[Bug c++/111737] New: Object holding a pointer to an uninitialized c-array not usable in a constant expression

[Bug c++/111737] New: Object holding a pointer to an uninitialized c-array not usable in a constant expression

[benoit.labrique at endress dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111737

            Bug ID: 111737
           Summary: Object holding a pointer to an uninitialized c-array
                    not usable 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: benoit.labrique at endress dot com
  Target Milestone: ---

Following code does not compile (see details below) as soon as there is an
indirection, in this case the function MakeBuffer().

This kind of construct is very common in the embedded world, where dynamic
memory allocation is not permitted. As in the code example, buffers of
different size can be passed to more generic functionality via a reference to
the base class "Collection", which is buffer-size agnostic.

// Compiled with -O2 -std=c++17

class Collection
{
protected:
    constexpr Collection(void * iBuffer)
        : m_buffer(iBuffer)
    {
    }

protected:
    void * m_buffer = nullptr;
};

class Buffer final : public Collection
{
public:
    explicit constexpr Buffer()
        : Collection(m_buffer), m_buffer{0} // gcc doesn't like that m_buffer
is used before it is initialized. Using std::array instead won't work at all.
    {
    }

private:
    int    m_buffer[1];
};

namespace
{
    inline constexpr auto MakeBuffer()
    {
        return Buffer();    // C++17: shall definitively perform copy elision
    }

    constexpr Buffer VIA_MAKE_BUFFER = MakeBuffer();        // Only gcc has a
problem with, neither clang nor msvc
    constexpr Buffer VIA_BUFFER_CONSTRUCTOR = Buffer();     // Why is it now OK
for gcc here?
}

int main()
{
    constexpr auto a = VIA_MAKE_BUFFER;
    constexpr auto b = VIA_BUFFER_CONSTRUCTOR;
    return 0;
}

https://godbolt.org/z/erPsooP4b

Compiling this code issues (ARM GCC 13.2 (or earlier)):

<source>: In function 'int main()':
<source>:38:24: error: the value of '{anonymous}::VIA_MAKE_BUFFER' is not
usable in a constant expression
   38 |     constexpr auto a = VIA_MAKE_BUFFER;
      |                        ^~~~~~~~~~~~~~~
<source>:32:22: note: '{anonymous}::VIA_MAKE_BUFFER' used in its own
initializer
   32 |     constexpr Buffer VIA_MAKE_BUFFER = MakeBuffer();        // Only gcc
has a problem with, neither clang nor msvc
      |                      ^~~~~~~~~~~~~~~

Alternatively: on x86-64 GCC 13.2 (or earlier):

<source>:32:51: error:
'Buffer{Collection{((void*)(&<anonymous>.Buffer::m_buffer))}, int [1]{0}}' is
not a constant expression
   32 |     constexpr Buffer VIA_MAKE_BUFFER = MakeBuffer();        // Only gcc
has a problem with, neither clang nor msvc
      |