c++: Extend -Wredundant-move for const-qual objects [PR90428]

In this PR, Jon suggested extending the -Wredundant-move warning
to warn when the user is moving a const object as in:

  struct T { };

  T f(const T& t)
  {
    return std::move(t);
  }

where the std::move is redundant, because T does not have
a T(const T&&) constructor (which is very unlikely).  Even with
the std::move, T(T&&) would not be used because it would mean
losing the const.  Instead, T(const T&) will be called.

I had to restructure the function a bit, but it's better now.  This patch
depends on my other recent patches to maybe_warn_pessimizing_move.

	PR c++/90428

gcc/cp/ChangeLog:

	* typeck.cc (can_do_rvo_p): Rename to ...
	(can_elide_copy_prvalue_p): ... this.
	(maybe_warn_pessimizing_move): Extend the
	-Wredundant-move warning to warn about std::move on a
	const-qualified object.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/Wredundant-move1.C: Adjust dg-warning.
	* g++.dg/cpp0x/Wredundant-move9.C: Likewise.
	* g++.dg/cpp0x/Wredundant-move10.C: New test.

gcc/cp/typeck.cc

@@ -10301,7 +10301,7 @@ can_do_nrvo_p (tree retval, tree functype)
    prvalue.  */
 
 static bool
-can_do_rvo_p (tree retval, tree functype)
+can_elide_copy_prvalue_p (tree retval, tree functype)
 {
   if (functype == error_mark_node)
     return false;
@@ -10415,36 +10415,38 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
 	return;
     }
 
-  /* We're looking for *std::move<T&> ((T &) &arg).  */
-  if (REFERENCE_REF_P (expr)
-      && TREE_CODE (TREE_OPERAND (expr, 0)) == CALL_EXPR)
-    {
+  /* First, check if this is a call to std::move.  */
+  if (!REFERENCE_REF_P (expr)
+      || TREE_CODE (TREE_OPERAND (expr, 0)) != CALL_EXPR)
+    return;
   tree fn = TREE_OPERAND (expr, 0);
-      if (is_std_move_p (fn))
-	{
+  if (!is_std_move_p (fn))
+    return;
   tree arg = CALL_EXPR_ARG (fn, 0);
-	  tree moved;
   if (TREE_CODE (arg) != NOP_EXPR)
     return;
+  /* If we're looking at *std::move<T&> ((T &) &arg), do the pessimizing N/RVO
+     and implicitly-movable warnings.  */
+  if (TREE_CODE (TREE_OPERAND (arg, 0)) == ADDR_EXPR)
+    {
       arg = TREE_OPERAND (arg, 0);
-	  if (TREE_CODE (arg) != ADDR_EXPR)
-	    return;
       arg = TREE_OPERAND (arg, 0);
       arg = convert_from_reference (arg);
-	  if (can_do_rvo_p (arg, type))
+      if (can_elide_copy_prvalue_p (arg, type))
 	{
 	  auto_diagnostic_group d;
 	  if (warning_at (loc, OPT_Wpessimizing_move,
-			      "moving a temporary object prevents copy "
-			      "elision"))
+			  "moving a temporary object prevents copy elision"))
 	    inform (loc, "remove %<std::move%> call");
 	}
-	  /* The rest of the warnings is only relevant for when we are
-	     returning from a function.  */
-	  else if (!return_p)
+      /* The rest of the warnings is only relevant for when we are returning
+	 from a function.  */
+      if (!return_p)
 	return;
+
+      tree moved;
       /* Warn if we could do copy elision were it not for the move.  */
-	  else if (can_do_nrvo_p (arg, type))
+      if (can_do_nrvo_p (arg, type))
 	{
 	  auto_diagnostic_group d;
 	  if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
@@ -10470,7 +10472,15 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
 					       tf_none);
 	  /* If this worked, implicit rvalue would work, so the call to
 	     std::move is redundant.  */
-	      if (t != error_mark_node)
+	  if (t != error_mark_node
+	      /* Trying to move something const will never succeed unless
+		 there's T(const T&&), which it almost never is, and if
+		 so, T wouldn't be error_mark_node now: the above convert_
+		 call with LOOKUP_PREFER_RVALUE returns an error if a const T&
+		 overload is selected.  */
+	      || (CP_TYPE_CONST_P (TREE_TYPE (arg))
+		  && same_type_ignoring_top_level_qualifiers_p
+		  (TREE_TYPE (arg), type)))
 	    {
 	      auto_diagnostic_group d;
 	      if (warning_at (loc, OPT_Wredundant_move,
@@ -10479,6 +10489,35 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
 	    }
 	}
      }
+  /* Also try to warn about redundant std::move in code such as
+      T f (const T& t)
+      {
+	return std::move(t);
+      }
+    for which EXPR will be something like
+      *std::move<const T&> ((const struct T &) (const struct T *) t)
+     and where the std::move does nothing if T does not have a T(const T&&)
+     constructor, because the argument is const.  It will not use T(T&&)
+     because that would mean losing the const.  */
+  else if (TYPE_REF_P (TREE_TYPE (arg))
+	   && CP_TYPE_CONST_P (TREE_TYPE (TREE_TYPE (arg))))
+    {
+      tree rtype = TREE_TYPE (TREE_TYPE (arg));
+      if (!same_type_ignoring_top_level_qualifiers_p (rtype, type))
+	return;
+      /* Check for the unlikely case there's T(const T&&) (we don't care if
+	 it's deleted).  */
+      for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (rtype)))
+	if (move_fn_p (fn))
+	  {
+	    tree t = TREE_VALUE (FUNCTION_FIRST_USER_PARMTYPE (fn));
+	    if (UNLIKELY (CP_TYPE_CONST_P (TREE_TYPE (t))))
+	      return;
+	  }
+      auto_diagnostic_group d;
+      if (warning_at (loc, OPT_Wredundant_move,
+		      "redundant move in return statement"))
+	inform (loc, "remove %<std::move%> call");
     }
 }
 

gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C

@@ -60,7 +60,8 @@ fn4 (const T t)
 {
   // t is const: will decay into copy despite std::move, so it's redundant.
   // We used to warn about this, but no longer since c++/87378.
-  return std::move (t);	 // { dg-warning "redundant move" "" { target c++20 } }
+  // Now we warn again since c++/90428.
+  return std::move (t);	 // { dg-warning "redundant move" }
 }
 
 int

gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C

@@ -0,0 +1,61 @@
+// PR c++/90428
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wredundant-move" }
+
+// Define std::move.
+namespace std {
+  template<typename _Tp>
+    struct remove_reference
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    constexpr typename std::remove_reference<_Tp>::type&&
+    move(_Tp&& __t) noexcept
+    { return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }
+}
+
+struct T { T(); T(const T&); T(T&&) = delete; };
+struct S : T { };
+struct W { W(const W&); W(W&&) = delete; W(const W&&); };
+
+T f1(T t)
+{
+  const T& rt = t;
+  return std::move(rt); // { dg-warning "redundant move" }
+}
+
+T f2(const T& t)
+{
+  return std::move(t); // { dg-warning "redundant move" }
+}
+
+W f3(const W& w)
+{
+  return std::move(w);
+}
+
+T f4(const S& s)
+{
+  return std::move(s);
+}
+
+T f5(const T t)
+{
+  return std::move(t); // { dg-warning "redundant move" }
+}
+
+struct S1 { S1(S1 &&) = delete; S1(const S1&); };
+struct S2: S1 {};
+
+S1 f3(const S2 s)
+{
+  return std::move(s); // { dg-warning "redundant move" "" { target c++20 } }
+}

gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C

@@ -61,7 +61,8 @@ fn4 (const T<int> t)
 {
   // t is const: will decay into copy despite std::move, so it's redundant.
   // We used to warn about this, but no longer since c++/87378.
-  return std::move (t);	 // { dg-warning "redundant move" "" { target c++20 } }
+  // Now we warn again since c++/90428.
+  return std::move (t);	 // { dg-warning "redundant move" }
 }
 
 int