[v3] c++: Implement -Wdangling-reference [PR106393]

  On Tue, Oct 25, 2022 at 11:53:51AM -0400, Jason Merrill via Gcc-patches wrote:
> On 10/25/22 11:21, Marek Polacek wrote:
> > On Mon, Oct 24, 2022 at 01:30:42PM -0400, Jason Merrill wrote:
> > > On 10/21/22 19:28, Marek Polacek wrote:
> > > > It doesn't warn when the function in question is a member function, otherwise
> > > > it'd emit loads of warnings for valid code like obj.emplace<T>({0}, 0).
> > > 
> > > We had discussed warning if the object argument is a temporary (and for the
> > > above check, the function returns *this)?
> > 
> > Presumably you mean detecting something like this:
> > 
> > struct S {
> >    const S& self () { return *this; }
> > };
> > const S& s = S().self();
> 
> Yes.  Or
> 
> struct S {
>   int ar[4];
>   int& operator[](int i) { return ar[i]; }
> };
> const int &r = S()[2];

Ok, I've extended the warning to check if the object argument is a temporary
as well, so we get a warning here now.
 
> > I don't currently have a way to detect it, can I steal a METHOD_TYPE flag
> > that says "this member function returns *this"?  Alternatively, walk its
> > DECL_SAVED_TREE and look for RETURN_EXPR?
> 
> Like you limited the above check to TREE_STATIC, let's also forget about
> checking for return *this.

I don't follow, but it looks like I don't need to change anything and just
keep the TREE_STATIC check?
 
> > > > It warns in member initializer lists as well:
> > > > 
> > > >     const int& f(const int& i) { return i; }
> > > >     struct S {
> > > >       const int &r; // { dg-warning "dangling reference" }
> > > >       S() : r(f(10)) { } // { dg-message "destroyed" }
> > > >     };
> > > > 
> > > > I've run the testsuite/bootstrap with the warning enabled by default.
> > > > There were just a few FAILs:
> > > > * g++.dg/warn/Wdangling-pointer-2.C
> > > > * 20_util/any/misc/any_cast.cc
> > > > * 20_util/forward/c_neg.cc
> > > > * 20_util/forward/f_neg.cc
> > > > * experimental/any/misc/any_cast.cc
> > > > all of these look like genuine bugs.  A bootstrap with the warning
> > > > enabled by default passed.
> > > > 
> > > > When testing a previous version of the patch, there were many FAILs in
> > > > libstdc++'s 22_locale/; all of them because the warning triggered on
> > > > 
> > > >     const test_type& obj = std::use_facet<test_type>(std::locale());
> > > > 
> > > > but this code looks valid -- std::use_facet doesn't return a reference
> > > > to its parameter.  Therefore I added code to suppress the warning when
> > > > the call is std::use_facet.  Now 22_locale/* pass even with the warning
> > > > on.  We could exclude more std:: functions like this if desirable.
> > > 
> > > Instead of adding special cases in the compiler, let's disable the warning
> > > around the definition of use_facet (and adjust the compiler as needed so
> > > that avoids the warning).
> > 
> > As I said in
> > <https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604307.html>
> > I don't think it's possible without inventing an attribute (?).
> 
> Replied.

Fixed.
 
> > > > +  tree fndecl = cp_get_callee_fndecl_nofold (call);
> > > > +  if (!fndecl
> > > > +      || warning_suppressed_p (fndecl, OPT_Wdangling_reference)
> > > > +      /* Don't warn about member functions; the warning would trigger in
> > > > +	 valid code like
> > > > +	   std::any a(...);
> > > > +	   S& s = a.emplace<S>({0}, 0);
> > > > +	 which constructs a new object and returns a reference to it.  */
> > > > +      || DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > > > +      /* It seems unreasonable to warn about operator functions.  */
> > > > +      || DECL_OVERLOADED_OPERATOR_P (fndecl)
> > > 
> > > I guess I'd expect false positives on << and >> because of iostreams, do you
> > > see false positives with other operators?
> > 
> > This was just a guess.  The warning triggered in g++.dg/overload/operator6.C.
> 
> That looks like a true positive.

Ok, then...

> > I suppose this could be limited to << and >>?  I'm not sure.
> 
> I'm not sure either.  Another possibility would be to only consider the LHS
> of binary operators, since returning a reference to the RHS is very unusual.

...I've removed the DECL_OVERLOADED_OPERATOR_P check altogether and don't
really see any false positives.  It's probably better to start with a more
general warning and then tweak it based on empirical evidence.
 
> > > > +// Invalid, but we don't warn here yet.
> > > > +// r12 = f (f ((const int &) &TARGET_EXPR <D.2459, 1>))
> > > > +const int& r12 = f(f(1));
> > > 
> > > This should be a simple recursion?
> > 
> > Hmm, the inner call is just a sub-expression of the full-expression so
> > there you can still use the returned temporary.  But in this case the
> > temporary is used beyond the full-expression so it's invalid.  I've added
> > 
> >           if (tree r = find_initializing_call_expr (arg))
> >             if (cp_tree_equal (CALL_EXPR_FN (r), CALL_EXPR_FN (expr)))
> >               return expr;
> > 
> > so that we detect f(f(1)) but I'm dubious that this is actually useful.
> 
> Indeed, but why limit it to checking for the same function rather than also
> warning for
> 
> f(g(1))
> 
> or
> 
> A().f().g()
> 
> ?

