Interpreter warns when calling [[nodiscard]] functions

[hahnjo]

Similar to #8622, but in the interpreter:

root [0] std::vector<int> v;
root [1] v.size()
ROOT_prompt_1:1:1: warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
v.size()
^~~~~~
(unsigned long) 0

(with libstdc++12)

This currently makes roottest-cling-stl-default-VectorSort fail with GCC 12.

[jalopezg-git]

Thanks for the report, @hahnjo!

IIRC, this one was not trivial to fix, as in principle, we only want to ignore diag::warn_unused_result if (i) value capture is enabled, and (ii) only for the last parsed statement.
However, as the diagnostic is emitted during parsing (see SemaStmt.cpp), I think the only way of conditionally emitting this warning -given the two conditions above-, would be to buffer all the diagnostics and then filter this one based on the cling::CompilationOptions and the source location.

Otherwise, we can always fallback to the naive fix of just disable/filter this diagnostic in every case.

[hahnjo]

Yes, I agree this is non-trivial to fix after reading through #8622. But I decided to open an issue just to keep track of the problem because it affects a test with GCC 12; not sure if we can do anything for that one in particular in the test code...