c++: Silence -Wuseless-cast warnings during move [PR103480]

This is maybe just a shot in the dark, but IMHO we shouldn't be diagnosing
-Wuseless-cast on casts the compiler adds on its own when calling its move
function.  We don't seem to warn when user calls std::move either.
We call move on elinit (*NON_LVALUE_EXPR <(struct C[2] &&) &D.2497->b>)[0]
so it is already an xvalue_p and try to static_cast it to struct C &&.
But we don't warn e.g. on std::move (std::move (whatever)).

Fixed by not doing the static cast and just returning expr from move
if expr is already an xvalue.

2022-01-11  Jakub Jelinek  <jakub@redhat.com>
	    Jason Merrill  <jason@redhat.com>

	PR c++/103480
	* tree.c (move): If expr is xvalue_p, just return expr without
	build_static_cast.

	* g++.dg/warn/Wuseless-cast2.C: New test.

2 files changed, 26 insertions, 0 deletions

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index d0c6490..7f7de86 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -1304,6 +1304,8 @@ move (tree expr)
 {
   tree type = TREE_TYPE (expr);
   gcc_assert (!TYPE_REF_P (type));
+  if (xvalue_p (expr))
+    return expr;
   type = cp_build_reference_type (type, /*rval*/true);
   return build_static_cast (input_location, type, expr,
 			    tf_warning_or_error);
diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast2.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast2.C
new file mode 100644
index 0000000..22e4039
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast2.C
@@ -0,0 +1,24 @@
+// PR c++/103480
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wuseless-cast" }
+
+template <typename T, int N>
+struct A { typedef T t[N]; };
+template <typename T, int N>
+struct B { typename A<T, N>::t b; };
+struct C {
+  constexpr C (C &&) {}
+  template <int N>
+  static auto bar ()
+  {
+    B<C, N> r;
+    return r;		// { dg-bogus "useless cast to type" }
+  }
+  C () = default;
+};
+
+void
+foo ()
+{
+  C::bar<2> ();
+}