Matcher combination does not work outside macro call

[gonzaponte]

Describe the bug
Matchers (at least floating point matchers) cannot be combined outside of a macro call.

Expected behavior
To be able to combine matchers arbitrarily and use them in a macro.

Reproduction steps
Given the following two definitions

auto is_close_to_zero = WithinAbs(0, 2e-2);
auto is_zero          = WithinULP(0., 1);

I can combine them in the macro like

CHECK_THAT(a_value, is_close_to_zero && ! is_zero);

but not outside of the macro like

auto is_close_to_but_not_zero = is_close_to_zero && ! is_zero;
CHECK_THAT(a_value, is_close_to_but_not_zero);

The latter gives a segfault.

Platform information:

• OS: Ubuntu 20.04
• Compiler+version: clang 16.0.1
• Catch version: v3.3.2

Additional context
None

[horenmar]

I assume you found the part of docs that says

IMPORTANT: The combining operators do not take ownership of the matcher objects being combined. This means that if you store combined matcher object, you have to ensure that the matchers being combined outlive its last use. What this means is that the following code leads to a use-after-free (UAF):

since you have the matchers live in their own vars. Note that ! also counts as combination operator for this purpose.

[gonzaponte]

Yes, I had read that. After some more thought, I think I misinterpreted that note.
I was assuming that the lifetime referred only to the initial matchers, not to every intermediate combination. So in

auto is_close_to_but_not_zero = is_close_to_zero && ! is_zero;

! is_zero leads to UAF because it's freed immediately after that line while in

auto is_not_zero = ! is_zero;
auto is_close_to_but_not_zero = is_close_to_zero && is_not_zero;

it is still in scope. Am I interpreting it correctly?

[horenmar]

Yes.

!is_zero creates a temporary of type MatchNotOf<double> that references the original is_zero matcher. && creates a MatchAllOf<double> that references this temporary and that then leads to UAF.

[gonzaponte]

Understood. Thanks and apologies for the noise.

[horenmar]

No problem. I'll probably add more examples to the docs to make this clearer.