OK, I've made the warning even more recursive so that now we detect
both of these, see Wdangling-reference3.C.  While at it, I've renamed
the functions.

As before, I've tested this patch twice, once with -Wdangling-reference
enabled by default, once with -Wdangling-reference enabled by -Wextra.
The couple of FAILs I saw were true positives (e.g., rv-conv1.C, rv-func2.C).

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
This patch implements a new experimental warning (enabled by -Wextra) to
detect references bound to temporaries whose lifetime has ended.  The
primary motivation is the Note in
<https://en.cppreference.com/w/cpp/algorithm/max>:

  Capturing the result of std::max by reference produces a dangling reference
  if one of the parameters is a temporary and that parameter is returned:

  int n = 1;
  const int& r = std::max(n-1, n+1); // r is dangling

That's because both temporaries for n-1 and n+1 are destroyed at the end
of the full expression.  With this warning enabled, you'll get:

g.C:3:12: warning: possibly dangling reference to a temporary [-Wdangling-reference]
    3 | const int& r = std::max(n-1, n+1);
      |            ^
g.C:3:24: note: the temporary was destroyed at the end of the full expression 'std::max<int>((n - 1), (n + 1))'
    3 | const int& r = std::max(n-1, n+1);
      |                ~~~~~~~~^~~~~~~~~~

The warning works by checking if a reference is initialized with a function
that returns a reference, and at least one parameter of the function is
a reference that is bound to a temporary.  It assumes that such a function
actually returns one of its arguments!  (I added code to check_return_expr
to suppress the warning when we've seen the definition of the function
and we can say that it can return a variable with static storage
duration.)

It warns when the function in question is a member function, but only if
the function is invoked on a temporary object, otherwise the warning
would emit loads of warnings for valid code like obj.emplace<T>({0}, 0).
It does detect the dangling reference in:

  struct S {
    const S& self () { return *this; }
  };
  const S& s = S().self();

It warns in member initializer lists as well:

  const int& f(const int& i) { return i; }
  struct S {
    const int &r;
    S() : r(f(10)) { }
  };

I've run the testsuite/bootstrap with the warning enabled by default.
There were just a few FAILs, all of which look like genuine bugs.
A bootstrap with the warning enabled by default passed as well.

When testing a previous version of the patch, there were many FAILs in
libstdc++'s 22_locale/; all of them because the warning triggered on

  const test_type& obj = std::use_facet<test_type>(std::locale());

but this code looks valid -- std::use_facet doesn't return a reference
to its parameter.  Therefore I added a #pragma and code to suppress the
warning.

	PR c++/106393

gcc/c-family/ChangeLog:

	* c.opt (Wdangling-reference): New.

gcc/cp/ChangeLog:

	* call.cc (expr_represents_temporary_p): New, factored out of...
	(conv_binds_ref_to_temporary): ...here.  Don't return false just
	because a ck_base is missing.  Use expr_represents_temporary_p.
	(do_warn_dangling_reference): New.
	(maybe_warn_dangling_reference): New.
	(extend_ref_init_temps): Call maybe_warn_dangling_reference.
	* typeck.cc (check_return_expr): Suppress -Wdangling-reference
	warnings.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wdangling-reference.

libstdc++-v3/ChangeLog:

	* include/bits/locale_classes.tcc: Add #pragma to disable
	-Wdangling-reference with std::use_facet.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/elision4.C: Use -Wdangling-reference, add dg-warning.
	* g++.dg/cpp23/elision7.C: Likewise.
	* g++.dg/warn/Wdangling-reference1.C: New test.
	* g++.dg/warn/Wdangling-reference2.C: New test.
	* g++.dg/warn/Wdangling-reference3.C: New test.
---
 gcc/c-family/c.opt                            |   4 +
 gcc/cp/call.cc                                | 148 ++++++++++++++++--
 gcc/cp/cp-tree.h                              |   4 +-
 gcc/cp/typeck.cc                              |  10 ++
 gcc/doc/invoke.texi                           |  40 ++++-
 gcc/testsuite/g++.dg/cpp23/elision4.C         |   5 +-
 gcc/testsuite/g++.dg/cpp23/elision7.C         |   3 +-
 .../g++.dg/warn/Wdangling-reference1.C        | 144 +++++++++++++++++
 .../g++.dg/warn/Wdangling-reference2.C        |  28 ++++
 .../g++.dg/warn/Wdangling-reference3.C        |  24 +++
 libstdc++-v3/include/bits/locale_classes.tcc  |   3 +
 11 files changed, 396 insertions(+), 17 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference2.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference3.C


