Print Predicate for Compile-time Assertions on Failure

[jwillikers]

Is it possible to have the predicate printed for constant expressions when there is a failure?

Take the following code as an example, which is taken from the tutorial but evaluated at compile-time.

int main() {
  using namespace boost::ut;
  expect(constant<1_i == 2>);
}

Is it possible to get output similar to the following at run-time where the predicate is shown on failure?

main.cpp:4:FAILED [1 == 2]
===============================================================================
tests:   0 | 0 failed
asserts: 1 | 0 passed | 1 failed

[kris-jusiak]

Not sure whether I fully understand, isn't the following what you are asking for 🤔

expect(1_i == 2);

[jwillikers]

If I understand correctly, constant is for evaluating an expression at compile time. Please correct me if I'm wrong. When using constant the output for a failure is either true or false instead of a more detailed message containing the assertion. This is most likely a rather niche concern and possible a terrible idea anyways, but if you want to let users do more at compile time it might make sense. Maybe. 😶

[kris-jusiak]

Ah, I see, yeah, you are right, constant is to ensure that expression can be evaluated at compile-time. It actually shows the expression (instead of true/false) if NTTP is supported (only gcc ATM, though, clang doesn't have it yet 😞).

NTTP branch -> https://github.com/boost-experimental/ut/blob/master/include/boost/ut.hpp#L1880
Full example -> https://godbolt.org/z/GJqhsE (it shows the expression in gcc but in clang it only shows true/false)

For the time being, for clang, I'd stick with the run-time version to see the expression and check whether it's constant on top of that (that works for both: clang and gcc)

constexpr auto expr = 1_i == 2;
expect(expr and constant<expr>);

Full example here -> https://godbolt.org/z/Zd-Ni9

[jwillikers]

Awesome! That's exactly what I was talking about. 😌

[kris-jusiak]

👍