[v2] c++: implement [[gnu::non_owning]] [PR110358]
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Commit Message
On Fri, Jan 26, 2024 at 04:04:35PM -0500, Jason Merrill wrote:
> On 1/25/24 20:37, Marek Polacek wrote:
> > 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. Such
> > an attribute should not be tied to this particular warning though. [*]
> >
> > The warning bogusly triggers for classes that are like std::span,
> > std::reference_wrapper, and std::ranges::ref_view. The common property
> > seems to be that these classes are only wrappers around some data. So
> > I chose the name non_owning, but I'm not attached to it. I hope that
> > in the future the attribute can be used for something other than this
> > diagnostic.
>
> You decided not to pursue Barry's request for a bool argument to the
> attribute?
At first I thought it'd be an unnecessary complication but it was actually
pretty easy. Better to accept the optional argument from the get-go
otherwise people would have to add > GCC 14 checks.
> Might it be more useful for the attribute to make reference_like_class_p
> return true, so that we still warn about a temporary of another type passing
> through it?
Good point. Fixed.
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. Such
an attribute should not be tied to this particular warning though.
The warning bogusly triggers for classes that are like std::span,
std::reference_wrapper, and std::ranges::ref_view. The common property
seems to be that these classes are only wrappers around some data. So
I chose the name non_owning, but I'm not attached to it. I hope that
in the future the attribute can be used for something other than this
diagnostic.
This attribute takes an optional bool argument to support cases like:
template <typename T>
struct [[gnu::non_owning(std::is_reference_v<T>)]] S {
// ...
};
PR c++/110358
PR c++/109642
gcc/cp/ChangeLog:
* call.cc (non_owning_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::non_owning.
* tree.cc (cxx_gnu_attributes): Add gnu::non_owning.
(handle_non_owning_attribute): New.
gcc/ChangeLog:
* doc/extend.texi: Document gnu::non_owning.
* doc/invoke.texi: Mention that gnu::non_owning disables
-Wdangling-reference.
gcc/testsuite/ChangeLog:
* g++.dg/ext/attr-non-owning1.C: New test.
* g++.dg/ext/attr-non-owning2.C: New test.
* g++.dg/ext/attr-non-owning3.C: New test.
* g++.dg/ext/attr-non-owning4.C: New test.
* g++.dg/ext/attr-non-owning5.C: New test.
* g++.dg/ext/attr-non-owning6.C: New test.
* g++.dg/ext/attr-non-owning7.C: New test.
* g++.dg/ext/attr-non-owning8.C: New test.
* g++.dg/ext/attr-non-owning9.C: New test.
---
gcc/cp/call.cc | 38 ++++++++++--
gcc/cp/tree.cc | 26 +++++++++
gcc/doc/extend.texi | 25 ++++++++
gcc/doc/invoke.texi | 21 +++++++
gcc/testsuite/g++.dg/ext/attr-non-owning1.C | 38 ++++++++++++
gcc/testsuite/g++.dg/ext/attr-non-owning2.C | 29 +++++++++
gcc/testsuite/g++.dg/ext/attr-non-owning3.C | 24 ++++++++
gcc/testsuite/g++.dg/ext/attr-non-owning4.C | 14 +++++
gcc/testsuite/g++.dg/ext/attr-non-owning5.C | 31 ++++++++++
gcc/testsuite/g++.dg/ext/attr-non-owning6.C | 65 +++++++++++++++++++++
gcc/testsuite/g++.dg/ext/attr-non-owning7.C | 31 ++++++++++
gcc/testsuite/g++.dg/ext/attr-non-owning8.C | 30 ++++++++++
gcc/testsuite/g++.dg/ext/attr-non-owning9.C | 25 ++++++++
13 files changed, 391 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning1.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning2.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning3.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning4.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning5.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning6.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning7.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning8.C
create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning9.C
base-commit: 98004ca00e4bf7a513cf3de65d3c3d9ad373872e
Comments
On 2/21/24 19:35, Marek Polacek wrote:
> On Fri, Jan 26, 2024 at 04:04:35PM -0500, Jason Merrill wrote:
>> On 1/25/24 20:37, Marek Polacek wrote:
>>> 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. Such
>>> an attribute should not be tied to this particular warning though. [*]
>>>
>>> The warning bogusly triggers for classes that are like std::span,
>>> std::reference_wrapper, and std::ranges::ref_view. The common property
>>> seems to be that these classes are only wrappers around some data. So
>>> I chose the name non_owning, but I'm not attached to it. I hope that
>>> in the future the attribute can be used for something other than this
>>> diagnostic.
>>
>> You decided not to pursue Barry's request for a bool argument to the
>> attribute?
>
> At first I thought it'd be an unnecessary complication but it was actually
> pretty easy. Better to accept the optional argument from the get-go
> otherwise people would have to add > GCC 14 checks.
