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, 12 insertions, 1 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index c176e2d..cd4d3c1 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