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

[egallager at gcc dot gnu.org]

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

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-08-21
                 CC|                            |egallager at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to TC from comment #0)
> 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;
>                                                          ^

The '_ZGRL1r0' is now '<temporary>':

$ /usr/local/bin/g++ -c -Wall -Wextra -pedantic 69103.cc
69103.cc:3:57: error: modification of ‘<temporary>’ is not a constant
expression
 static constexpr int *&&r = static_cast<int*&&>(temp) + 1;
                                                         ^
$

(In reply to Mikhail Maltsev from comment #1)
> Link to the original thread, for the record:
> https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/
> WS5pLaXwC8Y
> 
> _ZGRL1r0 demangles as "reference temporary #0 for r"

Confirmed that demangling it like that would still be better though.

(In reply to TC from comment #0)
> 
> 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;

...and of course doing like clang here would be the best.