Why constexpr implicit conversion doesn't always work?

Question

#include <iostream>

struct Index {
    constexpr operator int() const { return 666; }
};

template <int i> void foo() {
    std::cout << i << std::endl;
}

void wrapper(Index index) {
    foo<index>();
}

int main() {
    Index index;
//    foo<index>(); // error: the value of ‘index’ is not usable in a constant expression
    wrapper(index);
}

Hello, everyone.

I'm using a constexpr conversion of a variable "index" to an int value, which is substituted to a "foo" templated function. If I directly call foo<index>() from "main", I get a compiler error. If the same call is done from the "wrapper", then everything compiles and works fine.

What am I missing there?

Compile command: g++ -std=c++14 main.tex with g++ (GCC) 5.3.1 20160406 (Red Hat 5.3.1-6).

I think this is a bug in gcc. Clang appears to accept both: http://coliru.stacked-crooked.com/a/5f8b60e1f2dea54d — Brian Bi, Apr 14 '17 at 05:55
And gcc 6.3 also accepts it [Demo](http://coliru.stacked-crooked.com/a/cf0525ccda6957ae). — Jarod42, Apr 14 '17 at 08:01
If I change the code to "foo<(int)index>();" in main, then it passes the compilation. It seems that the compiler treats function parameter and variable declared in the function body in different way. — Jun Ge, Apr 14 '17 at 08:20
Then take a look here, I think you can find your answer: http://stackoverflow.com/questions/14116003/difference-between-constexpr-and-const — Ali.Ghodrat, Apr 14 '17 at 20:45
This is a problem with your compiler. The newest version of [gcc](https://godbolt.org/g/iu1QG7), [clang](https://godbolt.org/g/z7WSdc) and [msvc](https://godbolt.org/g/La3k9z) all compile your code effortlessly. — jan.sende, Aug 16 '18 at 12:33

score 0 · Accepted Answer · answered Aug 20 '18 at 12:17

A very similar error was reported here. It was first reported with GCC 4.9.0

In the analysis provided:

This is a GCC bug. It appears to be some sort of confusion in the way GCC handles internal linkage non-type template parameters of pointer type.

It has since been resolved.

Why constexpr implicit conversion doesn't always work?

1 Answers1