missing diagnostic for member access on null pointer in C++11 constant expression evaluation

[llvmbot]

Bugzilla Link	38201
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid

Extended Description

The code is as follow:

struct S { int a, b; } s;

constexpr S *p = (S*)0;
constexpr const int *q = &p->b;

The code is invalid not because of a cast (there is none) but because it attempts to use a null pointer to form the address of a member of an object. clang++ accepts it. I checked g++. It rejects the code:

error: dereferencing a null pointer in '*0'
 constexpr const int *q = &p->b;
                              ^

[zygoloid]

Confirmed. Slightly reduced:

struct S { int a, b; };
constexpr int *q = &((S*)nullptr)->b;

Interesting things: clang rejects accesses to 'a', but not to 'b'. Changing from forming a pointer to forming a reference results in a diagnostic but with no note explaining why the RHS is non-constant.

[wheatman]

This behavior is still the same in post 17 trunk(975deb3)
https://godbolt.org/z/5qsMxd8vz

code

struct S { int a, b; };
constexpr int *q = &((S*)nullptr)->b;

gcc 13.2 error

<source>:2:36: error: dereferencing a null pointer in '*0'
    2 | constexpr int *q = &((S*)nullptr)->b;
      |                                    ^
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [38201](https://llvm.org/bz38201) | | Version | unspecified | | OS | Linux | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor,@zygoloid |

Extended Description

The code is as follow:

struct S { int a, b; } s;

constexpr S p = (S)0;
constexpr const int *q = &p->b;

The code is invalid not because of a cast (there is none) but because it attempts to use a null pointer to form the address of a member of an object. clang++ accepts it. I checked g++. It rejects the code:

error: dereferencing a null pointer in '*0'
constexpr const int *q = &p->b;
^

[shafik]

This is related to: #75716

The address of unary-operator disables the diagnostic, w/o it we obtain the expected diagnostic: https://godbolt.org/z/Kqbn3dvh5