base-commit: a87819b8f1b890d36a3f05bd9de80be20e9525dd
  

On 10/26/22 12:10, Marek Polacek wrote:
> On Tue, Oct 25, 2022 at 11:53:51AM -0400, Jason Merrill via Gcc-patches wrote:
>> On 10/25/22 11:21, Marek Polacek wrote:
>>> On Mon, Oct 24, 2022 at 01:30:42PM -0400, Jason Merrill wrote:
>>>> On 10/21/22 19:28, Marek Polacek wrote:
>>>>> It doesn't warn when the function in question is a member function, otherwise
>>>>> it'd emit loads of warnings for valid code like obj.emplace<T>({0}, 0).
>>>>
>>>> We had discussed warning if the object argument is a temporary (and for the
>>>> above check, the function returns *this)?
>>>
>>> Presumably you mean detecting something like this:
>>>
>>> struct S {
>>>     const S& self () { return *this; }
>>> };
>>> const S& s = S().self();
>>
>> Yes.  Or
>>
>> struct S {
>>    int ar[4];
>>    int& operator[](int i) { return ar[i]; }
>> };
>> const int &r = S()[2];
> 
> Ok, I've extended the warning to check if the object argument is a temporary
> as well, so we get a warning here now.
>   
>>> I don't currently have a way to detect it, can I steal a METHOD_TYPE flag
>>> that says "this member function returns *this"?  Alternatively, walk its
>>> DECL_SAVED_TREE and look for RETURN_EXPR?
>>
>> Like you limited the above check to TREE_STATIC, let's also forget about
>> checking for return *this.
> 
> I don't follow, but it looks like I don't need to change anything and just
> keep the TREE_STATIC check?
>   
>>>>> It warns in member initializer lists as well:
>>>>>
>>>>>      const int& f(const int& i) { return i; }
>>>>>      struct S {
>>>>>        const int &r; // { dg-warning "dangling reference" }
>>>>>        S() : r(f(10)) { } // { dg-message "destroyed" }
>>>>>      };
>>>>>
>>>>> I've run the testsuite/bootstrap with the warning enabled by default.
>>>>> There were just a few FAILs:
>>>>> * g++.dg/warn/Wdangling-pointer-2.C
>>>>> * 20_util/any/misc/any_cast.cc
>>>>> * 20_util/forward/c_neg.cc
>>>>> * 20_util/forward/f_neg.cc
>>>>> * experimental/any/misc/any_cast.cc
>>>>> all of these look like genuine bugs.  A bootstrap with the warning
>>>>> enabled by default passed.
>>>>>
>>>>> When testing a previous version of the patch, there were many FAILs in
>>>>> libstdc++'s 22_locale/; all of them because the warning triggered on
>>>>>
>>>>>      const test_type& obj = std::use_facet<test_type>(std::locale());
>>>>>
>>>>> but this code looks valid -- std::use_facet doesn't return a reference
>>>>> to its parameter.  Therefore I added code to suppress the warning when
>>>>> the call is std::use_facet.  Now 22_locale/* pass even with the warning
>>>>> on.  We could exclude more std:: functions like this if desirable.
>>>>
>>>> Instead of adding special cases in the compiler, let's disable the warning
>>>> around the definition of use_facet (and adjust the compiler as needed so
>>>> that avoids the warning).
>>>
>>> As I said in
>>> <https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604307.html>
>>> I don't think it's possible without inventing an attribute (?).
>>
>> Replied.
> 
> Fixed.
>   
>>>>> +  tree fndecl = cp_get_callee_fndecl_nofold (call);
>>>>> +  if (!fndecl
>>>>> +      || warning_suppressed_p (fndecl, OPT_Wdangling_reference)
>>>>> +      /* Don't warn about member functions; the warning would trigger in
>>>>> +	 valid code like
>>>>> +	   std::any a(...);
>>>>> +	   S& s = a.emplace<S>({0}, 0);
>>>>> +	 which constructs a new object and returns a reference to it.  */
>>>>> +      || DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
>>>>> +      /* It seems unreasonable to warn about operator functions.  */
>>>>> +      || DECL_OVERLOADED_OPERATOR_P (fndecl)
>>>>
>>>> I guess I'd expect false positives on << and >> because of iostreams, do you
>>>> see false positives with other operators?
>>>
>>> This was just a guess.  The warning triggered in g++.dg/overload/operator6.C.
>>
>> That looks like a true positive.
> 
> Ok, then...
> 
>>> I suppose this could be limited to << and >>?  I'm not sure.
>>
>> I'm not sure either.  Another possibility would be to only consider the LHS
>> of binary operators, since returning a reference to the RHS is very unusual.
> 
> ...I've removed the DECL_OVERLOADED_OPERATOR_P check altogether and don't
> really see any false positives.  It's probably better to start with a more
> general warning and then tweak it based on empirical evidence.
>   
>>>>> +// Invalid, but we don't warn here yet.
>>>>> +// r12 = f (f ((const int &) &TARGET_EXPR <D.2459, 1>))
>>>>> +const int& r12 = f(f(1));
>>>>
>>>> This should be a simple recursion?
>>>
>>> Hmm, the inner call is just a sub-expression of the full-expression so
>>> there you can still use the returned temporary.  But in this case the
>>> temporary is used beyond the full-expression so it's invalid.  I've added
>>>
>>>            if (tree r = find_initializing_call_expr (arg))
>>>              if (cp_tree_equal (CALL_EXPR_FN (r), CALL_EXPR_FN (expr)))
>>>                return expr;
>>>
>>> so that we detect f(f(1)) but I'm dubious that this is actually useful.
>>
>> Indeed, but why limit it to checking for the same function rather than also
>> warning for
>>
>> f(g(1))
>>
>> or
>>
>> A().f().g()
>>
>> ?
> 
> OK, I've made the warning even more recursive so that now we detect
> both of these, see Wdangling-reference3.C.  While at it, I've renamed
> the functions.
> 
> As before, I've tested this patch twice, once with -Wdangling-reference
> enabled by default, once with -Wdangling-reference enabled by -Wextra.
> The couple of FAILs I saw were true positives (e.g., rv-conv1.C, rv-func2.C).

I might actually add it to -Wall, the false positive rate sounds pretty 
low at this point.

