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.

gcc/c-family/c.opt

@@ -1229,6 +1229,10 @@ Wselector
 ObjC ObjC++ Var(warn_selector) Warning
 Warn if a selector has multiple methods.
 
+Wself-move
+C++ ObjC++ Var(warn_self_move) Warning LangEnabledBy(C++ ObjC++, Wall)
+Warn when a value is moved to itself with std::move.
+
 Wsequence-point
 C ObjC C++ ObjC++ Var(warn_sequence_point) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn about possible violations of sequence point rules.

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);

gcc/doc/invoke.texi

@@ -264,7 +264,7 @@ in the following sections.
 -Wreorder  -Wregister @gol
 -Wstrict-null-sentinel  -Wno-subobject-linkage  -Wtemplates @gol
 -Wno-non-template-friend  -Wold-style-cast @gol
--Woverloaded-virtual  -Wno-pmf-conversions -Wsign-promo @gol
+-Woverloaded-virtual  -Wno-pmf-conversions -Wself-move -Wsign-promo @gol
 -Wsized-deallocation  -Wsuggest-final-methods @gol
 -Wsuggest-final-types  -Wsuggest-override  @gol
 -Wno-terminate  -Wuseless-cast  -Wno-vexing-parse  @gol
@@ -5838,6 +5838,7 @@ Options} and @ref{Objective-C and Objective-C++ Dialect Options}.
 -Wreorder   @gol
 -Wrestrict   @gol
 -Wreturn-type  @gol
+-Wself-move @r{(only for C++)}  @gol
 -Wsequence-point  @gol
 -Wsign-compare @r{(only in C++)}  @gol
 -Wsizeof-array-div @gol
@@ -6823,6 +6824,26 @@ of a declaration:
 
 This warning is enabled by @option{-Wall}.
 
+@item -Wno-self-move @r{(C++ and Objective-C++ only)}
+@opindex Wself-move
+@opindex Wno-self-move
+This warning warns when a value is moved to itself with @code{std::move}.
+Such a @code{std::move} typically has no effect.
+
+@smallexample
+struct T @{
+@dots{}
+@};
+void fn()
+@{
+  T t;
+  @dots{}
+  t = std::move (t);
+@}
+@end smallexample
+
+This warning is enabled by @option{-Wall}.
+
 @item -Wsequence-point
 @opindex Wsequence-point
 @opindex Wno-sequence-point

