On Fri, Mar 03, 2023 at 11:25:06AM -0500, Jason Merrill wrote:
> On 3/2/23 16:24, Marek Polacek wrote:
> > On Wed, Mar 01, 2023 at 04:53:23PM -0500, Jason Merrill wrote:
> > > > @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
> > > > const int& y = (f(1), 42); // NULL_TREE
> > > > const int& z = f(f(1)); // f(f(1))
> > > > - EXPR is the initializer. */
> > > > + EXPR is the initializer. If ARG_P is true, we're processing an
> > > > argument
> > > > + to a function; the point is to distinguish between, for example,
> > > > +
> > > > + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
> > > > +
> > > > + where we shouldn't warn, and
> > > > +
> > > > + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
> > > > +
> > > > + where we should warn (Ref is a reference_like_class_p so we see
> > > > through
> > > > + it. */
> > > > static tree
> > > > -do_warn_dangling_reference (tree expr)
> > > > +do_warn_dangling_reference (tree expr, bool arg_p)
> > > > {
> > > > STRIP_NOPS (expr);
> > > > + if (TREE_CODE (expr) == ADDR_EXPR)
> > > > + expr = TREE_OPERAND (expr, 0);
> > >
> > > I think if we move this here, we also need to check that expr before
> > > STRIP_NOPS had REFERENCE_TYPE. OK with that change.
> >
> > Sorry but I don't think I can do that. There can be CONVERT_EXPRs
> > that need to be stripped, whether arg_p or !arg_p. For example, we can get
> > (const int *) f ((const int &) &TARGET_EXPR <D.2765, NON_LVALUE_EXPR <10>>)
> > for
> > const int& r5 = (42, f(10));
>
> I meant that we only want to strip ADDR_EXPR if 'expr' at the start of the
> function had REFERENCE_TYPE, corresponding to
>
> > /* Check that this argument initializes a reference, except
> > for
> > the argument initializing the object of a member function. */
> > if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > && !TYPE_REF_P (TREE_TYPE (arg)))
> > continue;
>
> above the code for stripping an ADDR_EXPR from an argument that your patch
> removes.
I see.
> If the original expr is a pointer rather than a reference, we don't want to
> complain about it pointing to a temporary.
Ug, I can't make it work. When we recurse, I can no longer check
fndecl. How about just moving the stripping back where it was?
-- >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>.
This patch adjusts do_warn_dangling_reference so that we look through
reference wrapper classes (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 for them, supposing that the
member function returns a reference to a non-temporary object.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Add new bool parameter. See through
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 | 92 ++++++++++++++++---
.../g++.dg/warn/Wdangling-reference8.C | 77 ++++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 +++++
3 files changed, 176 insertions(+), 14 deletions(-)
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 048b2b052f8..62536573633 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13779,6 +13779,45 @@ 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
@@ -13793,12 +13832,37 @@ std_pair_ref_ref_p (tree t)
const int& y = (f(1), 42); // NULL_TREE
const int& z = f(f(1)); // f(f(1))
- EXPR is the initializer. */
+ EXPR is the initializer. If ARG_P is true, we're processing an argument
+ to a function; the point is to distinguish between, for example,
+
+ Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
+
+ where we shouldn't warn, and
+
+ Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
+
+ where we should warn (Ref is a reference_like_class_p so we see through
+ it. */
static tree
-do_warn_dangling_reference (tree expr)
+do_warn_dangling_reference (tree expr, bool arg_p)
{
STRIP_NOPS (expr);
+
+ if (arg_p && expr_represents_temporary_p (expr))
+ {
+ /* 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. */
+ tree e = expr;
+ while (handled_component_p (e))
+ e = TREE_OPERAND (e, 0);
+ e = TREE_TYPE (e);
+ if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
+ return expr;
+ }
+
switch (TREE_CODE (expr))
{
case CALL_EXPR:
@@ -13831,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
std::pair<const int&, const int&> v = std::minmax(1, 2);
which also creates a dangling reference, because std::minmax
returns std::pair<const T&, const T&>(b, a). */
- if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
+ if (!arg_p
+ && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
return NULL_TREE;
/* Here we're looking to see if any of the arguments is a temporary
@@ -13844,14 +13909,13 @@ do_warn_dangling_reference (tree expr)
if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
&& !TYPE_REF_P (TREE_TYPE (arg)))
continue;
- /* It could also be another call taking a temporary and returning
- it and initializing this reference parameter. */
- if (do_warn_dangling_reference (arg))
- return expr;
STRIP_NOPS (arg);
if (TREE_CODE (arg) == ADDR_EXPR)
arg = TREE_OPERAND (arg, 0);
- if (expr_represents_temporary_p (arg))
+ /* Recurse to see if the argument is a temporary. It could also
+ be another call taking a temporary and returning it and
+ initializing this reference parameter. */
+ if (do_warn_dangling_reference (arg, /*arg_p=*/true))
return expr;
/* Don't warn about member function like:
std::any a(...);
@@ -13868,15 +13932,15 @@ do_warn_dangling_reference (tree expr)
return NULL_TREE;
}
case COMPOUND_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
case COND_EXPR:
- if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
+ if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
return t;
- return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
case PAREN_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
case TARGET_EXPR:
- return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
+ return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
default:
return NULL_TREE;
}
@@ -13919,7 +13983,7 @@ maybe_warn_dangling_reference (const_tree decl, tree
init)
= make_temp_override (global_dc->dc_warn_system_headers,
(!in_system_header_at (DECL_SOURCE_LOCATION (decl))
|| global_dc->dc_warn_system_headers));
- if (tree call = do_warn_dangling_reference (init))
+ if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
{
auto_diagnostic_group d;
if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
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..330de1fd05d
--- /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(); // { dg-warning
"dangling reference" }
+ 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(); // { dg-warning "dangling
reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling
reference" }
+ (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(); // { dg-warning "dangling reference"
}
+ (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..9ad83f7365e
--- /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(); // { dg-warning "dangling reference" }
+ (void) a2;
+}
base-commit: ce1c99f1ccd7b1229a4f8531d6b6de6cf571a9ef
--
2.39.2
