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* [gcc r13-3388] c++: Mitigate -Wuseless-cast with classes [PR85043]
@ 2022-10-19 19:30 Marek Polacek
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From: Marek Polacek @ 2022-10-19 19:30 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:b3c98d6a59a6dcd5b0b52bd5676b586ef4fe785f
commit r13-3388-gb3c98d6a59a6dcd5b0b52bd5676b586ef4fe785f
Author: Marek Polacek <polacek@redhat.com>
Date: Tue Oct 18 12:20:14 2022 -0400
c++: Mitigate -Wuseless-cast with classes [PR85043]
-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.
Diff:
---
gcc/cp/typeck.cc | 12 +++++++-----
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, 54 insertions(+), 12 deletions(-)
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index da0e1427b97..16e7d85793d 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -8104,11 +8104,13 @@ maybe_warn_about_useless_cast (location_t loc, tree type, tree expr,
if (warn_useless_cast
&& complain & tf_warning)
{
- 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)))
- || same_type_p (TREE_TYPE (expr), type))
+ 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)))
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" }
+}
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