[Bug c++/96877] Erroneous warning when default initializing function pointer types defined using std::declval

[insertinterestingnamehere at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96877

--- Comment #6 from Ian Henriksen  
---
Thanks, this makes sense. I originally got this idea from
https://stackoverflow.com/a/27489923. The discussion there implied there was
some kind of ambiguity in the standard and showed some examples where exception
specification appeared to be carried through as a part of the type in order to
satisfy the rules about assigning to function pointers with noexcept
specifiers. After revisiting their examples I've found that there's actually
inconsistency with how actual values are handled vs how values from
std::declval are handled. For example, the following is accepted as valid C++11
by both gcc and clang:


#include 
#include 

void (*function_ptr)(void *) noexcept = nullptr;
using function_type = decltype(function_ptr);

using function_type_2 = std::remove_reference())>::type;

function_type thing1;
function_type_2 thing2;
static_assert(noexcept(thing1(nullptr)), "");
static_assert(!noexcept(thing2(nullptr)), "");
static_assert(std::is_same::value, "");


See https://godbolt.org/z/YbzGM3.

It's not obvious to me what the correct behavior would be based off of the
actual wording of the standard, but this is bizarre. This behavior is
consistent across clang and the last 3 major gcc releases, but the implicit
inclusion of the exception specification in the type does not seem like
something that should be relied upon.

[Bug c++/96877] Erroneous warning when default initializing function pointer types defined using std::declval

[insertinterestingnamehere at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96877

--- Comment #4 from Ian Henriksen  
---
It's worth noting that, with g++

using function_type = void (*)(void*) noexcept;

actually works, but

typedef void(*function_type)(void*) noexcept;

does not. clang++ rejects both though.

[Bug c++/96877] Erroneous warning when default initializing function pointer types defined using std::declval

[insertinterestingnamehere at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96877

--- Comment #3 from Ian Henriksen  
---
The goal of doing it that way was get the exception specification onto the
pointer type in C++11 and C++14. The intent was to get the equivalent of

typedef void(*function_type)(void*) noexcept;

but with standards earlier than C++17.

[Bug c++/96877] Erroneous warning when default initializing function pointer types defined using std::declval

[insertinterestingnamehere at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96877

Ian Henriksen  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |INVALID

--- Comment #1 from Ian Henriksen  
---
Actually this is my mistake. std::declval returns by reference, so this is
actually a reference type and the compiler is right to warn about the bad
initialization. In this case the correct thing to do is to remove the reference
after getting the type from decltype.

[Bug c++/96877] New: Erroneous warning when default initializing function pointer types defined using std::declval

[insertinterestingnamehere at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96877

Bug ID: 96877
   Summary: Erroneous warning when default initializing function
pointer types defined using std::declval
   Product: gcc
   Version: 10.2.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: insertinterestingnamehere at gmail dot com
  Target Milestone: ---

When compiling the following with -Wextra (C++11 and later, all recent version
of gcc) the compiler emits an erroneous warning about binding a temporary in a
constructor. Note: this works fine if function_type is declared as a plain
typedef instead of using decltype and std::declval. See
https://godbolt.org/z/sjKdP5.



#include 

using function_type = decltype(std::declval());

struct S {
  function_type fptr = nullptr;
};

int main() {
S thing;
}

[Bug c++/85775] New: False positive with -Wparentheses

[insertinterestingnamehere at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85775

Bug ID: 85775
   Summary: False positive with -Wparentheses
   Product: gcc
   Version: 8.1.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: insertinterestingnamehere at gmail dot com
  Target Milestone: ---

g++ 8.1 warns about unnecessary parentheses in some cases where they are
actually needed to designate something as a single argument to a macro. Here's
a minimal example:

#define M(x) x

template 
struct A {
static const volatile char *M(B);
};

template 
const volatile char *M((A<T, S>::B)) = nullptr;


As it stands, the compiler warns that the double parentheses are unnecessary,
but if they are removed the preprocessor complains that two arguments are being
passed to a macro that only expects one. The only work around, for now, is to
locally disable the warning with pragmas.