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
