On Thu, Feb 29, 2024 at 07:30:02PM -0500, Jason Merrill wrote:
> On 2/29/24 19:12, Marek Polacek wrote:
> > On Wed, Feb 28, 2024 at 06:03:54PM -0500, Jason Merrill wrote:
> >
> > > Hmm, if we're also going to allow the attribute to be applied to a
> > > function,
> > > the name doesn't make so much sense. For a class, it says that the class
> > > refers to its initializer; for a function, it says that the function
> > > return
> > > value *doesn't* refer to its argument.
> >
> > Yeah, that's a fair point; I guess "non_owning" would be too perplexing.
> >
> > > If we want something that can apply to both classes and functions, we're
> > > probably back to an attribute that just suppresses the warning, with a
> > > different name.
> > >
> > > Or I guess we could have two attributes, but that seems like a lot.
> > >
> > > WDYT?
> >
> > I think we don't want two separate attributes, and we do want that one
> > attribute to apply to both fns and classes. We could implement something
> > like
> >
> > [[gnu::no_warning("Wdangling-reference")]]
> > [[gnu::no_warning("Wdangling-reference", bool)]]
> >
> > but first, that's a lot of typing, second, it would be confusing because
> > it wouldn't work for any other warning. We already have [[unused]] and
> > [[maybe_unused]] whose effect is to suppress a warning. It think our
> > best bet is to do the most straightforward thing: [[gnu::no_dangling]],
> > which this patch implements. I didn't call it no_dangling_reference in
> > the hope that it can, some day, be also used for some -Wdangling-pointer
> > purposes.
> >
> >
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > Since -Wdangling-reference has false positives that can't be
> > prevented, we should offer an easy way to suppress the warning.
> > Currently, that is only possible by using a #pragma, either around the
> > enclosing class or around the call site. But #pragma GCC diagnostic tend
> > to be onerous. A better solution would be to have an attribute.
> >
> > To that end, this patch adds a new attribute, [[gnu::no_dangling]].
> > This attribute takes an optional bool argument to support cases like:
> >
> > template <typename T>
> > struct [[gnu::no_dangling(std::is_reference_v<T>)]] S {
> > // ...
> > };
> >
> > PR c++/110358
> > PR c++/109642
> >
> > gcc/cp/ChangeLog:
> >
> > * call.cc (no_dangling_p): New.
> > (reference_like_class_p): Use it.
> > (do_warn_dangling_reference): Use it. Don't warn when the function
> > or its enclosing class has attribute gnu::no_dangling.
> > * tree.cc (cxx_gnu_attributes): Add gnu::no_dangling.
> > (handle_no_dangling_attribute): New.
> >
> > gcc/ChangeLog:
> >
> > * doc/extend.texi: Document gnu::no_dangling.
> > * doc/invoke.texi: Mention that gnu::no_dangling disables
> > -Wdangling-reference.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/ext/attr-no-dangling1.C: New test.
> > * g++.dg/ext/attr-no-dangling2.C: New test.
> > * g++.dg/ext/attr-no-dangling3.C: New test.
> > * g++.dg/ext/attr-no-dangling4.C: New test.
> > * g++.dg/ext/attr-no-dangling5.C: New test.
> > * g++.dg/ext/attr-no-dangling6.C: New test.
> > * g++.dg/ext/attr-no-dangling7.C: New test.
> > * g++.dg/ext/attr-no-dangling8.C: New test.
> > * g++.dg/ext/attr-no-dangling9.C: New test.
