On Mon, Oct 24, 2022 at 01:30:42PM -0400, Jason Merrill wrote:
> On 10/21/22 19:28, Marek Polacek wrote:
> > This patch implements a new experimental warning (enabled by -Wextra) to
> > detect references bound to temporaries whose lifetime has ended. The
>
> Great!
>
> > 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 something other than its parameter.)
>
> Hmm, that misses returning a reference to a subobject or container element
> that will also go away when the object is destroyed.
Yes :-(. Fixed, tests added. I'm just checking TREE_STATIC now.
> Does it also avoid a lot of false positives?
Not at all, I just thought it may be worth it.
> > 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();
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?
> > 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 (?).
> I was remembering range adaptors being a stated motivation for Nico's P2012,
> but looking back at the paper I now see that this problem was avoided for
> them by disallowing rvalue arguments to range composition.
Aha, and if you can't pass a temporary, you will not have this problem.
> > 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.
>
> Could use a test with a virtual base.
Added (fns yum/lox in Wdangling-reference1.C).
> > +static tree
> > +find_initializing_call_expr (tree expr)
> > +{
> > + STRIP_NOPS (expr);
> > + switch (TREE_CODE (expr))
> > + {
> > + case CALL_EXPR:
> > + return expr;
> > + case COMPOUND_EXPR:
> > + return find_initializing_call_expr (TREE_OPERAND (expr, 1));
> > + case COND_EXPR:
> > + if (tree t = find_initializing_call_expr (TREE_OPERAND (expr, 1)))
> > + return t;
> > + return find_initializing_call_expr (TREE_OPERAND (expr, 2));
>
> For COND_EXPR I think we want to check both sides, in case there are calls
> on both sides but only the second one has a problematic temporary.
Wow, good catch. That was a bug. I've fixed it by moving the
expr_represents_temporary_p checking into find_initializing_call_expr.
> > + case PAREN_EXPR:
> > + return find_initializing_call_expr (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.
> > +
> > + This warning doesn't warn when the function in question is a member
> > + function.
> > +
> > + DECL is the reference being initialized, CALL is the initializer. */
> > +
> > +static void
> > +do_warn_dangling_reference (const_tree decl, tree call)
> > +{
> > + if (!warn_dangling_reference)
> > + return;
> > + if (!TYPE_REF_P (TREE_TYPE (decl)))
> > + return;
> > + call = find_initializing_call_expr (call);
> > + if (call == NULL_TREE)
> > + return;
> > +
> > + 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.
I suppose this could be limited to << and >>? I'm not sure.
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> > @@ -0,0 +1,103 @@
> > +// PR c++/106393
> > +// { dg-do compile { target c++11 } }
> > +// { dg-options "-Wdangling-reference" }
> > +
> > +const int& f(const int& i) { return i; }
> > +const int& h(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.
>
> We could suppress specifically for the case of returning a variable with
> static storage duration?
Done.
> > +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.
> > +const int& r11 = g ? f(10) : 9;
>
> Why no warning?
I don't think there's a dangling reference here, we get:
r11 = _ZGR3r11_ = g != 0 ? (int) *f ((const int &) &TARGET_EXPR <D.2389, 10>) :
9;, (const int &) &_ZGR3r11_;
> > +// 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.
I've added
const int& r15 = rp(&f(1));
to the test where I think we can't warn. Hence the cp_tree_equal.
Here's a v2 which fixes bugs in v1, but still doesn't handle member
functions in any way.
FWIW, bootstrapped/regtested on x86_64-pc-linux-gnu.
-- >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 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).
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.
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.
(conv_binds_ref_to_temporary): Don't return false just because a ck_base
is missing. Use expr_represents_temporary_p.
(find_initializing_call_expr): New.
(do_warn_dangling_reference): New.
(extend_ref_init_temps): Call do_warn_dangling_reference.
* typeck.cc (check_return_expr): Suppress -Wdangling-reference
warnings.
gcc/ChangeLog:
* doc/invoke.texi: Document -Wdangling-reference.
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.
