https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96765
Bug ID: 96765
Summary: Base class constructor cast to derived should cause a
warning
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jzwinck at gmail dot com
Target Milestone: ---
If I cast "this" in a base class constructor to a derived class type, there is
no warning even with -Wall -Wextra. Such a cast is undefined behavior, and
seems like it should be diagnosed at compile time.
For example:
struct Base
{
Base();
int num;
};
struct Derived : Base
{
int calc() const { return 42; }
};
Base::Base()
// UB: Derived not yet constructed
: num(static_cast<Derived*>(this)->calc())
{
}
int main()
{
Derived d;
return d.num;
}
It compiles cleanly with "-Wall -Wextra -Werror" in GCC 8 and 10, with and
without optimization. I think it should produce a diagnostic such as "invalid
static_cast from type ‘Base*’ to type ‘Derived*’ before the latter is
constructed".
UBSan reports the undefined behavior if Base has a virtual method (e.g. if you
add "virtual ~Base() = default;"), but not with the code as written, making it
even more advantageous to diagnose at compile time.
Live demo: https://godbolt.org/z/x3fa4Y
