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

[Marek Polacek <polacek@redhat.com>]

-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