[SOLVED] Template ignores [[nodiscard]] attribute

Issue

When applied to a function, the [[nodiscard]] attribute encourages the compiler to issue a warning if it is used in a discarded expression other than a cast to void. Example:

[[nodiscard]] int callable_return_not_discardable(int n)
{ return n; }

int main()
{
    callable_return_not_discardable(0); // warning/error:
        // ignoring return value of 'int callable_return_not_discardable(int)',
        // declared with attribute nodiscard [-Wunused-result]
    (void) callable_return_not_discardable(0); // OK
}

Live demo on gcc-8 and clang-7.

---

This is nice and useful, until an additional indirection layer is added: templates:

template<class Callable>
void invoke_with_answer(Callable&& callable)
{ callable(42); }

[[nodiscard]] int callable_return_not_discardable(int n)
{ return n; }

int main()
{
    invoke_with_answer(callable_return_not_discardable); // OK
}

Live demo on gcc-8 and clang-7.

My question is then:
Is it a missing feature, a consequence of what templates are or should clang and gcc be fixed to issue a warning here?

Solution

[[nodiscard]] is not part of a function's signature or type, and not at all preserved when said function is converted to a pointer or bound to a reference. Which is exactly what your example does.

The template, for all intents and purposes cannot "see" the attribute.

Answered By - StoryTeller - Unslander Monica