>
>> Might it be more useful for the attribute to make reference_like_class_p
>> return true, so that we still warn about a temporary of another type passing
>> through it?
>
> Good point. Fixed.
>
> 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. Such
> an attribute should not be tied to this particular warning though.
>
> The warning bogusly triggers for classes that are like std::span,
> std::reference_wrapper, and std::ranges::ref_view. The common property
> seems to be that these classes are only wrappers around some data. So
> I chose the name non_owning, but I'm not attached to it. I hope that
> in the future the attribute can be used for something other than this
> diagnostic.
>
> This attribute takes an optional bool argument to support cases like:
>
> template <typename T>
> struct [[gnu::non_owning(std::is_reference_v<T>)]] S {
> // ...
> };
>
> PR c++/110358
> PR c++/109642
>
> gcc/cp/ChangeLog:
>
> * call.cc (non_owning_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::non_owning.
> * tree.cc (cxx_gnu_attributes): Add gnu::non_owning.
> (handle_non_owning_attribute): New.
>
> gcc/ChangeLog:
>
> * doc/extend.texi: Document gnu::non_owning.
> * doc/invoke.texi: Mention that gnu::non_owning disables
> -Wdangling-reference.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/ext/attr-non-owning1.C: New test.
> * g++.dg/ext/attr-non-owning2.C: New test.
> * g++.dg/ext/attr-non-owning3.C: New test.
> * g++.dg/ext/attr-non-owning4.C: New test.
> * g++.dg/ext/attr-non-owning5.C: New test.
> * g++.dg/ext/attr-non-owning6.C: New test.
> * g++.dg/ext/attr-non-owning7.C: New test.
> * g++.dg/ext/attr-non-owning8.C: New test.
> * g++.dg/ext/attr-non-owning9.C: New test.
> ---
> gcc/cp/call.cc | 38 ++++++++++--
> gcc/cp/tree.cc | 26 +++++++++
> gcc/doc/extend.texi | 25 ++++++++
> gcc/doc/invoke.texi | 21 +++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning1.C | 38 ++++++++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning2.C | 29 +++++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning3.C | 24 ++++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning4.C | 14 +++++
> gcc/testsuite/g++.dg/ext/attr-non-owning5.C | 31 ++++++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning6.C | 65 +++++++++++++++++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning7.C | 31 ++++++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning8.C | 30 ++++++++++
> gcc/testsuite/g++.dg/ext/attr-non-owning9.C | 25 ++++++++
> 13 files changed, 391 insertions(+), 6 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning1.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning2.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning3.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning4.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning5.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning6.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning7.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning8.C
> create mode 100644 gcc/testsuite/g++.dg/ext/attr-non-owning9.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 1dac1470d3b..e4bf9c963bd 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
> + non-owning. */
> +
> +static bool
> +non_owning_p (tree t)
> +{
> + t = lookup_attribute ("non_owning", 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 (non_owning_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))
> + || non_owning_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..017da7e294a 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_non_owning_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 },
> + { "non_owning", 0, 1, false, true, false, false,
> + handle_non_owning_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 "non_owning" attribute; arguments as in
> + struct attribute_spec.handler. */
> +
> +tree
> +handle_non_owning_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 2135dfde9c8..9132add8267 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -29320,6 +29320,31 @@ Some_Class B __attribute__ ((init_priority (543)));
> Note that the particular values of @var{priority} do not matter; only their
> relative ordering.
>
> +@cindex @code{non_owning} type attribute
> +@item non_owning
> +
> +This attribute can be applied on a class type, function, or member
> +function and indicates that it does not own its associated data. For
> +example, classes like @code{std::span} or @code{std::reference_wrapper}
> +are considered non-owning.
> +
> +@smallexample
> +class [[gnu::non_owning]] 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::non_owning(std::is_reference_v<T>)]] S @{
> + @dots{}
> +@};
> +@end smallexample
> +
> +Currently, the only effect this attribute has is to suppress the
> +@option{-Wdangling-reference} diagnostic.
> +
> @cindex @code{warn_unused} type attribute
> @item warn_unused
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index b4e4ee9fb81..0df2e2ded34 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::non_owning} attribute,
> +which can be applied on the enclosing class type (in which case it disables
> +the warning for all its member functions), member function, or a regular
> +function. For example:
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.
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?
Jason
@@ -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
+ non-owning. */
+
+static bool
+non_owning_p (tree t)
+{
+ t = lookup_attribute ("non_owning", 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 (non_owning_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))
+ || non_owning_p (TREE_TYPE (fndecl)))
return NULL_TREE;
tree rettype = TREE_TYPE (TREE_TYPE (fndecl));
@@ -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_non_owning_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 },
+ { "non_owning", 0, 1, false, true, false, false,
+ handle_non_owning_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 "non_owning" attribute; arguments as in
+ struct attribute_spec.handler. */
+
+tree
+handle_non_owning_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. */
@@ -29320,6 +29320,31 @@ Some_Class B __attribute__ ((init_priority (543)));
Note that the particular values of @var{priority} do not matter; only their
relative ordering.