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> -- >8 --
> This patch implements a new experimental warning (enabled by -Wextra) to
> detect references bound to temporaries whose lifetime has ended.  The
> primary motivation is the Note in
> <https://en.cppreference.com/w/cpp/algorithm/max>:
> 
>    Capturing the result of std::max by reference produces a dangling reference
>    if one of the parameters is a temporary and that parameter is returned:
> 
>    int n = 1;
>    const int& r = std::max(n-1, n+1); // r is dangling
> 
> That's because both temporaries for n-1 and n+1 are destroyed at the end
> of the full expression.  With this warning enabled, you'll get:
> 
> g.C:3:12: warning: possibly dangling reference to a temporary [-Wdangling-reference]
>      3 | const int& r = std::max(n-1, n+1);
>        |            ^
> g.C:3:24: note: the temporary was destroyed at the end of the full expression 'std::max<int>((n - 1), (n + 1))'
>      3 | const int& r = std::max(n-1, n+1);
>        |                ~~~~~~~~^~~~~~~~~~
> 
> The warning works by checking if a reference is initialized with a function
> that returns a reference, and at least one parameter of the function is
> a reference that is bound to a temporary.  It assumes that such a function
> actually returns one of its arguments!  (I added code to check_return_expr
> to suppress the warning when we've seen the definition of the function
> and we can say that it can return a variable with static storage
> duration.)
> 
> It warns when the function in question is a member function, but only if
> the function is invoked on a temporary object, otherwise the warning
> would emit loads of warnings for valid code like obj.emplace<T>({0}, 0).
> It does detect the dangling reference in:
> 
>    struct S {
>      const S& self () { return *this; }
>    };
>    const S& s = S().self();
> 
> It warns in member initializer lists as well:
> 
>    const int& f(const int& i) { return i; }
>    struct S {
>      const int &r;
>      S() : r(f(10)) { }
>    };
> 
> I've run the testsuite/bootstrap with the warning enabled by default.
> There were just a few FAILs, all of which look like genuine bugs.
> A bootstrap with the warning enabled by default passed as well.
> 
> When testing a previous version of the patch, there were many FAILs in
> libstdc++'s 22_locale/; all of them because the warning triggered on
> 
>    const test_type& obj = std::use_facet<test_type>(std::locale());
> 
> but this code looks valid -- std::use_facet doesn't return a reference
> to its parameter.  Therefore I added a #pragma and code to suppress the
> warning.
> 
> 	PR c++/106393
> 
> gcc/c-family/ChangeLog:
> 
> 	* c.opt (Wdangling-reference): New.
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (expr_represents_temporary_p): New, factored out of...
> 	(conv_binds_ref_to_temporary): ...here.  Don't return false just
> 	because a ck_base is missing.  Use expr_represents_temporary_p.
> 	(do_warn_dangling_reference): New.
> 	(maybe_warn_dangling_reference): New.
> 	(extend_ref_init_temps): Call maybe_warn_dangling_reference.
> 	* typeck.cc (check_return_expr): Suppress -Wdangling-reference
> 	warnings.
> 
> gcc/ChangeLog:
> 
> 	* doc/invoke.texi: Document -Wdangling-reference.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/locale_classes.tcc: Add #pragma to disable
> 	-Wdangling-reference with std::use_facet.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp23/elision4.C: Use -Wdangling-reference, add dg-warning.
> 	* g++.dg/cpp23/elision7.C: Likewise.
> 	* g++.dg/warn/Wdangling-reference1.C: New test.
> 	* g++.dg/warn/Wdangling-reference2.C: New test.
> 	* g++.dg/warn/Wdangling-reference3.C: New test.
> ---
>   gcc/c-family/c.opt                            |   4 +
>   gcc/cp/call.cc                                | 148 ++++++++++++++++--
>   gcc/cp/cp-tree.h                              |   4 +-
>   gcc/cp/typeck.cc                              |  10 ++
>   gcc/doc/invoke.texi                           |  40 ++++-
>   gcc/testsuite/g++.dg/cpp23/elision4.C         |   5 +-
>   gcc/testsuite/g++.dg/cpp23/elision7.C         |   3 +-
