c++: Implement -Wself-move warning [PR81159]

About 5 years ago we got a request to implement -Wself-move, which
warns about useless moves like this:

  int x;
  x = std::move (x);

This patch implements that warning.

	PR c++/81159

gcc/c-family/ChangeLog:

	* c.opt (Wself-move): New option.

gcc/cp/ChangeLog:

	* typeck.cc (maybe_warn_self_move): New.
	(cp_build_modify_expr): Call maybe_warn_self_move.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wself-move.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wself-move1.C: New test.

1 files changed, 52 insertions, 1 deletions

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 7fde65a..b99947c 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -8897,7 +8897,56 @@ cp_build_c_cast (location_t loc, tree type, tree expr,
 
   return error_mark_node;
 }
-
+
+/* Warn when a value is moved to itself with std::move.  LHS is the target,
+   RHS may be the std::move call, and LOC is the location of the whole
+   assignment.  */
+
+static void
+maybe_warn_self_move (location_t loc, tree lhs, tree rhs)
+{
+  if (!warn_self_move)
+    return;
+
+  /* C++98 doesn't know move.  */
+  if (cxx_dialect < cxx11)
+    return;
+
+  if (processing_template_decl)
+    return;
+
+  if (!REFERENCE_REF_P (rhs)
+      || TREE_CODE (TREE_OPERAND (rhs, 0)) != CALL_EXPR)
+    return;
+  tree fn = TREE_OPERAND (rhs, 0);
+  if (!is_std_move_p (fn))
+    return;
+
+  /* Just a little helper to strip * and various NOPs.  */
+  auto extract_op = [] (tree &op) {
+    STRIP_NOPS (op);
+    while (INDIRECT_REF_P (op))
+      op = TREE_OPERAND (op, 0);
+    op = maybe_undo_parenthesized_ref (op);
+    STRIP_ANY_LOCATION_WRAPPER (op);
+  };
+
+  tree arg = CALL_EXPR_ARG (fn, 0);
+  extract_op (arg);
+  if (TREE_CODE (arg) == ADDR_EXPR)
+    arg = TREE_OPERAND (arg, 0);
+  tree type = TREE_TYPE (lhs);
+  tree orig_lhs = lhs;
+  extract_op (lhs);
+  if (cp_tree_equal (lhs, arg))
+    {
+      auto_diagnostic_group d;
+      if (warning_at (loc, OPT_Wself_move,
+		      "moving %qE of type %qT to itself", orig_lhs, type))
+	inform (loc, "remove %<std::move%> call");
+    }
+}
+
 /* For use from the C common bits.  */
 tree
 build_modify_expr (location_t location,
@@ -9101,6 +9150,8 @@ cp_build_modify_expr (location_t loc, tree lhs, enum tree_code modifycode,
 
       if (modifycode == NOP_EXPR)
 	{
+	  maybe_warn_self_move (loc, lhs, rhs);
+
 	  if (c_dialect_objc ())
 	    {
 	      result = objc_maybe_build_modify_expr (lhs, rhs);