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List:       gcc-patches
Subject:    [PATCH] c++: Silence -Wuseless-cast warnings during move [PR103480]
From:       Jakub Jelinek via Gcc-patches <gcc-patches () gcc ! gnu ! org>
Date:       2021-12-31 9:30:06
Message-ID: 20211231093006.GE2646553 () tucnak
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Hi!

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)).

Bootstrapped/regtested on x86_64-linux and i686-linux.

2021-12-31  Jakub Jelinek  <jakub@redhat.com>

	PR c++/103480
	* tree.c (move): Add warn_useless_cast warning sentinel.

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

--- gcc/cp/tree.c.jj	2021-12-30 15:12:42.739157171 +0100
+++ gcc/cp/tree.c	2021-12-30 18:39:08.050679041 +0100
@@ -1288,6 +1288,7 @@ move (tree expr)
   tree type = TREE_TYPE (expr);
   gcc_assert (!TYPE_REF_P (type));
   type = cp_build_reference_type (type, /*rval*/true);
+  warning_sentinel w (warn_useless_cast);
   return build_static_cast (input_location, type, expr,
 			    tf_warning_or_error);
 }
--- gcc/testsuite/g++.dg/warn/Wuseless-cast2.C.jj	2021-12-30 18:46:17.437651681 +0100
+++ gcc/testsuite/g++.dg/warn/Wuseless-cast2.C	2021-12-30 18:45:41.044162541 +0100
@@ -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> ();
+}

	Jakub

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