>   .../g++.dg/warn/Wdangling-reference1.C        | 144 +++++++++++++++++
>   .../g++.dg/warn/Wdangling-reference2.C        |  28 ++++
>   .../g++.dg/warn/Wdangling-reference3.C        |  24 +++
>   libstdc++-v3/include/bits/locale_classes.tcc  |   3 +
>   11 files changed, 396 insertions(+), 17 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference2.C
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference3.C
> 
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 01d480759ae..02d79991aeb 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -555,6 +555,10 @@ Wdangling-pointer=
>   C ObjC C++ ObjC++ Joined RejectNegative UInteger Var(warn_dangling_pointer) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall, 2, 0) IntegerRange(0, 2)
>   Warn for uses of pointers to auto variables whose lifetime has ended.
>   
> +Wdangling-reference
> +C++ ObjC++ Var(warn_dangling_reference) Warning LangEnabledBy(C++ ObjC++, Wextra)
> +Warn when a reference is bound to a temporary whose lifetime has ended.
> +
>   Wdate-time
>   C ObjC C++ ObjC++ CPP(warn_date_time) CppReason(CPP_W_DATE_TIME) Var(cpp_warn_date_time) Init(0) Warning
>   Warn about __TIME__, __DATE__ and __TIMESTAMP__ usage.
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 6a34e9c2ae1..dae7a33fb89 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -9313,6 +9313,16 @@ conv_binds_ref_to_prvalue (conversion *c)
>     return conv_is_prvalue (next_conversion (c));
>   }
>   
> +/* True iff EXPR represents a (subobject of a) temporary.  */
> +
> +static bool
> +expr_represents_temporary_p (tree expr)
> +{
> +  while (handled_component_p (expr))
> +    expr = TREE_OPERAND (expr, 0);
> +  return TREE_CODE (expr) == TARGET_EXPR;
> +}
> +
>   /* True iff C is a conversion that binds a reference to a temporary.
>      This is a superset of conv_binds_ref_to_prvalue: here we're also
>      interested in xvalues.  */
> @@ -9330,18 +9340,14 @@ conv_binds_ref_to_temporary (conversion *c)
>          struct Derived : Base {};
>          const Base& b(Derived{});
>        where we bind 'b' to the Base subobject of a temporary object of type
> -     Derived.  The subobject is an xvalue; the whole object is a prvalue.  */
> -  if (c->kind != ck_base)
> -    return false;
> -  c = next_conversion (c);
> -  if (c->kind == ck_identity && c->u.expr)
> -    {
> -      tree expr = c->u.expr;
> -      while (handled_component_p (expr))
> -	expr = TREE_OPERAND (expr, 0);
> -      if (TREE_CODE (expr) == TARGET_EXPR)
> -	return true;
> -    }
> +     Derived.  The subobject is an xvalue; the whole object is a prvalue.
> +
> +     The ck_base doesn't have to be present for cases like X{}.m.  */
> +  if (c->kind == ck_base)
> +    c = next_conversion (c);
> +  if (c->kind == ck_identity && c->u.expr
> +      && expr_represents_temporary_p (c->u.expr))
> +    return true;
>     return false;
>   }
>   
> @@ -13428,6 +13434,121 @@ initialize_reference (tree type, tree expr,
>     return expr;
>   }
>   
> +/* 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
> +   if none found.  For instance:
> +
> +     const S& s = S().self(); // S::self (&TARGET_EXPR <...>)
> +     const int& r = (42, f(1)); // f(1)
> +     const int& t = b ? f(1) : f(2); // f(1)
> +     const int& u = b ? f(1) : f(g); // f(1)
> +     const int& v = b ? f(g) : f(2); // f(2)
> +     const int& w = b ? f(g) : f(g); // NULL_TREE
> +     const int& y = (f(1), 42); // NULL_TREE
> +     const int& z = f(f(1)); // f(f(1))
> +
> +   EXPR is the initializer.  */
> +
> +static tree
> +do_warn_dangling_reference (tree expr)
> +{
> +  STRIP_NOPS (expr);
> +  switch (TREE_CODE (expr))
> +    {
> +    case CALL_EXPR:
> +      {
> +	tree fndecl = cp_get_callee_fndecl_nofold (expr);
> +	if (!fndecl
> +	    || warning_suppressed_p (fndecl, OPT_Wdangling_reference)
> +	    || !warning_enabled_at (DECL_SOURCE_LOCATION (fndecl),
> +				    OPT_Wdangling_reference)
> +	    /* If the function doesn't return a reference, don't warn.  This
> +	       can be e.g.
> +		 const int& z = std::min({1, 2, 3, 4, 5, 6, 7});
> +	       which doesn't dangle: std::min here returns an int.  */
> +	    || !TYPE_REF_P (TREE_TYPE (TREE_TYPE (fndecl))))

