[Bug c++/96557] New: Diagnostics: Can you tell me why it's not a constant expression?

["barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96557

            Bug ID: 96557
           Summary: Diagnostics: Can you tell me why it's not a constant
                    expression?
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider:

struct X {
    char storage[100] = {};
    char const* head = storage;
};

void f() {
    constexpr X x = {};
}

gcc correctly rejects this with:

error: 'X{"", ((const char*)(& x.X::storage))}' is not a constant expression
    7 |     constexpr X x = {};
      |                      ^

It'd be great if the error here actually indicated _why_ it's not a constant
expression (or how to fix it). clang does a little bit better:

<source>:7:17: error: constexpr variable 'x' must be initialized by a constant
expression
    constexpr X x = {};
                ^   ~~
<source>:7:17: note: pointer to subobject of 'x' is not a constant expression
<source>:7:17: note: declared here

But ideally we get some message about specifically 'head' pointing to 'storage'
and a fixup suggesting making the variable x static.