Re: [PATCH] c++: Tweaks for -Wredundant-move [PR107363]

[Jason Merrill via Gcc-patches]

On 10/28/22 10:42, Marek Polacek wrote:
Two things here:
1) when we're pointing out that std::move on a constant object is
    redundant, don't say "in return statement" when we aren't in a
    return statement;
2) suppress the warning when the std::move call was dependent, because
    removing the std::move may not be correct for a different
    instantiation of the original template.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.  I'd probably do #2 with a warning_sentinel in 
tsubst_copy_and_build/CALL_EXPR, but this way makes sense, too.

        PR c++/107363

gcc/cp/ChangeLog:

        * semantics.cc (finish_call_expr): Suppress OPT_Wpessimizing_move.
        * typeck.cc (maybe_warn_pessimizing_move): Check warn_redundant_move
        and warning_suppressed_p.  Adjust a message depending on return_p.
        (check_return_expr): Don't suppress OPT_Wpessimizing_move here.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/Wredundant-move13.C: New test.
---
  gcc/cp/semantics.cc                           |  4 ++
  gcc/cp/typeck.cc                              | 16 ++---
  .../g++.dg/cpp0x/Wredundant-move13.C          | 61 +++++++++++++++++++
  3 files changed, 73 insertions(+), 8 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wredundant-move13.C

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 36aa9c4499f..caaa40fde19 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -2738,6 +2738,10 @@ finish_call_expr (tree fn, vec<tree, va_gc> **args, bool 
disallow_virtual,
          result = build_min_nt_call_vec (orig_fn, *args);
          SET_EXPR_LOCATION (result, cp_expr_loc_or_input_loc (fn));
          KOENIG_LOOKUP_P (result) = koenig_p;
+         /* Disable the std::move warnings since this call was dependent
+            (c++/89780, c++/107363).  This also suppresses the
+            -Wredundant-move warning.  */
+         suppress_warning (result, OPT_Wpessimizing_move);
          if (is_overloaded_fn (fn))
            fn = get_fns (fn);
  
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 2e0fd8fbf17..5f5fb2a212b 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -10885,7 +10885,9 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool 
return_p)
       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))
+  else if (warn_redundant_move
+          && !warning_suppressed_p (expr, OPT_Wredundant_move)
+          && TYPE_REF_P (TREE_TYPE (arg))
           && CP_TYPE_CONST_P (TREE_TYPE (TREE_TYPE (arg))))
      {
        tree rtype = TREE_TYPE (TREE_TYPE (arg));
@@ -10901,8 +10903,11 @@ maybe_warn_pessimizing_move (tree expr, tree type, 
bool return_p)
              return;
          }
        auto_diagnostic_group d;
-      if (warning_at (loc, OPT_Wredundant_move,
-                     "redundant move in return statement"))
+      if (return_p
+         ? warning_at (loc, OPT_Wredundant_move,
+                       "redundant move in return statement")
+         : warning_at (loc, OPT_Wredundant_move,
+                       "redundant move in initialization"))
        inform (loc, "remove %<std::move%> call");
      }
  }
@@ -11126,11 +11131,6 @@ check_return_expr (tree retval, bool *no_warning)
        /* We don't know if this is an lvalue or rvalue use, but
         either way we can mark it as read.  */
        mark_exp_read (retval);
-      /* Disable our std::move warnings when we're returning
-        a dependent expression (c++/89780).  */
-      if (retval && TREE_CODE (retval) == CALL_EXPR)
-       /* This also suppresses -Wredundant-move.  */
-       suppress_warning (retval, OPT_Wpessimizing_move);
        return retval;
      }
  
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move13.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move13.C
new file mode 100644
index 00000000000..80e7d80cd02
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move13.C
@@ -0,0 +1,61 @@
+// PR c++/107363
+// { 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); }
+}
+
+template <typename T, typename U>
+struct Optional {
+  U &value();
+  T release_value() {
+    T t = std::move (value ());
+    return t;
+  }
+};
+
+struct Foo {};
+void test(Optional<const Foo, const Foo> o) { o.release_value(); }
+
+struct F {
+  F(const F&);
+  F(F&&) = delete;
+};
+
+struct Z {
+  Z(const Z&) = delete;
+  Z(Z&&) = delete;
+  Z(const Z&&);
+};
+
+const F& constfref();
+const Z& constzref();
+
+void
+g ()
+{
+  // Will call F::F(const F&) w/ and w/o std::move.  So it's redundant.
+  F f = std::move (constfref()); // { dg-warning "redundant move in 
initialization" }
+  (void) f;
+  // Will call Z::Z(const Z&&) w/ std::move, and Z::Z(const Z&) w/o.
+  // So it's not redundant.
+  Z z = std::move (constzref());
+  (void) z;
+}

base-commit: e583c86f49b9ef6991b25308a0ad60de9697f24a