Re: [PATCH] c++: Mitigate -Wuseless-cast with classes [PR85043]

[Jason Merrill <jason@redhat.com>]

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...."

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.

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/85043
> 
> gcc/cp/ChangeLog:
> 
> 	* typeck.cc (maybe_warn_about_useless_cast): Don't warn when a class
> 	object 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                           |  5 +++++
>   gcc/doc/invoke.texi                        | 13 ++++++++++-
>   gcc/testsuite/g++.dg/warn/Wuseless-cast.C  |  4 ++--
>   gcc/testsuite/g++.dg/warn/Wuseless-cast3.C | 25 ++++++++++++++++++++++
>   4 files changed, 44 insertions(+), 3 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..a7587f56720 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -8104,6 +8104,11 @@ maybe_warn_about_useless_cast (location_t loc, tree type, tree expr,
>     if (warn_useless_cast
>         && complain & tf_warning)
>       {
> +      /* Don't warn when converting a class object to a non-reference type,
> +	 because that's a common way to create a temporary.  */
> +      if (!TYPE_REF_P (type) && CLASS_TYPE_P (TREE_TYPE (expr)))
> +	return;
> +
>         if ((TYPE_REF_P (type)
>   	   && (TYPE_REF_IS_RVALUE (type)
>   	       ? xvalue_p (expr) : lvalue_p (expr))
> 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..6084b4cef49 100644
> --- a/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
> +++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
> @@ -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..2db07de731f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C
> @@ -0,0 +1,25 @@
> +// 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(static_cast<X&&>(arg));
> +}
> 
> base-commit: 885b6660c17fb91980b5682514ef54668e544b02