Maybe TYPE_REF_OBJ_P?  Either way is fine.

> +	  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)
> +	  {
> +	    tree arg = CALL_EXPR_ARG (expr, i);
> +	    /* 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;
> +	    /* 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))
> +	      return expr;
> +	  /* Don't warn about member function like:
> +	      std::any a(...);
> +	      S& s = a.emplace<S>({0}, 0);
> +	     which constructs a new object and returns a reference to it, but
> +	     we still want to detect:
> +	       struct S { const S& self () { return *this; } };
> +	       const S& s = S().self();
> +	     where 's' dangles.  If we've gotten here, the object this function
> +	     is invoked on is not a temporary.  */
> +	    if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl))
> +	      break;
> +	  }
> +	return NULL_TREE;
> +      }
> +    case COMPOUND_EXPR:
> +      return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
> +    case COND_EXPR:
> +      if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
> +	return t;
> +      return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
> +    case PAREN_EXPR:
> +      return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
> +    default:
> +      return NULL_TREE;
> +    }
> +}
> +
> +/* Implement -Wdangling-reference, to detect cases like
> +
> +     int n = 1;
> +     const int& r = std::max(n - 1, n + 1); // r is dangling
> +
> +   This creates temporaries from the arguments, returns a reference to
> +   one of the temporaries, but both temporaries are destroyed at the end
> +   of the full expression.
> +
> +   This works by checking if a reference is initialized with a function
> +   that returns a reference, and at least one parameter of the function
> +   is a reference that is bound to a temporary.  It assumes that such a
> +   function actually returns one of its arguments.
> +
> +   DECL is the reference being initialized, INIT is the initializer.  */
> +
> +static void
> +maybe_warn_dangling_reference (const_tree decl, tree init)
> +{
> +  if (!warn_dangling_reference)
> +    return;
> +  if (!TYPE_REF_P (TREE_TYPE (decl)))
> +    return;
> +  if (tree call = do_warn_dangling_reference (init))
> +    {
> +      auto_diagnostic_group d;
> +      if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
> +		      "possibly dangling reference to a temporary"))
> +	inform (EXPR_LOCATION (call), "the temporary was destroyed at "
> +		"the end of the full expression %qE", call);
> +    }
> +}
> +
>   /* If *P is an xvalue expression, prevent temporary lifetime extension if it
>      gets used to initialize a reference.  */
>   
> @@ -13525,6 +13646,9 @@ extend_ref_init_temps (tree decl, tree init, vec<tree, va_gc> **cleanups,
>     tree type = TREE_TYPE (init);
>     if (processing_template_decl)
>       return init;
> +
> +  maybe_warn_dangling_reference (decl, init);
> +
>     if (TYPE_REF_P (type))
>       init = extend_ref_init_temps_1 (decl, init, cleanups, cond_guard);
>     else
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 867096b08c6..40f5bf802c3 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -459,7 +459,6 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
>         TI_PENDING_TEMPLATE_FLAG.
>         TEMPLATE_PARMS_FOR_INLINE.
>         DELETE_EXPR_USE_VEC (in DELETE_EXPR).
> -      (TREE_CALLS_NEW) (in _EXPR or _REF) (commented-out).
>         ICS_ELLIPSIS_FLAG (in _CONV)
>         DECL_INITIALIZED_P (in VAR_DECL)
>         TYPENAME_IS_CLASS_P (in TYPENAME_TYPE)
> @@ -4567,6 +4566,9 @@ get_vec_init_expr (tree t)
>      When appearing in a CONSTRUCTOR, the expression is an unconverted
>      compound literal.
>   
> +   When appearing in a CALL_EXPR, it means that it is a call to
> +   a constructor.
> +
>      When appearing in a FIELD_DECL, it means that this field
>      has been duly initialized in its constructor.  */
>   #define TREE_HAS_CONSTRUCTOR(NODE) (TREE_LANG_FLAG_4 (NODE))
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index ab6979bcc50..54fac880d8c 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -11246,6 +11246,16 @@ check_return_expr (tree retval, bool *no_warning)
>     if (processing_template_decl)
>       return saved_retval;
>   
> +  /* A naive attempt to reduce the number of -Wdangling-reference false
> +     positives: if we know that this function can return a variable with
> +     static storage duration rather than one of its parameters, suppress
> +     the warning.  */
> +  if (warn_dangling_reference
> +      && TYPE_REF_P (functype)
> +      && bare_retval

You probably need to check VAR_P (bare_retval) here, static_flag is used 
for various other things in other tree codes.

> +      && TREE_STATIC (bare_retval))
> +    suppress_warning (current_function_decl, OPT_Wdangling_reference);
> +
>     /* Actually copy the value returned into the appropriate location.  */
>     if (retval && retval != result)
>       retval = cp_build_init_expr (result, retval);
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 64f77e8367a..0a3a174cdc8 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -249,7 +249,8 @@ in the following sections.
>   -Wno-class-conversion  -Wclass-memaccess @gol
>   -Wcomma-subscript  -Wconditionally-supported @gol
>   -Wno-conversion-null  -Wctad-maybe-unsupported @gol
> --Wctor-dtor-privacy  -Wno-delete-incomplete @gol
> +-Wctor-dtor-privacy  -Wdangling-reference @gol
> +-Wno-delete-incomplete @gol
>   -Wdelete-non-virtual-dtor  -Wno-deprecated-array-compare @gol
>   -Wdeprecated-copy -Wdeprecated-copy-dtor @gol
>   -Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion @gol
> @@ -3627,6 +3628,42 @@ public static member functions.  Also warn if there are no non-private
>   methods, and there's at least one private member function that isn't
>   a constructor or destructor.
>   
> +@item -Wdangling-reference @r{(C++ and Objective-C++ only)}
> +@opindex Wdangling-reference
> +@opindex Wno-dangling-reference
> +Warn when a reference is bound to a temporary whose lifetime has ended.
> +For example:
> +
> +@smallexample
> +int n = 1;
> +const int& r = std::max(n - 1, n + 1); // r is dangling
> +@end smallexample
> +
> +In the example above, two temporaries are created, one for each
> +argument, and a reference to one of the temporaries is returned.
> +However, both temporaries are destroyed at the end of the full
> +expression, so the reference @code{r} is dangling.  This warning
> +also detects dangling references in member initializer lists:
> +
> +@smallexample
> +const int& f(const int& i) @{ return i; @}
> +struct S @{
> +  const int &r; // r is dangling
> +  S() : r(f(10)) @{ @}
> +@};
> +@end smallexample
> +
> +Member functions are checked as well, but only their object argument:
> +
> +@smallexample
> +struct S @{
> +   const S& self () @{ return *this; @}
> +@};
> +const S& s = S().self(); // s is dangling
> +@end smallexample
> +
> +This warning is enabled by @option{-Wextra}.

It would be good to mention using #pragma to disable the warning around 
safe functions.

