[clang] Constexpr function of a function parameter cannot be used in a constant expression

[StefanPaulet]

The following code compiles on gcc, but fails on msvc and clang:

template <unsigned int N>
struct array {
    constexpr auto size() const -> unsigned int {
        return N;
    }
};

constexpr auto foo(array<5> const& arr) {
    return array<arr.size()>{}.size();
}

constexpr auto bar(array<5> arr) {
    return array<arr.size()>{}.size();
}

int main()
{
    array<5> arr {};
    static_assert(arr.size() == 5);
    static_assert(foo(arr) == 5);
    static_assert(bar(arr) == 5);
}

https://godbolt.org/z/q1xajW1Gq

However, if size is made static, msvc also compiles:
https://godbolt.org/z/Tfqe5E9z8

The problem seems to be with arr being passed by reference, as passing by value also get accepted.

[frederick-vs-ja]

This is being fixed by #95474, which implements P2280R4.

Closing as duplicate of #63139.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (StefanPaulet)

The following code compiles on gcc, but fails on msvc and clang: ``` C++ template <unsigned int N> struct array { constexpr auto size() const -> unsigned int { return N; } };

constexpr auto foo(array<5> const& arr) {
return array<arr.size()>{}.size();
}

constexpr auto bar(array<5> arr) {
return array<arr.size()>{}.size();
}

int main()
{
array<5> arr {};
static_assert(arr.size() == 5);
static_assert(foo(arr) == 5);
static_assert(bar(arr) == 5);
}

https://godbolt.org/z/q1xajW1Gq

However, if `size` is made static, msvc also compiles:
https://godbolt.org/z/Tfqe5E9z8

The problem seems to be with `arr` being passed by reference, as passing by value also get accepted.
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