[Bug c++/69103] New: Misleading diagnostic for invalid constexpr initialization

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69103

            Bug ID: 69103
           Summary: Misleading diagnostic for invalid constexpr
                    initialization
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

Consider:

static int i;
static int* temp = &i;
static constexpr int *&&r = static_cast<int*&&>(temp) + 1;

This correctly doesn't compile, but produces the misleading error message

prog.cc:3:57: error: modification of '_ZGRL1r0' is not a constant-expression
 static constexpr int *&&r = static_cast<int*&&>(temp) + 1;
                                                         ^

We aren't modifying anything in the initializer.

In contrast, Clang produces:

prog.cc:3:25: error: constexpr variable 'r' must be initialized by a constant
expression
static constexpr int *&&r = static_cast<int*&&>(temp) + 1;
                        ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
prog.cc:3:29: note: read of non-constexpr variable 'temp' is not allowed in a
constant expression
static constexpr int *&&r = static_cast<int*&&>(temp) + 1;
                            ^
prog.cc:2:13: note: declared here
static int* temp = &i;