Internal compiler error while working with variadic templates

Asked Jun 04 '20 at 08:23

Active Jun 04 '20 at 08:23

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I am trying to get the product of all variadic template arguments given to a basic struct. At looks like follows:

template<int a, int ... as>
struct A {

    int data[doIt<a, as...>()];
};

And I constructed doIt<>() in multiple levels. I found out that this is the only way it should work. I got this solution from this stackoverflow question. The code looks loke follows:

template<int ... is>
constexpr std::size_t doIt();

template<int i, int ... is>
constexpr std::size_t doIt_helper() {
    return i * doIt<is...>(); // this row throws the error
}

template<>
constexpr std::size_t doIt<>() {
    return 1;
}

template<int ... is>
constexpr std::size_t doIt() {
    return doIt_helper<is...>();
}

And when I try to compile the code I get: internal compiler error: in instantiate_decl, at cp/pt.c:22741 and I am supposed to report the bug to gcc. But I want to make sure that I didn't do the mistake. I am compiling with g++ -std=c++17. And the version is gcc version 7.5.0. What's going on?

asked Jun 04 '20 at 08:23

Moritz Seppelt

An internal compiler error is always a compiler bug, regardless of whether you made a mistake or not. – molbdnilo Jun 04 '20 at 08:31
But should this code actually run? – Moritz Seppelt Jun 04 '20 at 08:33
According to GCC 9.1 it does. And Clang agrees too. It seems valid – StoryTeller - Unslander Monica Jun 04 '20 at 08:35
As a workaround, since you are using C++17, you can just use a fold expression in lieu of the helper. `doIt` can be reduced to `return (1 * ... * is);`. This compiles fine – StoryTeller - Unslander Monica Jun 04 '20 at 08:38
Thanks that works. I will report the bug now :D – Moritz Seppelt Jun 04 '20 at 08:46
@StoryTeller-UnslanderMonica Can you please explain this syntax or a link to the documention, because this really seems like magic to me :D – Moritz Seppelt Jun 04 '20 at 09:08
https://en.cppreference.com/w/cpp/language/fold – StoryTeller - Unslander Monica Jun 04 '20 at 09:09

Internal compiler error while working with variadic templates

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