Clang fails to compile consteval because of conflicting rule for variable arrays (?)

[socketpair]

https://godbolt.org/z/dKE6sa4PW

#include <algorithm>
#include <array>

template <std::size_t N>
struct CompTimeStr {
    std::array<char, N> store;

    consteval CompTimeStr(const char (&str)[N]) {
        std::copy_n(str, store.size(), store.data());
    }

    template <std::size_t N2>
    consteval CompTimeStr<N + N2 - 1> operator+(
        const CompTimeStr<N2> str) const {
        char newchar[size() + str.size() + 1];
        std::copy_n(data(), size(), newchar);
        std::copy_n(str.data(), str.size() + 1, newchar + size());
        return newchar;
    }

    consteval std::size_t size() const { return store.size() - 1; }
    consteval const char* data() const { return store.data(); }
};

template <CompTimeStr str>
consteval auto addBar() {
    return str + str;
}

int main() {
    constexpr CompTimeStr str = addBar<"foo">();

    return str.size();
}

[frederick-vs-ja]

At this time you can write char newchar[N + N2 - 1]; instead, which is shorter.

Slightly reduced example (Godbolt link):

#include <cstddef>
#include <type_traits>

template <std::size_t N>
struct record {
    consteval record(std::integral_constant<std::size_t, N>) {}

    template <std::size_t N2>
    consteval std::size_t test_with(const record<N2> other) const {
        constexpr auto len = size() + other.size() + 1; // HERE
        return len;
    }

    consteval std::size_t size() const { return N - 1; }
};

template <record Foo>
consteval auto test_two_records() {
    return Foo.test_with(Foo);
}

int main() {
    constexpr auto n = test_two_records<std::integral_constant<std::size_t, 42>{}>();
}

I believe this is P2280R4 which is tracked in #63139 and will be fixed by #95474.

[socketpair]

@frederick-vs-ja Thank you, I already know how to fix. I just reported obvious bug in Clang.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Коренберг Марк (socketpair)

https://godbolt.org/z/dKE6sa4PW

#include <algorithm>
#include <array>

template <std::size_t N>
struct CompTimeStr {
    std::array<char, N> store;

    consteval CompTimeStr(const char (&str)[N]) {
        std::copy_n(str, store.size(), store.data());
    }

    template <std::size_t N2>
    consteval CompTimeStr<N + N2 - 1> operator+(
        const CompTimeStr<N2> str) const {
        char newchar[size() + str.size() + 1];
        std::copy_n(data(), size(), newchar);
        std::copy_n(str.data(), str.size() + 1, newchar + size());
        return newchar;
    }

    consteval std::size_t size() const { return store.size() - 1; }
    consteval const char* data() const { return store.data(); }
};

template <CompTimeStr str>
consteval auto addBar() {
    return str + str;
}

int main() {
    constexpr CompTimeStr str = addBar<"foo">();

    return str.size();
}

[AaronBallman]

Related: #102970

[cor3ntin]

@shafik

[shafik]

Reduced some more: https://godbolt.org/z/x7P9TjYce

template <typename, unsigned>
struct A{};

template <unsigned N>
struct record {
    consteval record(A<unsigned, N>) {}

    consteval unsigned test_with() const {
        constexpr auto len = size() + 1;
        return len;
    }

    consteval unsigned size() const { return N - 1; }
};

template <record F>
consteval auto test_two_records() {
    return F.test_with();
}

void g() {
    constexpr auto n = test_two_records<A<unsigned, 42>{}>();
}

[shafik]

I am not sure this is totally unknown refs and ptrs. It looks like we are not seeing up CurrentCall->This and I don't think that is purely part of the that proposal. I think we may be missing another feature here?

[shafik]

Based on this, I think this is an unrelated bug: https://godbolt.org/z/nMh8Mebqo