Wednesday, November 7, 2018

Digest for comp.lang.c++@googlegroups.com - 25 updates in 10 topics

Robert Wessel <robertwessel2@yahoo.com>: Nov 07 09:18AM -0600

On Tue, 6 Nov 2018 07:13:37 -0000 (UTC), Juha Nieminen
>> come to mind immediately.
 
>Does it even have a modern C++ compiler? Does anybody even know how to use
>such a thing?
 
 
IFAIK, it does not. It does have a C89 compiler.
 
There are also some DSPs still out there with CHAR_BIT as 16 or 32.
Juha Nieminen <nospam@thanks.invalid>: Nov 07 10:06AM

Is this a bug in clang, or is this intended behavior?
Consider the following program:
 
//-----------------------------------------------------------------
#include <iostream>
 
template<typename... Params_t>
constexpr int constexprValue(Params_t...) { return 5; }
 
int main()
{
const bool flag = true;
 
if constexpr(flag)
{
constexpr int value = constexprValue(1, 2, 3);
std::cout << value << "\n";
}
}
//-----------------------------------------------------------------
 
This compiles and works fine. However, change flag to false, and clang
gives an error:
 
error: constexpr variable 'value' must be initialized by a constant
expression
undefined function 'constexprValue<int, int, int>' cannot be used
in a constant expression
 
It must be a bug, right? Because if it's intended, it's incomprehensible
why they would want that to fail.
Chris Vine <chris@cvine--nospam--.freeserve.co.uk>: Nov 07 11:08AM

On Wed, 7 Nov 2018 10:06:32 -0000 (UTC)
> in a constant expression
 
> It must be a bug, right? Because if it's intended, it's incomprehensible
> why they would want that to fail.
 
Your code works for me with clang-7.0.0 and gcc-8.2.0 with the
-std=c++17 compiler flag. constexprValue() is defined whether or not
'flag' evaluates to false or true and is instantiated here for literal
types, and 'flag' is initialized by a literal, so I think it must be a
bug.
"Öö Tiib" <ootiib@hot.ee>: Nov 07 03:10AM -0800

On Wednesday, 7 November 2018 12:06:42 UTC+2, Juha Nieminen wrote:
> in a constant expression
 
> It must be a bug, right? Because if it's intended, it's incomprehensible
> why they would want that to fail.
 
Yes, it is most probably a bug.

It compiles with clang 3.9 , with clangs 4, 5 and 6 it fails and
with 7 it compiles again.
"Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Nov 07 12:40PM +0100

On 07.11.2018 11:06, Juha Nieminen wrote:
> expression
> undefined function 'constexprValue<int, int, int>' cannot be used
> in a constant expression
 
Unable to reproduce with g++ and msvc.
 
 
> It must be a bug, right? Because if it's intended, it's incomprehensible
> why they would want that to fail.
 
Agreed.
 
But consider:
 
-------------------------------------------------------------------------
#include <iostream>
using namespace std;
 
template< class >
struct Baluba;
 
template<> struct Baluba<int>{}; // Defined.
 
template< int x >
void foo()
{
if constexpr( x == 666 )
{
Baluba<double> bah;
}
}
 
void call_foo()
{
foo<42>(); // OK
#ifdef FOO
foo<666>(); // Ungood, doesn't compile.
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