From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH v2] c++: Mitigate -Wuseless-cast with classes [PR85043]
Date: Wed, 19 Oct 2022 14:37:23 -0400 [thread overview]
Message-ID: <b1494b7b-6adc-f038-5cf8-e7e2a9e7d002@redhat.com> (raw)
In-Reply-To: <Y1An8APGvWejfjHX@redhat.com>
On 10/19/22 12:38, Marek Polacek wrote:
> On Wed, Oct 19, 2022 at 09:27:27AM -0400, Jason Merrill wrote:
>> On 10/18/22 13:38, Marek Polacek wrote:
>>> -Wuseless-cast (not part of -Wall/-Wextra) warns here:
>>>
>>> struct S { };
>>> void g (S&&);
>>> void f (S&& arg)
>>> {
>>> g (S(arg)); // warning: useless cast to type 'struct S'
>>> }
>>>
>>> which is wrong: the code will not compile without the cast because
>>> "arg" is an lvalue which cannot bind to S&&.
>>>
>>> I'd like to disable the warning when a class object is cast to
>>> a non-reference type, which seems like a minimal patch that fixes the
>>> problems reported in our Bugzilla. Of course, the cast in "(int)i"
>>> may not be useless, either, but I'm not changing that here.
>>
>> As I commented on PR 14710, "I wouldn't warn about a cast that changes the
>> type or value category of an expression at all...."
>
> Ah, I didn't see that comment.
>
>> The code currently checks the value category when casting to a reference,
>> but not when casting to a non-reference; that seems like the thing to fix.
>
> OK, how about this? The nice thing is that it handles non-classes as well,
> while we still warn about truly useless casts as in "X(X{})". Thanks,
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> -Wuseless-cast (not part of -Wall/-Wextra) warns here:
>
> struct S { };
> void g (S&&);
> void f (S&& arg)
> {
> g (S(arg)); // warning: useless cast to type 'struct S'
> }
>
> which is wrong: the code will not compile without the cast because
> "arg" is an lvalue which cannot bind to S&&.
>
> This patch disables the warning when an object that isn't a prvalue
> is cast to a non-reference type. Therefore we still warn about the
> useless cast in "X(X{})".
>
> PR c++/85043
>
> gcc/cp/ChangeLog:
>
> * typeck.cc (maybe_warn_about_useless_cast): Don't warn when
> a glvalue is cast to a non-reference type.
>
> gcc/ChangeLog:
>
> * doc/invoke.texi: Update documentation of -Wuseless-cast.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wuseless-cast.C: Remove dg-warning.
> * g++.dg/warn/Wuseless-cast3.C: New test.
> ---
> gcc/cp/typeck.cc | 4 ++-
> gcc/doc/invoke.texi | 13 +++++++++-
> gcc/testsuite/g++.dg/warn/Wuseless-cast.C | 12 ++++-----
> gcc/testsuite/g++.dg/warn/Wuseless-cast3.C | 29 ++++++++++++++++++++++
> 4 files changed, 50 insertions(+), 8 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wuseless-cast3.C
>
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index da0e1427b97..8e1d14ee6b0 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -8108,7 +8108,9 @@ maybe_warn_about_useless_cast (location_t loc, tree type, tree expr,
> && (TYPE_REF_IS_RVALUE (type)
> ? xvalue_p (expr) : lvalue_p (expr))
> && same_type_p (TREE_TYPE (expr), TREE_TYPE (type)))
> - || same_type_p (TREE_TYPE (expr), type))
> + /* Don't warn when converting a class object to a non-reference type,
> + because that's a common way to create a temporary. */
> + || (!glvalue_p (expr) && same_type_p (TREE_TYPE (expr), type)))
Might use ?: instead of || so we check this only when !TYPE_REF_P. OK
with that change.
> warning_at (loc, OPT_Wuseless_cast,
> "useless cast to type %q#T", type);
> }
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index c176e2dc646..cd4d3c1d72c 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -4551,7 +4551,18 @@ pointers after reallocation.
> @item -Wuseless-cast @r{(C++ and Objective-C++ only)}
> @opindex Wuseless-cast
> @opindex Wno-useless-cast
> -Warn when an expression is casted to its own type.
> +Warn when an expression is cast to its own type. This warning does not
> +occur when a class object is converted to a non-reference type as that
> +is a way to create a temporary:
> +
> +@smallexample
> +struct S @{ @};
> +void g (S&&);
> +void f (S&& arg)
> +@{
> + g (S(arg)); // make arg prvalue so that it can bind to S&&
> +@}
> +@end smallexample
>
> @item -Wno-conversion-null @r{(C++ and Objective-C++ only)}
> @opindex Wconversion-null
> diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
> index 2fd6bc45102..d7cb89930a6 100644
> --- a/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
> +++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
> @@ -62,11 +62,11 @@ A prvalue();
>
> void f()
> {
> - int n;
> + int n;
>
> - (int)(n); // { dg-warning "3:useless cast" }
> - static_cast<int>(n); // { dg-warning "3:useless cast" }
> - reinterpret_cast<int>(n); // { dg-warning "3:useless cast" }
> + (int)(n);
> + static_cast<int>(n);
> + reinterpret_cast<int>(n);
>
> (int*)(&n); // { dg-warning "3:useless cast" }
> const_cast<int*>(&n); // { dg-warning "3:useless cast" }
> @@ -100,8 +100,8 @@ void f()
>
> A a;
>
> - (A)(a); // { dg-warning "3:useless cast" }
> - static_cast<A>(a); // { dg-warning "3:useless cast" }
> + (A)(a);
> + static_cast<A>(a);
>
> (A*)(&a); // { dg-warning "3:useless cast" }
> const_cast<A*>(&a); // { dg-warning "3:useless cast" }
> diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C
> new file mode 100644
> index 00000000000..43dd2ebfaea
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C
> @@ -0,0 +1,29 @@
> +// PR c++/85043
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wuseless-cast" }
> +
> +struct S { int s; void bump () { s++; } };
> +
> +void
> +foo ()
> +{
> + S s = { 1 };
> + s.bump ();
> + S (s).bump (); // { dg-bogus "useless" }
> + ((S) s).bump (); // { dg-bogus "useless" }
> + static_cast<S> (s).bump (); // { dg-bogus "useless" }
> +}
> +
> +struct X { };
> +void g(X&&);
> +
> +void
> +f (X&& arg)
> +{
> + g(X(arg)); // { dg-bogus "useless" }
> + g(X(X{})); // { dg-warning "useless" }
> + g(static_cast<X&&>(arg));
> +
> + int i = (int) 1; // { dg-warning "useless" }
> + const int &r = (int) i; // { dg-bogus "useless" }
> +}
>
> base-commit: 69a233610f6b27cd4283561569d8ce0f35044dc4
next prev parent reply other threads:[~2022-10-19 18:37 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-18 17:38 [PATCH] " Marek Polacek
2022-10-19 13:27 ` Jason Merrill
2022-10-19 16:38 ` [PATCH v2] " Marek Polacek
2022-10-19 18:37 ` Jason Merrill [this message]
2022-10-19 19:31 ` Marek Polacek
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