> > ---
> > gcc/cp/call.cc | 38 ++++++++++--
> > gcc/cp/tree.cc | 26 ++++++++
> > gcc/doc/extend.texi | 21 +++++++
> > gcc/doc/invoke.texi | 21 +++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling1.C | 38 ++++++++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling2.C | 29 +++++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling3.C | 24 ++++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling4.C | 14 +++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling5.C | 31 ++++++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling6.C | 65 ++++++++++++++++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling7.C | 31 ++++++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling8.C | 30 +++++++++
> > gcc/testsuite/g++.dg/ext/attr-no-dangling9.C | 25 ++++++++
> > 13 files changed, 387 insertions(+), 6 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling2.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling3.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling4.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling5.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling8.C
> > create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling9.C
> >
> > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > index c40ef2e3028..9e4c8073600 100644
> > --- a/gcc/cp/call.cc
> > +++ b/gcc/cp/call.cc
> > @@ -14033,11 +14033,7 @@ 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. We no
> > - longer check that it has a constructor taking the same reference type
> > - since that approach still generated too many false positives. */
> > +/* Return true if a class T has a reference member. */
> > static bool
> > class_has_reference_member_p (tree t)
> > @@ -14061,12 +14057,41 @@ class_has_reference_member_p_r (tree binfo, void
> > *)
> > ? integer_one_node : NULL_TREE);
> > }
> > +
> > +/* Return true if T (either a class or a function) has been marked as
> > + not-dangling. */
> > +
> > +static bool
> > +no_dangling_p (tree t)
> > +{
> > + t = lookup_attribute ("no_dangling", TYPE_ATTRIBUTES (t));
> > + if (!t)
> > + return false;
> > +
> > + t = TREE_VALUE (t);
> > + if (!t)
> > + return true;
> > +
> > + t = build_converted_constant_bool_expr (TREE_VALUE (t),
> > tf_warning_or_error);
> > + t = cxx_constant_value (t);
> > + return t == boolean_true_node;
> > +}
> > +
> > +/* 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. 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
> > reference_like_class_p (tree ctype)
> > {
> > if (!CLASS_TYPE_P (ctype))
> > return false;
> > + if (no_dangling_p (ctype))
> > + return true;
> > +
> > /* Also accept a std::pair<const T&, const T&>. */
> > if (std_pair_ref_ref_p (ctype))
> > return true;
> > @@ -14173,7 +14198,8 @@ do_warn_dangling_reference (tree expr, bool arg_p)
> > but probably not to one of its arguments. */
> > || (DECL_OBJECT_MEMBER_FUNCTION_P (fndecl)
> > && DECL_OVERLOADED_OPERATOR_P (fndecl)
> > - && DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF)))
> > + && DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF))
> > + || no_dangling_p (TREE_TYPE (fndecl)))
> > return NULL_TREE;
> > tree rettype = TREE_TYPE (TREE_TYPE (fndecl));
> > diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> > index ad312710f68..e75be9a4e66 100644
> > --- a/gcc/cp/tree.cc
> > +++ b/gcc/cp/tree.cc
> > @@ -47,6 +47,7 @@ static tree verify_stmt_tree_r (tree *, int *, void *);
> > static tree handle_init_priority_attribute (tree *, tree, tree, int, bool
> > *);
> > static tree handle_abi_tag_attribute (tree *, tree, tree, int, bool *);
> > static tree handle_contract_attribute (tree *, tree, tree, int, bool *);
> > +static tree handle_no_dangling_attribute (tree *, tree, tree, int, bool *);
> > /* If REF is an lvalue, returns the kind of lvalue that REF is.
> > Otherwise, returns clk_none. */
> > @@ -5102,6 +5103,8 @@ static const attribute_spec cxx_gnu_attributes[] =
> > handle_init_priority_attribute, NULL },
> > { "abi_tag", 1, -1, false, false, false, true,
> > handle_abi_tag_attribute, NULL },
> > + { "no_dangling", 0, 1, false, true, false, false,
> > + handle_no_dangling_attribute, NULL },
> > };
> > const scoped_attribute_specs cxx_gnu_attribute_table =
> > @@ -5391,6 +5394,29 @@ handle_contract_attribute (tree *ARG_UNUSED (node),
> > tree ARG_UNUSED (name),
> > return NULL_TREE;
> > }
> > +/* Handle a "no_dangling" attribute; arguments as in
> > + struct attribute_spec.handler. */
> > +
> > +tree
> > +handle_no_dangling_attribute (tree *node, tree name, tree args, int,
> > + bool *no_add_attrs)
> > +{
> > + if (args && TREE_CODE (TREE_VALUE (args)) == STRING_CST)
> > + {
> > + error ("%qE attribute argument must be an expression that evaluates "
> > + "to true or false", name);
> > + *no_add_attrs = true;
> > + }
> > + else if (!FUNC_OR_METHOD_TYPE_P (*node)
> > + && !RECORD_OR_UNION_TYPE_P (*node))
> > + {
> > + warning (OPT_Wattributes, "%qE attribute ignored", name);
> > + *no_add_attrs = true;
> > + }
> > +
> > + return NULL_TREE;
> > +}
> > +
> > /* Return a new PTRMEM_CST of the indicated TYPE. The MEMBER is the
> > thing pointed to by the constant. */
> > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> > index efd78014d1a..571655bf39a 100644
> > --- a/gcc/doc/extend.texi
> > +++ b/gcc/doc/extend.texi
> > @@ -29317,6 +29317,27 @@ Some_Class B __attribute__ ((init_priority
> > (543)));
> > Note that the particular values of @var{priority} do not matter; only
> > their
> > relative ordering.
