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

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

[Marek Polacek]

-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.

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
-- 
2.37.3

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

[Jason Merrill]

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

[PATCH v2] c++: Mitigate -Wuseless-cast with classes [PR85043]

[Marek Polacek]

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)))
 	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
-- 
2.37.3

Re: [PATCH v2] c++: Mitigate -Wuseless-cast with classes [PR85043]

[Jason Merrill]

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

Re: [PATCH v2] c++: Mitigate -Wuseless-cast with classes [PR85043]

[Marek Polacek]

On Wed, Oct 19, 2022 at 02:37:23PM -0400, Jason Merrill wrote:
> 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.

Ah nice, pushed with this instead of the ||:

      if (TYPE_REF_P (type)
          ? ((TYPE_REF_IS_RVALUE (type)
              ? xvalue_p (expr) : lvalue_p (expr))
             && same_type_p (TREE_TYPE (expr), TREE_TYPE (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)))


Thanks,

Marek