On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
> On 1/18/23 20:13, Marek Polacek wrote:
> > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> > > On 1/18/23 12:52, Marek Polacek wrote:
> > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > some grief. The code in question uses a reference wrapper with a member
> > > > function returning a reference to a subobject of a non-temporary object:
> > > >
> > > > const Plane & meta = fm.planes().inner();
> > > >
> > > > I've tried a few approaches, e.g., checking that the member function's
> > > > return type is the same as the type of the enclosing class (which is
> > > > the case for member functions returning *this), but that then breaks
> > > > Wdangling-reference4.C with std::optional<std::string>.
> > > >
> > > > So I figured that perhaps we want to look at the object we're invoking
> > > > the member function(s) on and see if that is a temporary, as in, don't
> > > > warn about
> > > >
> > > > const Plane & meta = fm.planes().inner();
> > > >
> > > > but do warn about
> > > >
> > > > const Plane & meta = FrameMetadata().planes().inner();
> > > >
> > > > It's ugly, but better than asking users to add #pragmas into their code.
> > >
> > > Hmm, that doesn't seem right; the former is only OK because Ref is in
> > > fact a
> > > reference-like type. If planes() returned a class that held data, we
> > > would
> > > want to warn.
> >
> > Sure, it's always some kind of tradeoff with warnings :/.
> > > In this case, we might recognize the reference-like class because it has a
> > > reference member and a constructor taking the same reference type.
> >
> > That occurred to me too, but then I found out that std::reference_wrapper
> > actually uses T*, not T&, as you say. But here's a patch to do that
> > (I hope).
> > > That wouldn't help with std::reference_wrapper or std::ref_view because
> > > they
> > > have pointer members instead of references, but perhaps loosening the
> > > check
> > > to include that case would make sense?
> >
> > Sorry, I don't understand what you mean by loosening the check. I could
> > hardcode std::reference_wrapper and std::ref_view but I don't think that's
> > what you meant.
>
> Indeed that's not what I meant, but as I was saying in our meeting I think
> it's worth doing; the compiler has various tweaks to handle specific
> standard-library classes better.
Okay, done in the patch below. Except that I'm not including a test for
std::ranges::ref_view because I don't really know how that works.
> > Surely I cannot _not_ warn for any class that contains a T*.
>
> I was thinking if a constructor takes a T& and the class has a T* that would
> be close enough, though this also wouldn't handle the standard library
> classes so the benefit is questionable.
>
> > Here's the patch so that we have some actual code to discuss... Thanks.
> >
> > -- >8 --
> > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > some grief. The code in question uses a reference wrapper with a member
> > function returning a reference to a subobject of a non-temporary object:
> >
> > const Plane & meta = fm.planes().inner();
> >
> > I've tried a few approaches, e.g., checking that the member function's
> > return type is the same as the type of the enclosing class (which is
> > the case for member functions returning *this), but that then breaks
> > Wdangling-reference4.C with std::optional<std::string>.
> >
> > Perhaps we want to look at the member function's enclosing class
> > to see if it's a reference wrapper class (meaning, has a reference
> > member and a constructor taking the same reference type) and don't
> > warn if so, supposing that the member function returns a reference
> > to a non-temporary object.
> >
> > It's ugly, but better than asking users to add #pragmas into their code.
> >
> > PR c++/107532
> >
> > gcc/cp/ChangeLog:
> >
> > * call.cc (do_warn_dangling_reference): Don't warn when the
> > member function comes from a reference wrapper class.
>
> Let's factor the new code out into e.g. reference_like_class_p
Done. Thanks,
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
Perhaps we want to look at the member function's enclosing class
to see if it's a reference wrapper class (meaning, has a reference
member and a constructor taking the same reference type, or is
std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
supposing that the member function returns a reference to a non-temporary
object.
It's ugly, but better than asking users to add #pragmas into their code.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Don't warn when the member function comes
from a reference_like_class_p.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
* g++.dg/warn/Wdangling-reference9.C: New test.
---
gcc/cp/call.cc | 48 ++++++++++++
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 +++++
3 files changed, 146 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 991730713e6..672722998ee 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
return true;
}
+/* 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. */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+ if (decl_in_std_namespace_p (tdecl))
+ {
+ tree name = DECL_NAME (tdecl);
+ return (name
+ && (id_equal (name, "reference_wrapper")
+ || 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;
+ }
+ }
+ return false;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
return NULL_TREE;
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Here we suppose that a member function of such a reference
+ wrapper class returns a reference to a non-temporary object. */
+ if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+ && !DECL_OVERLOADED_OPERATOR_P (fndecl)
+ && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
+ return NULL_TREE;
+
/* Here we're looking to see if any of the arguments is a temporary
initializing a reference parameter. */
for (int i = 0; i < call_expr_nargs (expr); ++i)
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..4d585891fae
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner();
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner();
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner();
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner();
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..15c1f6b9dd2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+ std::reference_wrapper<const X> wrapit() const { return x; }
+ X x;
+};
+
+void
+g (const S& s)
+{
+ const auto& a1 = s.wrapit().get();
+ (void) a1;
+ const auto& a2 = S().wrapit().get();
+ (void) a2;
+}
base-commit: 0846336de56119777861e02bf68f92a6af466000
--
2.39.0