> > +@cindex @code{no_dangling} type attribute
> > +@item no_dangling
> > +
> > +This attribute can be applied on a class type, function, or member
> > +function. Entities marked with this attribute will have the
> > +@option{-Wdangling-reference} diagnostic suppressed.
> > +
> > +@smallexample
> > +class [[gnu::no_dangling]] S @{ @dots{} @};
> > +@end smallexample
> > +
> > +This attribute takes an optional argument, which must be an expression that
> > +evaluates to true or false:
> > +
> > +@smallexample
> > +template <typename T>
> > +struct [[gnu::no_dangling(std::is_reference_v<T>)]] S @{
> > + @dots{}
> > +@};
> > +@end smallexample >
> > @cindex @code{warn_unused} type attribute
> > @item warn_unused
> > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > index 7862c751801..9022a413dd4 100644
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -3908,6 +3908,9 @@ const T& foo (const T&) @{ @dots{} @}
> > #pragma GCC diagnostic pop
> > @end smallexample
> > +The @code{#pragma} can also surround the class; in that case, the warning
> > +will be disabled for all the member functions.
> > +
> > @option{-Wdangling-reference} also warns about code like
> > @smallexample
> > @@ -3932,6 +3935,24 @@ struct Span @{
> > as @code{std::span}-like; that is, the class is a non-union class
> > that has a pointer data member and a trivial destructor.
> > +The warning can be disabled by using the @code{gnu::no_dangling} attribute,
>
> Can this be an xref, and put all the documentation for the usage and effect
> of the attribute in the attribute section?
Yes. And I need an entry both for the type and function attribute.
> > +which can be applied on the enclosing class type (in which case it disables
> > +the warning for all its member functions),
>
> I think this misrepresents the effect of putting the attribute on the class.
> Rather, it means that we won't warn about a dangling reference to the class.
Updated, NFC:
-- >8 --
Since -Wdangling-reference has false positives that can't be
prevented, we should offer an easy way to suppress the warning.
Currently, that is only possible by using a #pragma, either around the
enclosing class or around the call site. But #pragma GCC diagnostic tend
to be onerous. A better solution would be to have an attribute.
To that end, this patch adds a new attribute, [[gnu::no_dangling]].
This attribute takes an optional bool argument to support cases like:
template <typename T>
struct [[gnu::no_dangling(std::is_reference_v<T>)]] S {
// ...
};
PR c++/110358
PR c++/109642
gcc/cp/ChangeLog:
* call.cc (no_dangling_p): New.
(reference_like_class_p): Use it.
(do_warn_dangling_reference): Use it. Don't warn when the function
or its enclosing class has attribute gnu::no_dangling.
* tree.cc (cxx_gnu_attributes): Add gnu::no_dangling.
(handle_no_dangling_attribute): New.
gcc/ChangeLog:
* doc/extend.texi: Document gnu::no_dangling.
* doc/invoke.texi: Mention that gnu::no_dangling disables
-Wdangling-reference.
gcc/testsuite/ChangeLog:
* g++.dg/ext/attr-no-dangling1.C: New test.
* g++.dg/ext/attr-no-dangling2.C: New test.
* g++.dg/ext/attr-no-dangling3.C: New test.
* g++.dg/ext/attr-no-dangling4.C: New test.
* g++.dg/ext/attr-no-dangling5.C: New test.
* g++.dg/ext/attr-no-dangling6.C: New test.
* g++.dg/ext/attr-no-dangling7.C: New test.
* g++.dg/ext/attr-no-dangling8.C: New test.
* g++.dg/ext/attr-no-dangling9.C: New test.
