c++pointerslanguage-lawyerconstexprrvalue

May object pointer be always compared with nullptr in a constant expression?

The following program was reduced to demonstrate the question. struct A has a constexpr member function that compares this against some pointer, which can be nullptr. Then this member function is called for a temporary object (taking address of rvalue) inside a constant expression:

struct A {
    A * a = nullptr; // in real program this pointer can be changed
    constexpr bool f() {
        return a != this;
    }
};

static_assert( A{}.f() );

Clang and MSVC are ok with it, but GCC does not like this code, complaining:

<source>:8:21: error: non-constant condition for static assertion
    8 | static_assert( A{}.f() );
      |                ~~~~~^~
<source>:4:18: error: '((&<anonymous>) != 0)' is not a constant expression
    4 |         return a != this;

Online demo: https://gcc.godbolt.org/z/evK1Y1r8e

Is it a well formed program? Or is it always necessary to check that pointers are not nullptr before comparison in a constant expression?

Solution

  • This is just a GCC bug: of course the (symbolic) address of a temporary constructed during constant evaluation is known to that evaluation.