>   @item -Wdelete-non-virtual-dtor @r{(C++ and Objective-C++ only)}
>   @opindex Wdelete-non-virtual-dtor
>   @opindex Wno-delete-non-virtual-dtor
> @@ -5936,6 +5973,7 @@ name is still supported, but the newer name is more descriptive.)
>   
>   @gccoptlist{-Wclobbered  @gol
>   -Wcast-function-type  @gol
> +-Wdangling-reference @r{(C++ only)} @gol
>   -Wdeprecated-copy @r{(C++ only)} @gol
>   -Wempty-body  @gol
>   -Wenum-conversion @r{(C only)} @gol
> diff --git a/gcc/testsuite/g++.dg/cpp23/elision4.C b/gcc/testsuite/g++.dg/cpp23/elision4.C
> index c19b86b8b5f..d39053ad741 100644
> --- a/gcc/testsuite/g++.dg/cpp23/elision4.C
> +++ b/gcc/testsuite/g++.dg/cpp23/elision4.C
> @@ -1,5 +1,6 @@
>   // PR c++/101165 - P2266R1 - Simpler implicit move
>   // { dg-do compile { target c++23 } }
> +// { dg-options "-Wdangling-reference" }
>   // Test from P2266R1, $ 5.2. LibreOffice OString constructor.
>   
>   struct X {
> @@ -33,6 +34,6 @@ T& temporary2(T&& x) { return static_cast<T&>(x); }
>   void
>   test ()
>   {
> -  int& r1 = temporary1 (42);
> -  int& r2 = temporary2 (42);
> +  int& r1 = temporary1 (42); // { dg-warning "dangling reference" }
> +  int& r2 = temporary2 (42); // { dg-warning "dangling reference" }
>   }
> diff --git a/gcc/testsuite/g++.dg/cpp23/elision7.C b/gcc/testsuite/g++.dg/cpp23/elision7.C
> index 19fa89ae133..0045842b34f 100644
> --- a/gcc/testsuite/g++.dg/cpp23/elision7.C
> +++ b/gcc/testsuite/g++.dg/cpp23/elision7.C
> @@ -1,5 +1,6 @@
>   // PR c++/101165 - P2266R1 - Simpler implicit move
>   // { dg-do compile { target c++23 } }
> +// { dg-options "-Wdangling-reference" }
>   
>   struct X {
>     X ();
> @@ -68,5 +69,5 @@ f7 (T &&t)
>   void
>   do_f7 ()
>   {
> -  const int &x = f7 (0);
> +  const int &x = f7 (0); // { dg-warning "dangling reference" }
>   }
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> new file mode 100644
> index 00000000000..97c81ee716c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> @@ -0,0 +1,144 @@
> +// PR c++/106393
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +const int& f(const int& i) { return i; }
> +const int& f_(const int& i) { return i; }
> +const int& h(int);
> +const int& rp(const int *);
> +int g;
> +const int& globref(const int&) { return g; }
> +struct X {
> +  int* i;
> +  operator const int&() const { return *i; }
> +};
> +X x{&g};
> +
> +const int& r1 = f(10); // { dg-warning "dangling reference" }
> +// r2 = _ZGR2r2_ = (int) *f ((const int &) &TARGET_EXPR <D.2429, 10>) + 1; (const int &) &_ZGR2r2_
> +const int& r2 = f(10) + 1;
> +// Don't warn here, we have
> +//   r3 = f (X::operator const int& (&x))
> +const int& r3 = f(x);
> +// Don't warn here, because we've seen the definition of globref
> +// and could figure out that it may not return one of its parms.
> +// Questionable -- it can also hide bugs --, but it helps here.
> +const int& r4 = globref(1);
> +const int& r5 = (42, f(10)); // { dg-warning "dangling reference" }
> +const int& r6 = (f(10), 42);
> +const int& r7 = (f(10)); // { dg-warning "dangling reference" }
> +const int& r8 = g ? f(10) : f(9); // { dg-warning "dangling reference" }
> +const int& r9 = (42, g ? f(10) : f(9)); // { dg-warning "dangling reference" }
> +const int& r10 = (g ? f(10) : f(9), 42);
> +// Binds to a reference temporary for r11.  No dangling reference.
> +const int& r11 = g ? f(10) : 9;
> +const int& r12 = g ? 9 : f(10);
> +// r12 = f (f ((const int &) &TARGET_EXPR <D.2459, 1>))
> +const int& r13 = f(f(1)); // { dg-warning "dangling reference" }
> +const int& r14 = f(f_(1)); // { dg-warning "dangling reference" }
> +const int& r15 = f(g ? f(1) : f(2)); // { dg-warning "dangling reference" }
> +const int& r16 = f(*&f(1)); // { dg-warning "dangling reference" }
> +const int& r17 = rp(&f(1));
> +const int& r18 = rp(&f(g));
> +const int& r19 = h(f(1));
> +// Other forms of initializers.
> +const int& r20(f(10)); // { dg-warning "dangling reference" }
> +const int& r21(f(10)); // { dg-warning "dangling reference" }
> +// Returns a ref, but doesn't have a parameter of reference type.
> +const int& r22 = h(10);
> +const int& r23 = g ? h(10) : f(10); // { dg-warning "dangling reference" }
> +const int& r24 = g ? f(10) : h(10); // { dg-warning "dangling reference" }
> +const int& r25 = g ? h(10) : (1, f(10)); // { dg-warning "dangling reference" }
> +const int& r26 = g ? (1, f(10)) : h(10); // { dg-warning "dangling reference" }
> +const int& r29 = f((f_(1), 1)); // { dg-warning "dangling reference" }
> +const int& r30 = f((f_(1), g));
> +
> +struct Z {
> +  operator int() { return 42; }
> +};
> +
> +const int& r27 = f(Z()); // { dg-warning "dangling reference" }
> +const int& r28 = f(true ? Z() : Z()); // { dg-warning "dangling reference" }
> +
> +const int& operator|(const int &, Z);
> +const int& r31 = 1 | Z(); // { dg-warning "dangling reference" }
> +
> +// OK: the reference is bound to the 10 so still valid at the point
> +// where it's copied into i1.
