On 3/20/23 18:06, Marek Polacek wrote:
On Sat, Mar 18, 2023 at 08:35:36AM -0400, Jason Merrill wrote:
On 3/17/23 16:29, Marek Polacek wrote:
Based on <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107532#c24>,
it seems like we should treat *any* class with a reference member
as a reference wrapper. This simplifies the code so I'm happy to
make that change.
The patch, however, does not suppress the warning in
int i = 42;
auto const& v = std::get<0>(std::tuple<int&>(i));
Why not? tuple<int&> has an int& member, doesn't it? Do we need to look
into bases as well?
Indeed. I don't know why I didn't do it right away; it's really not that
complicated:
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
OK.
-- >8 --
Based on <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107532#c24>,
it seems like we should treat *any* class with a reference member
as a reference wrapper. To suppress the warning in
int i = 42;
auto const& v = std::get<0>(std::tuple<int&>(i));
we have to look into base classes as well. For std::tuple, this means
that we have to check the _Head_base subobject, which is a non-direct
base class of std::tuple. So I've employed a DFS walk.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (class_has_reference_member_p): New.
(class_has_reference_member_p_r): New.
(reference_like_class_p): Don't look for a specific constructor.
Use a DFS walk with class_has_reference_member_p_r.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference11.C: New test.
* g++.dg/warn/Wdangling-reference12.C: New test.
---
gcc/cp/call.cc | 63 +++++++++++--------
.../g++.dg/warn/Wdangling-reference11.C | 23 +++++++
.../g++.dg/warn/Wdangling-reference12.C | 12 ++++
3 files changed, 72 insertions(+), 26 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference12.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index c52a09b9be2..429170e43ea 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13783,8 +13783,31 @@ std_pair_ref_ref_p (tree t)
/* Return true if a class CTYPE is either std::reference_wrapper or
std::ref_view, or a reference wrapper class. We consider a class
- a reference wrapper class if it has a reference member and a
- constructor taking the same reference type. */
+ a reference wrapper class if it has a reference member. We no
+ longer check that it has a constructor taking the same reference type
+ since that approach still generated too many false positives. */
+
+static bool
+class_has_reference_member_p (tree t)
+{
+ for (tree fields = TYPE_FIELDS (t);
+ fields;
+ fields = DECL_CHAIN (fields))
+ if (TREE_CODE (fields) == FIELD_DECL
+ && !DECL_ARTIFICIAL (fields)
+ && TYPE_REF_P (TREE_TYPE (fields)))
+ return true;
+ return false;
+}
+
+/* A wrapper for the above suitable as a callback for dfs_walk_once. */
+
+static tree
+class_has_reference_member_p_r (tree binfo, void *)
+{
+ return (class_has_reference_member_p (BINFO_TYPE (binfo))
+ ? integer_one_node : NULL_TREE);
+}
static bool
reference_like_class_p (tree ctype)
@@ -13800,31 +13823,19 @@ reference_like_class_p (tree ctype)
if (decl_in_std_namespace_p (tdecl))
{
tree name = DECL_NAME (tdecl);
- return (name
- && (id_equal (name, "reference_wrapper")
- || id_equal (name, "span")
- || id_equal (name, "ref_view")));
- }
- for (tree fields = TYPE_FIELDS (ctype);
- fields;
- fields = DECL_CHAIN (fields))
- {
- if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
- continue;
- tree type = TREE_TYPE (fields);
- if (!TYPE_REF_P (type))
- continue;
- /* OK, the field is a reference member. Do we have a constructor
- taking its type? */
- for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
- {
- tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
- if (args
- && same_type_p (TREE_VALUE (args), type)
- && TREE_CHAIN (args) == void_list_node)
- return true;
- }
+ if (name
+ && (id_equal (name, "reference_wrapper")
+ || id_equal (name, "span")
+ || id_equal (name, "ref_view")))
+ return true;
}
+
+ /* Some classes, such as std::tuple, have the reference member in its
+ (non-direct) base class. */
+ if (dfs_walk_once (TYPE_BINFO (ctype), class_has_reference_member_p_r,
+ nullptr, nullptr))
+ return true;
+
return false;
}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
new file mode 100644
index 00000000000..667618e7196
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
@@ -0,0 +1,23 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct R
+{
+ int& r;
+ int& get() { return r; }
+ int&& rget() { return static_cast<int&&>(r); }
+};
+
+int main()
+{
+ int i = 42;
+ int& l = R{i}.get(); // { dg-bogus "dangling reference" }
+ int const& cl = R{i}.get(); // { dg-bogus "dangling reference" }
+ int&& r = R{i}.rget(); // { dg-bogus "dangling reference" }
+ int const&& cr = R{i}.rget(); // { dg-bogus "dangling reference" }
+ (void) l;
+ (void) r;
+ (void) cr;
+ (void) cl;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference12.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference12.C
new file mode 100644
index 00000000000..85e01f01a50
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference12.C
@@ -0,0 +1,12 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <tuple>
+
+int main()
+{
+ int i = 42;
+ auto const& v = std::get<0>(std::tuple<int&>(i)); // { dg-bogus "dangling
reference" }
+ (void) v;
+}
base-commit: a226590fefb35ed66adf73d85cefe49048a78ab8
