[clang] Array access during constant evaluation is inconsistently rejected

[cookiestarfish]

I hope it is obvious from the example, clang should either reject both element accesses or accept both during constant evaluation.

struct Base
{};

struct Derived : Base
{
    int value;
};

static_assert([]() {
    Derived d[4] {};
    d[0] = Derived{.value=10};
    d[1] = Derived{.value=20};
    d[2] = Derived{.value=30};
    d[3] = Derived{.value=40};

    Base* b = d;
    
    // does not compile
    // return static_cast<Derived*>(b)[2].value;
    
    // using a temporary compiles.
    Derived* temp = static_cast<Derived*>(b);
    return temp[2].value;
}() == 30);

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: marco (cookiestarfish)

I hope it is obvious from the example, clang should either reject both element accesses or accept both during constant evaluation. ``` struct Base {};

struct Derived : Base
{
int value;
};

static_assert( {
Derived d[4] {};
d[0] = Derived{.value=10};
d[1] = Derived{.value=20};
d[2] = Derived{.value=30};
d[3] = Derived{.value=40};

Base* b = d;

// does not compile
// return static_cast<Derived*>(b)[2].value;

// using a temporary compiles.
Derived* temp = static_cast<Derived*>(b);
return temp[2].value;

}() == 30);

</details>

[cor3ntin]

Confirmed - https://godbolt.org/z/MEeW9neKr