Clang cannot compute at compile time typeid of a variable of concrete class type with vtable

[ilya-biryukov]

On this code:

// Build with -std=c++20
#include <typeinfo>

struct base { virtual void enable_vtable(); };
struct derived : base {};

derived d;
static_assert(&typeid(d) == &typeid(derived));

Clang produces the following error:

error: static assertion expression is not an integral constant expression
note: typeid applied to object 'd' whose dynamic type is not constant

Even though typeid can be computed at compile time (since d is not a reference and has a concrete object type).
GCC and MSVC compile this code with no errors, see Godbolt.

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[AaronBallman]

Doesn't require C++20, you can see the same "not constant expression" behavior with an array index: https://godbolt.org/z/P54G99a53

[AaronBallman]

Oops, constexpr typeid, however, is a C++20 feature, so this does require C++20 after all. :-)

[llvmbot]

@llvm/issue-subscribers-c-20

[AaronBallman]

http://eel.is/c++draft/expr.const#5.26 disallows use of typeid when the dynamic type is constexpr-unknown.
http://eel.is/c++draft/expr.const#8 defines this type as not being constexpr-unknown, so I believe this code should be accepted.

It's worth noting that GCC does not seem to properly note when the type is constexpr-unknown, though MSVC does: https://godbolt.org/z/x8a4c5xdq

[cor3ntin]

Related #63139

[cor3ntin]

Fixed by #95474