Compilation error when trying to cast to pointer-to-array with typeof() in C++

[Michael137]

Came up in #152113

When trying to perform following cast:

int main() {
    char *s = "abc";
    (typeof(*s)(*)[3])s;
}

Clang (and GCC FWIW), produce following error (godbolt):

main.cpp:6:18: error: expected expression
    6 |     (typeof(*s)(*)[3])s;

But this works fine in C (godbolt).

However, wrapping the typeof in another typeof works fine in both C (godbolt) and C++ (godbolt):

int main() {
    char *s = "abc";
    *(typeof(typeof(*s))(*)[3])s;
}

What's the correct behaviour here?

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Michael Buch (Michael137)

Came up in https://github.com//issues/152113

When trying to perform following cast:

int main() {
    char *s = "abc";
    (typeof(*s)(*)[3])s;
}

Clang (and GCC FWIW), produce following error (godbolt):

main.cpp:6:18: error: expected expression
    6 |     (typeof(*s)(*)[3])s;

But this works fine in C (godbolt).

However, wrapping the typeof in another typeof works fine in both C (godbolt) and C++ (godbolt):

int main() {
    char *s = "abc";
    *(typeof(typeof(*s))(*)[3])s;
}

What's the correct behaviour here?

[AaronBallman]

I think it's an ambiguous parenthesized expression in C++ because typeof can accept either an expression or a type, but not in C. That said, I've not stepped through the parser in a debugger to verify that guess. Would be worth looking around ParseCXXAmbiguousParenExpression() in case anyone is interested in investigating further.