constexpr dynamic_cast failure

[gbMattN]

As of c++20, dynamic_cast expressions can appear in constexpr expressions, but the following code

struct Base {
    virtual int foo() {
        return 1;
    }
};
struct Derived : public Base {};

Derived d;

int test_it (void) {
    constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
    return x;
}

int main(void) {
    return 0;
}

Gives the error

<source>:11:26: error: constexpr variable 'x' must be initialized by a constant expression
   11 |     constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
      |                          ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:11:30: note: dynamic_cast applied to object 'd' whose dynamic type is not constant
   11 |     constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
      |                              ^
1 error generated.

This was built on Ubuntu 22.04 using the main branch of llvm at time of reporting (75f295c) with the -stdc++20 flag.

It builds successfully on MSVC since v19.36 and GCC since 10.1

[llvmbot]

@llvm/issue-subscribers-c-20

Details

As of c++20, dynamic_cast expressions can appear in constexpr expressions, but the following code ``` struct Base { virtual int foo() { return 1; } }; struct Derived : public Base {};

Derived d;

int test_it (void) {
constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
return x;
}

int main(void) {
return 0;
}

Gives the error

<source>:11:26: error: constexpr variable 'x' must be initialized by a constant expression
11 | constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:11:30: note: dynamic_cast applied to object 'd' whose dynamic type is not constant
11 | constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
| ^
1 error generated.

This was built on Ubuntu 22.04 using the main branch of llvm at time of reporting (75f295c2455131993f9c945320736b84c5fcf99c) with the -stdc++20 flag.

It builds successfully on MSVC since v19.36 and GCC since 10.1
</details>

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Details

As of c++20, dynamic_cast expressions can appear in constexpr expressions, but the following code ``` struct Base { virtual int foo() { return 1; } }; struct Derived : public Base {};

Derived d;

int test_it (void) {
constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
return x;
}

int main(void) {
return 0;
}

Gives the error

<source>:11:26: error: constexpr variable 'x' must be initialized by a constant expression
11 | constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:11:30: note: dynamic_cast applied to object 'd' whose dynamic type is not constant
11 | constexpr auto const x= (dynamic_cast<void *>(&d)!=0);
| ^
1 error generated.

This was built on Ubuntu 22.04 using the main branch of llvm at time of reporting (75f295c2455131993f9c945320736b84c5fcf99c) with the -stdc++20 flag.

It builds successfully on MSVC since v19.36 and GCC since 10.1
</details>

[cor3ntin]

I think this yet another example of #63139 wich Clang does not implement yet
@shafik

[cor3ntin]

@shafik can you add that to your tests?

[cor3ntin]

@shafik this still reproduces

[cor3ntin]

Actually, this seems to boil down to the examples here, which are supposed to be invalid
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html#still-further-cases

So I'm not sure there is a bug here - and that matches my understanding of https://eel.is/c++draft/expr.const#10.26 @brevzin

[frederick-vs-ja]

Actually, this seems to boil down to the examples here, which are supposed to be invalid https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html#still-further-cases

So I'm not sure there is a bug here - and that matches my understanding of https://eel.is/c++draft/expr.const#10.26 @brevzin

This seems to be related to a regression introduced by P2280R4, for which @zygoloid reported CWG2740.

But it's curious that Clang rejected the example even before implementing P2280R4.