---
gcc/cp/call.cc | 38 ++++++++++--
gcc/cp/tree.cc | 26 ++++++++
gcc/doc/extend.texi | 51 +++++++++++++++
gcc/doc/invoke.texi | 7 +++
gcc/testsuite/g++.dg/ext/attr-no-dangling1.C | 38 ++++++++++++
gcc/testsuite/g++.dg/ext/attr-no-dangling2.C | 29 +++++++++
gcc/testsuite/g++.dg/ext/attr-no-dangling3.C | 24 ++++++++
gcc/testsuite/g++.dg/ext/attr-no-dangling4.C | 14 +++++
gcc/testsuite/g++.dg/ext/attr-no-dangling5.C | 31 ++++++++++
gcc/testsuite/g++.dg/ext/attr-no-dangling6.C | 65 ++++++++++++++++++++
gcc/testsuite/g++.dg/ext/attr-no-dangling7.C | 31 ++++++++++
gcc/testsuite/g++.dg/ext/attr-no-dangling8.C | 30 +++++++++
gcc/testsuite/g++.dg/ext/attr-no-dangling9.C | 25 ++++++++
13 files changed, 403 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling2.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling3.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling4.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling5.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling8.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-no-dangling9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index c40ef2e3028..9e4c8073600 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -14033,11 +14033,7 @@ 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. We no
- longer check that it has a constructor taking the same reference type
- since that approach still generated too many false positives. */
+/* Return true if a class T has a reference member. */
static bool
class_has_reference_member_p (tree t)
@@ -14061,12 +14057,41 @@ class_has_reference_member_p_r (tree binfo, void *)
? integer_one_node : NULL_TREE);
}
+
+/* Return true if T (either a class or a function) has been marked as
+ not-dangling. */
+
+static bool
+no_dangling_p (tree t)
+{
+ t = lookup_attribute ("no_dangling", TYPE_ATTRIBUTES (t));
+ if (!t)
+ return false;
+
+ t = TREE_VALUE (t);
+ if (!t)
+ return true;
+
+ t = build_converted_constant_bool_expr (TREE_VALUE (t), tf_warning_or_error);
+ t = cxx_constant_value (t);
+ return t == boolean_true_node;
+}
+
+/* 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. 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
reference_like_class_p (tree ctype)
{
if (!CLASS_TYPE_P (ctype))
return false;
+ if (no_dangling_p (ctype))
+ return true;
+
/* Also accept a std::pair<const T&, const T&>. */
if (std_pair_ref_ref_p (ctype))
return true;
@@ -14173,7 +14198,8 @@ do_warn_dangling_reference (tree expr, bool arg_p)
but probably not to one of its arguments. */
|| (DECL_OBJECT_MEMBER_FUNCTION_P (fndecl)
&& DECL_OVERLOADED_OPERATOR_P (fndecl)
- && DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF)))
+ && DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF))
+ || no_dangling_p (TREE_TYPE (fndecl)))
return NULL_TREE;
tree rettype = TREE_TYPE (TREE_TYPE (fndecl));
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index ad312710f68..e75be9a4e66 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -47,6 +47,7 @@ static tree verify_stmt_tree_r (tree *, int *, void *);
static tree handle_init_priority_attribute (tree *, tree, tree, int, bool *);
static tree handle_abi_tag_attribute (tree *, tree, tree, int, bool *);
static tree handle_contract_attribute (tree *, tree, tree, int, bool *);
+static tree handle_no_dangling_attribute (tree *, tree, tree, int, bool *);
/* If REF is an lvalue, returns the kind of lvalue that REF is.
Otherwise, returns clk_none. */
@@ -5102,6 +5103,8 @@ static const attribute_spec cxx_gnu_attributes[] =
handle_init_priority_attribute, NULL },
{ "abi_tag", 1, -1, false, false, false, true,
handle_abi_tag_attribute, NULL },
+ { "no_dangling", 0, 1, false, true, false, false,
+ handle_no_dangling_attribute, NULL },
};
const scoped_attribute_specs cxx_gnu_attribute_table =
@@ -5391,6 +5394,29 @@ handle_contract_attribute (tree *ARG_UNUSED (node), tree
ARG_UNUSED (name),
return NULL_TREE;
}
+/* Handle a "no_dangling" attribute; arguments as in
+ struct attribute_spec.handler. */
+
+tree
+handle_no_dangling_attribute (tree *node, tree name, tree args, int,
+ bool *no_add_attrs)
+{
+ if (args && TREE_CODE (TREE_VALUE (args)) == STRING_CST)
+ {
+ error ("%qE attribute argument must be an expression that evaluates "
+ "to true or false", name);
+ *no_add_attrs = true;
+ }
+ else if (!FUNC_OR_METHOD_TYPE_P (*node)
+ && !RECORD_OR_UNION_TYPE_P (*node))
+ {
+ warning (OPT_Wattributes, "%qE attribute ignored", name);
+ *no_add_attrs = true;
+ }
+
+ return NULL_TREE;
+}
+
/* Return a new PTRMEM_CST of the indicated TYPE. The MEMBER is the
thing pointed to by the constant. */
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 6c2c7ae5d8a..5bd3933f011 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -4380,6 +4380,36 @@ visibility of their template.
