Re: [PATCH] c++: suppress -Waddress warnings with *_cast [PR105569]

[Jason Merrill <jason@redhat.com>]

On 5/16/22 13:06, Marek Polacek wrote:
> dynamic_cast can legally return nullptr, so I don't think it's helpful
> for -Waddress to warn for
> 
>    if (dynamic_cast<A*>(&ref))
>      // ...
> 
> More generally, it's likely not useful to warn for the artificial
> POINTER_PLUS_EXPRs created by build_base_path.

Normally the POINTER_PLUS_EXPR is guarded by if (nonnull).  But 
build_base_path isn't adding that guard in this case because the operand 
is known to be a reference, which cannot be null 
(http://eel.is/c++draft/dcl.ref#5).  So a warning is indicated for this 
testcase, though it would be good to give a more informative one 
("comparing address of reference to null").

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/105569
> 
> gcc/cp/ChangeLog:
> 
> 	* class.cc (build_base_path): Suppress -Waddress warning.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/warn/Waddress-9.C: New test.
> ---
>   gcc/cp/class.cc                        |  2 ++
>   gcc/testsuite/g++.dg/warn/Waddress-9.C | 34 ++++++++++++++++++++++++++
>   2 files changed, 36 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Waddress-9.C
> 
> diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> index 3c195b35396..06167380423 100644
> --- a/gcc/cp/class.cc
> +++ b/gcc/cp/class.cc
> @@ -518,6 +518,8 @@ build_base_path (enum tree_code code,
>   
>     expr = build1 (NOP_EXPR, ptr_target_type, expr);
>   
> +  suppress_warning (expr, OPT_Waddress);
> +
>    indout:
>     if (!want_pointer)
>       {
> diff --git a/gcc/testsuite/g++.dg/warn/Waddress-9.C b/gcc/testsuite/g++.dg/warn/Waddress-9.C
> new file mode 100644
> index 00000000000..2ec41949ccf
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Waddress-9.C
> @@ -0,0 +1,34 @@
> +// PR c++/105569
> +// { dg-do compile { target c++11 } }
> +// { dg-options -Waddress }
> +
> +class A {};
> +
> +class B : public virtual A {};
> +
> +class C : public A {};
> +
> +int main() {
> +    B* object = new B();
> +    B &ref = *object;
> +
> +    // -Waddress warns here
> +    bool b = nullptr == dynamic_cast<A*>(&ref); // { dg-bogus "-Waddress" }
> +    bool b4 = nullptr == static_cast<A*>(&ref); // { dg-bogus "-Waddress" }
> +    if (dynamic_cast<A*>(&ref)) // { dg-bogus "-Waddress" }
> +      {
> +      }
> +    if (static_cast<A*>(&ref)) // { dg-bogus "-Waddress" }
> +      {
> +      }
> +
> +    // -Waddress doesn't warn anymore
> +    auto ptr = dynamic_cast<A*>(&ref);
> +    bool b2 = ptr == nullptr;
> +
> +    C* cobject = new C();
> +    C &cref = *cobject;
> +
> +    // -Waddress also doesn't warn anymore
> +    bool b3 = nullptr == dynamic_cast<A*>(&cref);
> +}
> 
> base-commit: 682e587f1021241758f7dfe0b22651008622a312