https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102307
--- Comment #7 from jyhgekyfbkjsyebf at protonmail dot ch ---
Note that:
* based on repro: if I add constexpr to the first ctor and provide body, it
will compile without error. Like this:
constexpr Matrix(double const (&arr)[N][M]) {}
* based on repro: if I comment out the second ctor declaration, compiler will
produce an error (instead of a crash). Like this:
// constexpr Matrix(std::array<std::array<double, M>, N> const &arr);
* based on repro: if I add a data member, and
* provide body to the first ctor, and
* inside use std::copy to fill the member
the compiler will crash. Like this:
```
#include <algorithm>
#include <array>
template <unsigned N, unsigned M> struct Matrix {
std::array<double, N * M> mat;
Matrix(double const (&arr)[N][M]) {
std::copy(&arr[0][0], &arr[0][0] + N * M, std::begin(mat));
}
constexpr Matrix(std::array<std::array<double, M>, N> const &arr);
};
int main() { constexpr Matrix<2, 3> mat{{{1.0, 2.0, 3.0}, {4.0, 5.0, 6.0}}}; }
```
* based on last case: if I replace std::copy call with 2 for loops, and
* add constexpr to first ctor, and
* add -std=c++20 flag
the code will compile. Like this:
```
constexpr Matrix(double const (&arr)[N][M]) {
for (auto n{0U}; n < N; ++n) {
for (auto m{0U}; m < M; ++m)
mat[n * M + m] = arr[n][m];
}
}
```
As you can see, the results due to slight changes are all over the place. I
leave it up to you to interpret them. All of the above is reproducible on
godbolt. I only used local gcc to generate backtrace, everything else was
tinkered with there.