If both the template and enclosing class have explicit visibility, the
visibility from the template is used.
+@cindex @code{no_dangling} function attribute
+@item no_dangling
+
+The @code{gnu::no_dangling} attribute can be used to suppress the
+@option{-Wdangling-reference} diagnostic on a function or a member
+function. For example:
+
+@smallexample
+class A @{
+ int *p;
+ [[gnu::no_dangling]] int &foo() @{ return *p; @}
+@};
+
+[[gnu::no_dangling]] const int &
+foo (const int &i)
+@{
+ @dots{}
+@}
+@end smallexample
+
+This attribute takes an optional argument, which must be an expression that
+evaluates to true or false:
+
+@smallexample
+template <typename T>
+[[gnu::no_dangling(std::is_reference_v<T>)]] int foo (T& t) @{
+ @dots{}
+@};
+@end smallexample
+
@cindex @code{warn_unused_result} function attribute
@item warn_unused_result
The @code{warn_unused_result} attribute causes a warning to be emitted
@@ -29327,6 +29357,27 @@ Some_Class B __attribute__ ((init_priority (543)));
Note that the particular values of @var{priority} do not matter; only their
relative ordering.
+@cindex @code{no_dangling} type attribute
+@item no_dangling
+
+This attribute can be applied on a class type. Dangling references to
+classes marked with this attribute will have the @option{-Wdangling-reference}
+diagnostic suppressed.
+
+@smallexample
+class [[gnu::no_dangling]] S @{ @dots{} @};
+@end smallexample
+
+This attribute takes an optional argument, which must be an expression that
+evaluates to true or false:
+
+@smallexample
+template <typename T>
+struct [[gnu::no_dangling(std::is_reference_v<T>)]] S @{
+ @dots{}
+@};
+@end smallexample
+
@cindex @code{warn_unused} type attribute
@item warn_unused
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index dc5fd863ca4..0422b2c400a 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -3908,6 +3908,9 @@ const T& foo (const T&) @{ @dots{} @}
#pragma GCC diagnostic pop
@end smallexample
+The @code{#pragma} can also surround the class; in that case, the warning
+will be disabled for all the member functions.
+
@option{-Wdangling-reference} also warns about code like
@smallexample
@@ -3932,6 +3935,10 @@ struct Span @{
as @code{std::span}-like; that is, the class is a non-union class
that has a pointer data member and a trivial destructor.
+The warning can be disabled by using the @code{gnu::no_dangling} attribute
+on a function (@pxref{Common Function Attributes}), or a class type
+(@pxref{C++ Attributes}).
+
This warning is enabled by @option{-Wall}.
@opindex Wdelete-non-virtual-dtor
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
new file mode 100644
index 00000000000..02eabbc5003
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling1.C
@@ -0,0 +1,38 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+int g = 42;
+
+struct [[gnu::no_dangling]] A {
+ int *i;
+ int &foo() { return *i; }
+};
+
+struct A2 {
+ int *i;
+ [[gnu::no_dangling]] int &foo() { return *i; }
+ [[gnu::no_dangling]] static int &bar (const int &) { return *&g; }
+};
+
+union [[gnu::no_dangling]] U { };
+
+A a() { return A{&g}; }
+A2 a2() { return A2{&g}; }
+
+class X { };
+const X x1;
+const X x2;
+
+[[gnu::no_dangling]] const X& get(const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+void
+test ()
+{
+ [[maybe_unused]] const X& x = get (10); // { dg-bogus "dangling" }
+ [[maybe_unused]] const int &i = a().foo(); // { dg-bogus "dangling" }
+ [[maybe_unused]] const int &j = a2().foo(); // { dg-bogus "dangling" }
+ [[maybe_unused]] const int &k = a2().bar(10); // { dg-bogus
"dangling" }
+}
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling2.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling2.C
new file mode 100644
index 00000000000..4cdc97ea7c4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling2.C
@@ -0,0 +1,29 @@
+// { dg-do compile { target c++11 } }
+// Negative tests.
