[Bug c++/113047] New: dereferencing a null pointer in a constant expression

[Bug c++/113047] New: dereferencing a null pointer in a constant expression

[gcc_bz at brnz dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113047

            Bug ID: 113047
           Summary: dereferencing a null pointer in a constant expression
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc_bz at brnz dot org
  Target Milestone: ---

Should it be possible to dereference a null pointer in a C++20 constant
expression?

Consider this c++20 code:

struct T {
  constexpr bool isNull() {
    return !this;
  }
};
static_assert(((T*)nullptr)->isNull());

constexpr T* t = nullptr;
static_assert((*t).isNull());


I would expect this to not compile due to the dereferencing of a null pointer
in a constant expression.

With reference to https://godbolt.org/z/q53rz5d69

By default, gcc-13.2 compiles without warning. -Wall yields a -Wnonnull
warning.

clang-17.0.1 produces errors and warning, by default.

msvc 19.38 compiles without warning.

Potentially relevant context:
2748. Accessing static data members via null pointer
https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2748

2823. Implicit undefined behavior when dereferencing pointers
https://cplusplus.github.io/CWG/issues/2823.html

[Bug c++/113047] dereferencing a null pointer in a constant expression

[gcc_bz at brnz dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113047

--- Comment #1 from gcc_bz at brnz dot org ---
I have submitted this same question for clang: 
https://github.com/llvm/llvm-project/issues/75716

[Bug c++/113047] dereferencing a null pointer in a constant expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113047

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=102420

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect this is a dup of bug 102420 .

[Bug c++/113047] dereferencing a null pointer in a constant expression

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113047

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #3 from Jiang An <de34 at live dot cn> ---
CWG2823 has been voted into the standard (see https://wg21.link/p3046r0,
https://wg21.link/n4972).

> Should it be possible to dereference a null pointer in a C++20 constant expression?

Now such derefencing is clearly UB and needs to cause constant evaluation
failure (not only in C++20, IMO).

[Bug c++/113047] dereferencing a null pointer in a constant expression

[nathanieloshead at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113047

Nathaniel Shead <nathanieloshead at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nathanieloshead at gmail dot com

--- Comment #4 from Nathaniel Shead <nathanieloshead at gmail dot com> ---
I've pushed a fix for PR102420 (and hence comment #1), but looking at the DR
this isn't sufficient for the result of CWG2823, for which presumably all of
the following should also start erroring as well (note none of these error in
Clang yet either):

  struct X {
    static constexpr int f() { return 0; }
  };

  constexpr int g(X* x) { return (*x).f(); }  // error
  constexpr int a = g(nullptr);

  constexpr int h(X* x) { return x->f(); }  // error
  constexpr int b = h(nullptr);

  // and similarly
  constexpr int test() {
    int* p = nullptr;
    *p;  // error
    return 0;
  }
  constexpr int t = test();

[Bug c++/113047] dereferencing a null pointer in a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113047

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-01-18
                 CC|                            |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1