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

["potswa at mac dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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); }