++20, none of the standard library stringification functions are constexpr enabled. While implementing this for integers is straight forward (and Bugspray does that), implementing it for floating point types is a much bigger headache. Therefore, Bugspray currently can't display floating point types during constant evaluation.

Compiletime failure reporting

Bugspray currently places its compiletime test results in the stack trace of a function that will always fail to compile. The error report is in the string-like template argument of the bs::compile_time_error function. This comes with certain limitations:

  • It is not possible to have line breaks. Instead, Bugspray currently uses the semicolon as a separator.
  • There is a maximum message length. At the time of writing, Bugspray defines this limit to be 2048 characters. Anything beyond this will be truncated.

On the other hand, gcc formats the message in a very usable way (scroll right):

/proj/bugspray/include/bugspray/test_evaluation/evaluate_test_case.hpp: In instantiation of ‘constexpr void bs::compile_time_error() [with structural_string<...auto...> ...Messages = {bs::structural_string<277>{"FAILURE: /home/jan/CLionProjects/bugspray/test/failing-examples/00-simple/test.cpp:31: REQUIRE(variable == 0); WITH EXPANSION: 42 == 0; WITH: other_var: 6; BEFORE REACHING TARGET; IN: /home/jan/CLionProjects/bugspray/test/failing-examples/00-simple/test.cpp:26: This will fail"}}]’:
/proj/bugspray/test/failing-examples/00-simple/test.cpp:26:1:   required from here
/proj/bugspray/test/failing-examples/00-simple/test.cpp:33:1:   in ‘constexpr’ expansion of ‘bs::evaluate_compiletime_test<bs::structural_string<15>{"This will fail"}, 0>{}.bs::evaluate_compiletime_test<bs::structural_string<15>{"This will fail"}, 0>::operator()()’
/proj/bugspray/include/bugspray/test_evaluation/evaluate_test_case.hpp:92:39: error: static assertion failed: Test evaluation failed.
   92 |     static_assert((sizeof...(Messages), false), "Test evaluation failed.");
      |                   ~~~~~~~~~~~~~~~~~~~~^~~~~~~~

Clang unfortunately elides almost the entire compile-time error report, and I have not yet found a way to convince it otherwise.

Compiler support

Bugspray is tested against gcc (11, 12, 13), and clang (17). Last time I checked, msvc's C++20 support wasn't sufficient to compile Bugspray. I am re-evaluating this as soon as new versions are published.