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.

1 files changed, 7 insertions, 5 deletions

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index da0e142..16e7d85 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);
     }