[Bug c++/57857] New: argument of decltype used by no return value warning

[Bug c++/57857] New: argument of decltype used by no return value warning

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57857

            Bug ID: 57857
           Summary: argument of decltype used by no return value warning
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: potswa at mac dot com

The following program complains that "declval() must not be used!" in a static
assertion if -Wall is passed. But declval() is only present in the unevaluated
context of a decltype specifier.

The issue seems to be linked to the generation of the warning message. But the
warning message will be present without the static_assert if the function has
more than one exit point, such as if(0) throw; or if(0) return; (the latter
using -fpermissive).


#include <utility>

template <typename U>
auto foo() -> decltype(std::declval<U>() + std::declval<U>());

template <typename T>
decltype(foo<T>()) bar(T) {
//  if ( 0 ) throw;
}

int main()
{ bar(1); }

[Bug c++/57857] argument of decltype used by no return value warning

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57857

--- Comment #1 from David Krauss <potswa at mac dot com> ---
Narrowing down the cause, the statement { 0; } silences the error but {
void(0); } does not.

[Bug c++/57857] argument of decltype used by no return value warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57857

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |5.5.0
           Keywords|                            |rejects-valid
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=71187
      Known to work|                            |6.1.0
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC 6+ produces:
<source>: In instantiation of 'decltype (foo<T>()) bar(T) [with T = int;
decltype (foo<T>()) = int]':
<source>:13:8:   required from here
<source>:10:1: warning: no return statement in function returning non-void
[-Wreturn-type]
 }
 ^

And no calling of the static_assert .