+
+struct [[no_dangling]] A { // { dg-warning "ignored" }
+ [[no_dangling]] int &foo (int &); // { dg-warning "ignored" }
+};
+
+[[no_dangling]] int &bar (int &); // { dg-warning "ignored" }
+
+[[gnu::no_dangling]] int i; // { dg-warning "ignored" }
+[[gnu::no_dangling]] double d; // { dg-warning "ignored" }
+[[gnu::no_dangling]] typedef int T; // { dg-warning "ignored" }
+
+[[gnu::no_dangling()]] int &fn1 (int &); // { dg-error "parentheses"
}
+[[gnu::no_dangling("a")]] int &fn2 (int &); // { dg-error "must be an
expression" }
+[[gnu::no_dangling(true, true)]] int &fn3 (int &); // { dg-error "wrong
number of arguments" }
+
+enum [[gnu::no_dangling]] E { // { dg-warning "ignored" }
+ X [[gnu::no_dangling]] // { dg-warning "ignored" }
+};
+
+[[gnu::no_dangling]]; // { dg-warning "ignored" }
+
+void
+g ()
+{
+ goto L;
+[[gnu::no_dangling]] L:; // { dg-warning "ignored" }
+}
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling3.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling3.C
new file mode 100644
index 00000000000..764b104fd3c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling3.C
@@ -0,0 +1,24 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+template <typename T>
+struct [[gnu::no_dangling]] Span {
+ T* data_;
+ int len_;
+ // So that our heuristic doesn't suppress the warning anyway.
+ ~Span();
+
+ [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& {
return data_[n]; }
+ [[nodiscard]] constexpr auto front() const noexcept -> T& { return
data_[0]; }
+ [[nodiscard]] constexpr auto back() const noexcept -> T& { return
data_[len_ - 1]; }
+};
+
+auto get() -> Span<int>;
+
+auto f() -> int {
+ int const& a = get().front(); // { dg-bogus "dangling" }
+ int const& b = get().back(); // { dg-bogus "dangling" }
+ int const& c = get()[0]; // { dg-bogus "dangling" }
+
+ return a + b + c;
+}
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling4.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling4.C
new file mode 100644
index 00000000000..e910723d985
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling4.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++11 } }
+
+#if !__has_attribute(no_dangling)
+#error unsupported
+#endif
+
+#ifdef __has_cpp_attribute
+# if !__has_cpp_attribute(no_dangling)
+# error no_dangling
+# endif
+#endif
+
+struct [[gnu::no_dangling]] S { };
+static_assert (__builtin_has_attribute (S, no_dangling), "");
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling5.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling5.C
new file mode 100644
index 00000000000..ec5075482c4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling5.C
@@ -0,0 +1,31 @@
+// PR c++/110358
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+
+template <typename T>
+struct Span {
+ T* data_;
+ int len_;
+ ~Span();
+
+ [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& {
return data_[n]; }
+};
+
+template <>
+struct [[gnu::no_dangling]] Span<int> {
+ int* data_;
+ int len_;
+ ~Span();
+
+ [[nodiscard]] constexpr auto operator[](int n) const noexcept -> int& {
return data_[n]; }
+};
+
+auto getch() -> Span<char>;
+auto geti() -> Span<int>;
+
+void
+f ()
+{
+ [[maybe_unused]] const auto &a = getch()[0]; // { dg-warning "dangling
reference" }
+ [[maybe_unused]] const auto &b = geti()[0]; // { dg-bogus "dangling
reference" }
+}
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
new file mode 100644
index 00000000000..235a5fd86c5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
@@ -0,0 +1,65 @@
+// PR c++/110358
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+
+class X { };
+const X x1;
+const X x2;
+
+constexpr bool val () { return true; }
+struct ST { static constexpr bool value = true; };
+struct SF { static constexpr bool value = false; };
+
+template<typename T>
+[[gnu::no_dangling(T::value)]]
+const X& get (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+template<bool B = true>
+[[gnu::no_dangling(B)]]
+const X& foo (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+[[gnu::no_dangling(val ())]]
+const X& bar (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+[[gnu::no_dangling(!val ())]]
