Lazy template instantiation of unused reference arguments

Clang Frontend
1

Hi,

I’m having some issue with “opaque struct” being instantiated by Clang on unused reference arguments that have template types (that’s an annoyance but not the topic of this email). I assume Clang decide to be lazy on the instantiation of unused arguments to save compile time.

But now I am wondering if that is actually a legal behavior in C++.
For example, the following code does not trigger the static_assert:

template
struct test {
static_assert(N == 42);
};

int foo(test<69> &a) {
return 0;
}

Is that expected/desired behavior? (if you add “a = a;” then it will properly trigger it)

2

The C++ language rules do not permit instantiation of test<69> in this program. See http://eel.is/c++draft/temp.inst#1.sentence-1 – this program does not require test<69> to be a complete type, and its definition doesn’t affect the semantics of the program, so instantiation of that class is neither required nor permitted here.

3

Oh, wow, I learn every day.

Thank you for the pointer.