> +int i1 = f(10);
> +
> +int
> +test1 ()
> +{
> +  const int &lr = f(10); // { dg-warning "dangling reference" }
> +  int i2 = f(10);
> +  return lr;
> +}
> +
> +struct B { };
> +struct D : B { };
> +struct C {
> +  D d;
> +};
> +
> +C c;
> +D d;
> +
> +using U = D[3];
> +
> +const B& frotz(const D&);
> +const B& b1 = frotz(C{}.d); // { dg-warning "dangling reference" }
> +const B& b2 = frotz(D{}); // { dg-warning "dangling reference" }
> +const B& b3 = frotz(c.d);
> +const B& b4 = frotz(d);
> +const B& b5 = frotz(U{}[0]); // { dg-warning "dangling reference" }
> +
> +// Try returning a subobject.
> +const B& bar (const D& d) { return d; }
> +const B& b6 = bar (D{}); // { dg-warning "dangling reference" }
> +const B& baz (const C& c) { return c.d; }
> +const B& b7 = baz (C{}); // { dg-warning "dangling reference" }
> +const D& qux (const C& c) { return c.d; }
> +const D& d1 = qux (C{}); // { dg-warning "dangling reference" }
> +
> +struct E {
> +  E(int);
> +};
> +const E& operator*(const E&);
> +const E& b8 = *E(1); // { dg-warning "dangling reference" }
> +
> +struct F : virtual B { };
> +struct G : virtual B { };
> +struct H : F, G { };
> +const B& yum (const F& f) { return f; }
> +const B& b9 = yum (F{}); // { dg-warning "dangling reference" }
> +const B& lox (const H& h) { return h; }
> +const B& b10 = lox (H{}); // { dg-warning "dangling reference" }
> +
> +struct S {
> +  const int &r; // { dg-warning "dangling reference" }
> +  S() : r(f(10)) { } // { dg-message "destroyed" }
> +};
> +
> +// From cppreference.
> +template<class T>
> +const T& max(const T& a, const T& b)
> +{
> +    return (a < b) ? b : a;
> +}
> +
> +int n = 1;
> +const int& refmax = max(n - 1, n + 1); // { dg-warning "dangling reference" }
> +
> +struct Y {
> +  operator int&();
> +  operator int&&();
> +  const int& foo(const int&);
> +};
> +
> +// x1 = Y::operator int&& (&TARGET_EXPR <D.2410, {}>)
> +int&& x1 = Y(); // { dg-warning "dangling reference" }
> +int&& x2 = Y{}; // { dg-warning "dangling reference" }
> +int& x3 = Y(); // { dg-warning "dangling reference" }
> +int& x4 = Y{}; // { dg-warning "dangling reference" }
> +const int& t1 = Y().foo(10); // { dg-warning "dangling reference" }
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference2.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference2.C
> new file mode 100644
> index 00000000000..dafdb43f1b9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference2.C
> @@ -0,0 +1,28 @@
> +// PR c++/106393
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +namespace std {
> +struct any {};
> +template <typename _ValueType> _ValueType any_cast(any &&);
> +template <typename _Tp> struct remove_reference { using type = _Tp; };
> +template <typename _Tp> _Tp forward(typename remove_reference<_Tp>::type);
> +template <typename _Tp> typename remove_reference<_Tp>::type move(_Tp);
> +} // namespace std
> +
> +const int &r = std::any_cast<int&>(std::any()); // { dg-warning "dangling reference" }
> +
> +template <class T> struct C {
> +  T t_; // { dg-warning "dangling reference" }
> +  C(T);
> +  template <class U> C(U c) : t_(std::forward<T>(c.t_)) {}
> +};
> +struct A {};
> +struct B {
> +  B(A);
> +};
> +int main() {
> +  A a;
> +  C<A> ca(a);
> +  C<B &&>(std::move(ca));
> +}
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference3.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference3.C
> new file mode 100644
> index 00000000000..4bc20c13b3f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference3.C
> @@ -0,0 +1,24 @@
> +// PR c++/106393
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct A {
> +   int ar[4];
> +   int& operator[](int i) { return ar[i]; }
> +};
> +const int &r = A()[2]; // { dg-warning "dangling reference" }
> +
> +struct S {
> +  const S& self () { return *this; }
> +};
> +const S& s = S().self(); // { dg-warning "dangling reference" }
> +
> +struct G {
> +  const G& g() { return *this; }
> +};
> +
> +struct F {
> +  G& f();
> +};
> +
> +const G& g = F().f().g(); // { dg-warning "dangling reference" }
> diff --git a/libstdc++-v3/include/bits/locale_classes.tcc b/libstdc++-v3/include/bits/locale_classes.tcc
> index 64cd7534dc6..9cc4f238ee7 100644
> --- a/libstdc++-v3/include/bits/locale_classes.tcc
> +++ b/libstdc++-v3/include/bits/locale_classes.tcc
> @@ -127,6 +127,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      *  @return  Reference to facet of type Facet.
>      *  @throw  std::bad_cast if @p __loc doesn't contain a facet of type _Facet.
>     */
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wdangling-reference"
>     template<typename _Facet>
>       const _Facet&
>       use_facet(const locale& __loc)
> @@ -141,6 +143,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         return static_cast<const _Facet&>(*__facets[__i]);
>   #endif
>       }
> +#pragma GCC diagnostic pop
>   
>   
>     // Generic version does nothing.
> 
> base-commit: a87819b8f1b890d36a3f05bd9de80be20e9525dd