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[2603:7000:9500:2e39::1db4]) by smtp.gmail.com with ESMTPSA id w29-20020a05620a0e9d00b006ee7e223bb8sm5173341qkm.39.2022.10.19.09.38.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 19 Oct 2022 09:38:10 -0700 (PDT) Date: Wed, 19 Oct 2022 12:38:08 -0400 From: Marek Polacek To: Jason Merrill Cc: GCC Patches Subject: [PATCH v2] c++: Mitigate -Wuseless-cast with classes [PR85043] Message-ID: References: <20221018173829.392773-1-polacek@redhat.com> <84d8cddc-79f5-3920-fd34-4b2b28010363@redhat.com> MIME-Version: 1.0 In-Reply-To: <84d8cddc-79f5-3920-fd34-4b2b28010363@redhat.com> User-Agent: Mutt/2.2.7 (2022-08-07) X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Spam-Status: No, score=-12.5 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: 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(n); // { dg-warning "3:useless cast" } - reinterpret_cast(n); // { dg-warning "3:useless cast" } + (int)(n); + static_cast(n); + reinterpret_cast(n); (int*)(&n); // { dg-warning "3:useless cast" } const_cast(&n); // { dg-warning "3:useless cast" } @@ -100,8 +100,8 @@ void f() A a; - (A)(a); // { dg-warning "3:useless cast" } - static_cast(a); // { dg-warning "3:useless cast" } + (A)(a); + static_cast(a); (A*)(&a); // { dg-warning "3:useless cast" } const_cast(&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).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(arg)); + + int i = (int) 1; // { dg-warning "useless" } + const int &r = (int) i; // { dg-bogus "useless" } +} base-commit: 69a233610f6b27cd4283561569d8ce0f35044dc4 -- 2.37.3