+@cindex @code{non_owning} type attribute
+@item non_owning
+
+This attribute can be applied on a class type, function, or member
+function and indicates that it does not own its associated data. For
+example, classes like @code{std::span} or @code{std::reference_wrapper}
+are considered non-owning.
+
+@smallexample
+class [[gnu::non_owning]] 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::non_owning(std::is_reference_v<T>)]] S @{
+ @dots{}
+@};
+@end smallexample
+
+Currently, the only effect this attribute has is to suppress the
+@option{-Wdangling-reference} diagnostic.
+
@cindex @code{warn_unused} type attribute
@item warn_unused
@@ -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::non_owning} attribute,
+which can be applied on the enclosing class type (in which case it disables
+the warning for all its member functions), member function, or a regular
+function. For example:
+
+@smallexample
+class [[gnu::non_owning]] A @{
+ int *p;
+ int &foo() @{ return *p; @}
+@};
+
+[[gnu::non_owning]] const int &
+foo (const int &i)
+@{
+ @dots{}
+@}
+@end smallexample
+
This warning is enabled by @option{-Wall}.
@opindex Wdelete-non-virtual-dtor
new file mode 100644
@@ -0,0 +1,38 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+int g = 42;
+
+struct [[gnu::non_owning]] A {
+ int *i;
+ int &foo() { return *i; }
+};
+
+struct A2 {
+ int *i;
+ [[gnu::non_owning]] int &foo() { return *i; }
+ [[gnu::non_owning]] static int &bar (const int &) { return *&g; }
+};
+
+union [[gnu::non_owning]] U { };
+
+A a() { return A{&g}; }
+A2 a2() { return A2{&g}; }
+
+class X { };
+const X x1;
+const X x2;
+
+[[gnu::non_owning]] 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" }
+}
new file mode 100644
@@ -0,0 +1,29 @@
+// { dg-do compile { target c++11 } }
+// Negative tests.
+
+struct [[non_owning]] A { // { dg-warning "ignored" }
+ [[non_owning]] int &foo (int &); // { dg-warning "ignored" }
+};
+
+[[non_owning]] int &bar (int &); // { dg-warning "ignored" }
+
+[[gnu::non_owning]] int i; // { dg-warning "ignored" }
+[[gnu::non_owning]] double d; // { dg-warning "ignored" }
+[[gnu::non_owning]] typedef int T; // { dg-warning "ignored" }
+
+[[gnu::non_owning()]] int &fn1 (int &); // { dg-error "parentheses" }
+[[gnu::non_owning("a")]] int &fn2 (int &); // { dg-error "must be an expression" }
+[[gnu::non_owning(true, true)]] int &fn3 (int &); // { dg-error "wrong number of arguments" }
+
+enum [[gnu::non_owning]] E { // { dg-warning "ignored" }
+ X [[gnu::non_owning]] // { dg-warning "ignored" }
+};
+
+[[gnu::non_owning]]; // { dg-warning "ignored" }
+
+void
+g ()
+{
+ goto L;
+[[gnu::non_owning]] L:; // { dg-warning "ignored" }
+}
new file mode 100644
@@ -0,0 +1,24 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+template <typename T>
+struct [[gnu::non_owning]] 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;
+}
new file mode 100644
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++11 } }
+
+#if !__has_attribute(non_owning)
+#error unsupported
+#endif
+
+#ifdef __has_cpp_attribute
+# if !__has_cpp_attribute(non_owning)
+# error non_owning
+# endif
+#endif
+
+struct [[gnu::non_owning]] S { };
+static_assert (__builtin_has_attribute (S, non_owning), "");
new file mode 100644
@@ -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::non_owning]] 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" }
+}
new file mode 100644
@@ -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::non_owning(T::value)]]
+const X& get (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+template<bool B = true>
+[[gnu::non_owning(B)]]
+const X& foo (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+[[gnu::non_owning(val ())]]
+const X& bar (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+[[gnu::non_owning(!val ())]]
+const X& baz (const int& i)
+{
+ return i == 0 ? x1 : x2;
+}
+
+template <typename T>
+struct [[gnu::non_owning(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" }
+}
new file mode 100644
@@ -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::non_owning(N)]] const X& get(const int& i); // { dg-error "parameter packs not expanded" }
+
+template<typename T>
+[[gnu::non_owning(T::x)]] // { dg-error "member" }
+const X& foo(const int& i);
+
+bool val () { return true; }
+
+[[gnu::non_owning(val ())]] // { dg-error "call" }
+const X& bar (const int& i);
+
+[[gnu::non_owning(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" }
+}
+
new file mode 100644
@@ -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::non_owning(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::non_owning(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" }
+}
+
new file mode 100644
@@ -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::non_owning(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" }
+}
+