gcc/testsuite/g++.dg/warn/Wself-move1.C

@@ -0,0 +1,125 @@
+// PR c++/81159
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wself-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); }
+}
+
+int g;
+
+struct S {
+  int x;
+  S(S&& o) {
+    x = std::move (x); // { dg-warning "moving '\[^\n\r]*S::x' of type .int. to itself" }
+    x = std::move (o.x);
+    o.x = std::move (x);
+    o.x = std::move (o.x); // { dg-warning "moving 'o.S::x' of type .int. to itself" }
+  }
+  void foo (int x) {
+    x = std::move (x); // { dg-warning "moving 'x' of type .int. to itself" }
+  }
+};
+
+struct X {
+  int x;
+  X(int x) : x(std::move (x)) { }
+};
+
+struct A {};
+struct B { A a; };
+struct C { C(); ~C(); };
+struct D { D(); D(const D&); D(D&&); D& operator=(const D&); };
+
+void
+test ()
+{
+  int i = 42;
+  i = std::move (i); // { dg-warning "moving 'i' of type .int. to itself" }
+  (i) = std::move (i); // { dg-warning "moving 'i' of type .int. to itself" }
+
+  g = std::move (g); // { dg-warning "moving 'g' of type .int. to itself" }
+  (g) = std::move (g); // { dg-warning "moving 'g' of type .int. to itself" }
+
+  A a;
+  a = std::move (a); // { dg-warning "moving 'a' of type .A. to itself" }
+  (a) = std::move (a); // { dg-warning "moving 'a' of type .A. to itself" }
+
+  B b;
+  b = std::move (b); // { dg-warning "moving 'b' of type .B. to itself" }
+  (b) = std::move (b); // { dg-warning "moving 'b' of type .B. to itself" }
+  b.a = std::move (b.a); // { dg-warning "moving 'b.B::a' of type .A. to itself" }
+  (b.a) = std::move (b.a); // { dg-warning "moving 'b.B::a' of type .A. to itself" }
+
+  C c;
+  c = std::move (c); // { dg-warning "moving 'c' of type .C. to itself" }
+  D d;
+  d = std::move (d); // { dg-warning "moving 'd' of type .D. to itself" }
+}
+
+template<typename T>
+void ttest ()
+{
+  T t;
+  t = std::move (t); // { dg-warning "moving 't' of type .A. to itself" }
+}
+
+template void ttest<A>();
+
+void
+testref (int &r, int &&rr)
+{
+  r = std::move (r); // { dg-warning "moving 'r' of type .int. to itself" }
+  rr = std::move (rr); // { dg-warning "moving 'rr' of type .int. to itself" }
+}
+
+// Test various other arguments to std::move.
+template<typename T>
+void
+testargs (T *Tptr, T **Tpptr, T& Tref, T&& Trref, const T *Tcptr)
+{
+  Tptr = std::move (Tptr); // { dg-warning "moving 'Tptr' of type 'int\\*' to itself" }
+  *Tptr = std::move (*Tptr); // { dg-warning "moving '\\* Tptr' of type 'int' to itself" }
+  *Tptr = std::move (*(Tptr)); // { dg-warning "moving '\\* Tptr' of type 'int' to itself" }
+  *(Tptr) = std::move (*Tptr); // { dg-warning "moving '\\* Tptr' of type 'int' to itself" }
+  *(Tptr + 1) = std::move (*(Tptr + 1)); // { dg-warning "moving '\[^\n\r]*Tptr\[^\n\r]*' of type 'int' to itself" }
+  *(Tptr + 1) = std::move (*(Tptr + 2));
+  (*(Tptr)) = std::move (*Tptr); // { dg-warning "moving '\\* Tptr' of type 'int' to itself" }
+  *Tpptr = std::move (*Tpptr); // { dg-warning "moving '\\* Tpptr' of type 'int\\*' to itself" }
+  **Tpptr = std::move (**Tpptr); // { dg-warning "moving '\\* \\* Tpptr' of type 'int' to itself" }
+  Tref = std::move (Tref); // { dg-warning "moving 'Tref' of type 'int' to itself" }
+  Trref = std::move (Trref); // { dg-warning "moving 'Trref' of type 'int' to itself" }
+  Tcptr = std::move (Tcptr); // { dg-warning "moving 'Tcptr' of type 'const int\\*' to itself" }
+  (Tptr) = std::move (Tptr); // { dg-warning "moving 'Tptr' of type 'int\\*' to itself" }
+  (*Tptr) = std::move (*Tptr); // { dg-warning "moving '\\* Tptr' of type 'int' to itself" }
+  (*Tpptr) = std::move (*Tpptr); // { dg-warning "moving '\\* Tpptr' of type 'int\\*' to itself" }
+  (**Tpptr) = std::move (**Tpptr); // { dg-warning "moving '\\* \\* Tpptr' of type 'int' to itself" }
+  (*(*(Tpptr))) = std::move (**Tpptr); // { dg-warning "moving '\\* \\* Tpptr' of type 'int' to itself" }
+  (Tref) = std::move (Tref); // { dg-warning "moving 'Tref' of type 'int' to itself" }
+  (Trref) = std::move (Trref); // { dg-warning "moving 'Trref' of type 'int' to itself" }
+  (Tcptr) = std::move (Tcptr); // { dg-warning "moving 'Tcptr' of type 'const int\\*' to itself" }
+}
+
+void
+call_testargs ()
+{
+  int i = 42;
+  int *p = &i;
+  testargs<int>(&i, &p, i, 42, &i);
+}