See:

constexpr _Complex double d = {1.0, 2.0, 3.0, 45};
constexpr  double e[2]      = {1.0, 2.0, 3.0, 45};

Godbolt: https://godbolt.org/z/P7vqM4dq9

Output:

<source>:3:37: error: excess elements in scalar initializer
constexpr _Complex double d = {1.0, 2.0, 3.0, 45};
                                    ^~~
<source>:4:42: error: excess elements in array initializer
constexpr  double e[2]      = {1.0, 2.0, 3.0, 45};
                                         ^~~

Clang highlights the second entry for the _Complex variable, but it should highlight the third I think.

I have other issues with the diagnostics here, but since this is about _Complex and that is a fringe feature anyway, I don't care much about those and fixing the off-by-one error should be enough.