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From: "barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96557] New: Diagnostics: Can you tell me why it's not a
 constant expression?
Date: Mon, 10 Aug 2020 16:25:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96557

            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] =3D {};
    char const* head =3D storage;
};

void f() {
    constexpr X x =3D {};
}

gcc correctly rejects this with:

error: 'X{"", ((const char*)(& x.X::storage))}' is not a constant expression
    7 |     constexpr X x =3D {};
      |                      ^

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 const=
ant
expression
    constexpr X x =3D {};
                ^   ~~
<source>:7:17: note: pointer to subobject of 'x' is not a constant expressi=
on
<source>:7:17: note: declared here

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