+const X& baz (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+template <typename T>
+struct [[gnu::no_dangling(T::value)]]
+Span {
+ T* data_;
+ int len_;
+ ~Span();
+
+ [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& {
return data_[n]; }
+};
+
+auto geti() -> Span<ST>;
+auto gety() -> Span<SF>;
+
+void
+test ()
+{
+ [[maybe_unused]] const X& x1 = get<ST> (10); // { dg-bogus "dangling" }
+ [[maybe_unused]] const X& x2 = get<SF> (10); // { dg-warning "dangling" }
+ [[maybe_unused]] const X& x3 = foo<true> (10); // { dg-bogus "dangling" }
+ [[maybe_unused]] const X& x4 = foo<false> (10); // { dg-warning "dangling" }
+ [[maybe_unused]] const X& x7 = foo<> (10); // { dg-bogus "dangling" }
+ [[maybe_unused]] const X& x5 = bar (10); // { dg-bogus "dangling" }
+ [[maybe_unused]] const X& x6 = baz (10); // { dg-warning "dangling" }
+
+ [[maybe_unused]] const auto &b1 = geti()[0]; // { dg-bogus "dangling" }
+ [[maybe_unused]] const auto &b2 = gety()[0]; // { dg-warning "dangling" }
+}
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
new file mode 100644
index 00000000000..3c392ed409f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
@@ -0,0 +1,31 @@
+// PR c++/110358
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+
+class X { };
+const X x1;
+const X x2;
+
+template<bool... N>
+[[gnu::no_dangling(N)]] const X& get(const int& i); // { dg-error "parameter
packs not expanded" }
+
+template<typename T>
+[[gnu::no_dangling(T::x)]] // { dg-error "member" }
+const X& foo(const int& i);
+
+bool val () { return true; }
+
+[[gnu::no_dangling(val ())]] // { dg-error "call" }
+const X& bar (const int& i);
+
+[[gnu::no_dangling(20)]] const X& fn1 (const int &);
+
+void
+test ()
+{
+ [[maybe_unused]] const X& x1 = bar (10); // { dg-warning "dangling" }
+ [[maybe_unused]] const X& x2 = foo<int> (10); // { dg-error "no
matching" }
+ [[maybe_unused]] const X& x3 // { dg-warning "dangling" }
+ = fn1 (10); // { dg-error
"narrowing" }
+}
+
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling8.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling8.C
new file mode 100644
index 00000000000..8208d751a4b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling8.C
@@ -0,0 +1,30 @@
+// PR c++/110358
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+
+template<class T> constexpr bool is_reference_v = false;
+template<class T> constexpr bool is_reference_v<T&> = true;
+template<class T> constexpr bool is_reference_v<T&&> = true;
+
+template <typename T>
+struct [[gnu::no_dangling(is_reference_v<T>)]] S {
+ int &foo (const int &);
+};
+
+template <typename T1, typename T2>
+struct X {
+ template <typename U1 = T1, typename U2 = T2>
+ struct [[gnu::no_dangling(is_reference_v<U1> && is_reference_v<U2>)]] Y {
+ int &foo (const int &);
+ };
+};
+
+void
+g ()
+{
+ [[maybe_unused]] const int &x0 = S<int&>().foo (42); // { dg-bogus
"dangling" }
+ [[maybe_unused]] const int &x1 = S<int>().foo (42); // { dg-warning
"dangling" }
+ [[maybe_unused]] const auto &x2 = X<int, int&>::Y<>().foo (42); // {
dg-warning "dangling" }
+ [[maybe_unused]] const auto &x3 = X<int&, int&>::Y<>().foo (42); // {
dg-bogus "dangling" }
+}
+
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling9.C
b/gcc/testsuite/g++.dg/ext/attr-no-dangling9.C
new file mode 100644
index 00000000000..65b4f7145a9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling9.C
@@ -0,0 +1,25 @@
+// PR c++/110358
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+
+template<bool B>
+struct bool_constant {
+ static constexpr bool value = B;
+ constexpr operator bool() const { return value; }
+};
+
+using true_type = bool_constant<true>;
+using false_type = bool_constant<false>;
+
+struct S {
+ template<bool B>
+ [[gnu::no_dangling(B)]] int &foo (const int &);
+};
+
+void
+g ()
+{
+ [[maybe_unused]] const int &x0 = S().foo<false_type{}> (42); // {
dg-warning "dangling" }
+ [[maybe_unused]] const int &x1 = S().foo<true_type{}> (42); // { dg-bogus
"dangling" }
+}
+
base-commit: 4894c82b0c3cf0d6ec4bc1e96709b6140ec11f6e
--
2.44.0