---
gcc/c-family/c.opt | 4 +
gcc/cp/call.cc | 144 ++++++++++++++++--
gcc/cp/cp-tree.h | 4 +-
gcc/cp/typeck.cc | 10 ++
gcc/doc/invoke.texi | 34 ++++-
gcc/testsuite/g++.dg/cpp23/elision4.C | 5 +-
gcc/testsuite/g++.dg/cpp23/elision7.C | 3 +-
.../g++.dg/warn/Wdangling-reference1.C | 128 ++++++++++++++++
.../g++.dg/warn/Wdangling-reference2.C | 28 ++++
9 files changed, 343 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
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..cf17ee04cf7 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 +13434,117 @@ initialize_reference (tree type, tree expr,
return expr;
}
+/* Helper for do_warn_dangling_reference to find a non-nested CALL_EXPR
+ that initializes the LHS (and at least one of its arguments represents
+ a temporary), or NULL_TREE if none found. For instance:
+
+ 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
+find_initializing_call_expr (tree expr)
+{
+ STRIP_NOPS (expr);
+ switch (TREE_CODE (expr))
+ {
+ case CALL_EXPR:
+ for (int i = 0; i < call_expr_nargs (expr); ++i)
+ {
+ /* We're looking to see if ARG is something like
+ (const int &) &TARGET_EXPR <...>. */
+ tree arg = CALL_EXPR_ARG (expr, i);
+ /* It could also be the same call taking a temporary. */
+ if (tree r = find_initializing_call_expr (arg))
+ if (cp_tree_equal (CALL_EXPR_FN (r), CALL_EXPR_FN (expr)))
+ return expr;
+ STRIP_NOPS (arg);
+ if (TREE_CODE (arg) == ADDR_EXPR)
+ arg = TREE_OPERAND (arg, 0);
+ if (expr_represents_temporary_p (arg))
+ return expr;
+ }
+ return NULL_TREE;
+ case COMPOUND_EXPR:
+ return find_initializing_call_expr (TREE_OPERAND (expr, 1));
+ case COND_EXPR:
+ if (tree t = find_initializing_call_expr (TREE_OPERAND (expr, 1)))
+ return t;
+ return find_initializing_call_expr (TREE_OPERAND (expr, 2));
+ case PAREN_EXPR:
+ return find_initializing_call_expr (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.
+
+ This warning doesn't warn when the function in question is a member
+ function.
+
+ DECL is the reference being initialized, CALL is the initializer. */
+
+static void
+do_warn_dangling_reference (const_tree decl, tree call)
+{
+ if (!warn_dangling_reference)
+ return;
+ if (!TYPE_REF_P (TREE_TYPE (decl)))
+ return;
+ call = find_initializing_call_expr (call);
+ if (call == NULL_TREE)
+ return;
+
+ 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)
+ /* 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))))
+ return;
+
+ /* Mitigate some known cases. */
+ if (decl_in_std_namespace_p (fndecl))
+ if (tree name = DECL_NAME (fndecl))
+ if (id_equal (name, "use_facet"))
+ return;
+
+ 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 +13642,9 @@ extend_ref_init_temps (tree decl, tree init, vec<tree,
va_gc> **cleanups,
tree type = TREE_TYPE (init);
if (processing_template_decl)
return init;
+
+ do_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 2cca20be6c1..d22c5e1b731 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
+ && 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..e94fe20779a 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 +3628,36 @@ 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 not checked. Certain standard functions, such
+as @code{std::use_facet}, are also excluded from checking.
+
+This warning is enabled by @option{-Wextra}.
+
@item -Wdelete-non-virtual-dtor @r{(C++ and Objective-C++ only)}
@opindex Wdelete-non-virtual-dtor
@opindex Wno-delete-non-virtual-dtor
@@ -5936,6 +5967,7 @@ 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..00bee232d9f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
@@ -0,0 +1,128 @@
+// PR c++/106393
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+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.
+const int& r11 = g ? f(10) : 9;
+const int& r11_ = g ? 9 : f(10);
+// r12 = f (f ((const int &) &TARGET_EXPR <D.2459, 1>))
+const int& r12 = f(f(1)); // { dg-warning "dangling reference" }
+const int& r13 = f(g ? f(1) : f(2)); // { dg-warning "dangling reference" }
+const int& r14 = f(*&f(1)); // { dg-warning "dangling reference" }
+const int& r15 = rp(&f(1));
+const int& r16 = rp(&f(g));
+const int& r17 = h(f(1));
+// Other forms of initializers.
+const int& r18(f(10)); // { dg-warning "dangling reference" }
+const int& r19(f(10)); // { dg-warning "dangling reference" }
+// Returns a ref, but doesn't have a parameter of reference type.
+const int& r20 = h(10);
+const int& r21 = g ? h(10) : f(10); // { dg-warning "dangling reference" }
+const int& r22 = g ? f(10) : h(10); // { dg-warning "dangling reference" }
+const int& r23 = g ? h(10) : (1, f(10)); // { dg-warning "dangling reference" }
+const int& r24 = g ? (1, f(10)) : h(10); // { 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);
+
+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" }
+
+// Don't warn about member functions.
+struct Y {
+ operator int&&();
+ const int& foo(const int&);
+};
+
+// x1 = Y::operator int&& (&TARGET_EXPR <D.2410, {}>)
+int&& x1 = Y();
+int&& x2 = Y{};
+const int& t1 = Y().foo(10);
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));
+}
base-commit: 4c5b1160776382772fc0a33130dfaf621699fdbf
--
2.37.3
