On Sun, Feb 05, 2023 at 05:25:25PM -0800, Jason Merrill wrote:
> On 1/24/23 17:49, Marek Polacek wrote:
> > On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote:
> > > On 1/19/23 21:03, Marek Polacek wrote:
> > > > On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
> > > > > On 1/18/23 20:13, Marek Polacek wrote:
> > > > > > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> > > > > > > On 1/18/23 12:52, Marek Polacek wrote:
> > > > > > > > Here, -Wdangling-reference triggers where it probably
> > > > > > > > shouldn't, causing
> > > > > > > > some grief. The code in question uses a reference wrapper with
> > > > > > > > a member
> > > > > > > > function returning a reference to a subobject of a
> > > > > > > > non-temporary object:
> > > > > > > >
> > > > > > > > const Plane & meta = fm.planes().inner();
> > > > > > > >
> > > > > > > > I've tried a few approaches, e.g., checking that the member
> > > > > > > > function's
> > > > > > > > return type is the same as the type of the enclosing class
> > > > > > > > (which is
> > > > > > > > the case for member functions returning *this), but that then
> > > > > > > > breaks
> > > > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > > > >
> > > > > > > > So I figured that perhaps we want to look at the object we're
> > > > > > > > invoking
> > > > > > > > the member function(s) on and see if that is a temporary, as
> > > > > > > > in, don't
> > > > > > > > warn about
> > > > > > > >
> > > > > > > > const Plane & meta = fm.planes().inner();
> > > > > > > >
> > > > > > > > but do warn about
> > > > > > > >
> > > > > > > > const Plane & meta = FrameMetadata().planes().inner();
> > > > > > > >
> > > > > > > > It's ugly, but better than asking users to add #pragmas into
> > > > > > > > their code.
> > > > > > >
> > > > > > > Hmm, that doesn't seem right; the former is only OK because Ref
> > > > > > > is in fact a
> > > > > > > reference-like type. If planes() returned a class that held
> > > > > > > data, we would
> > > > > > > want to warn.
> > > > > >
> > > > > > Sure, it's always some kind of tradeoff with warnings :/.
> > > > > > > In this case, we might recognize the reference-like class because
> > > > > > > it has a
> > > > > > > reference member and a constructor taking the same reference type.
> > > > > >
> > > > > > That occurred to me too, but then I found out that
> > > > > > std::reference_wrapper
> > > > > > actually uses T*, not T&, as you say. But here's a patch to do that
> > > > > > (I hope).
> > > > > > > That wouldn't help with std::reference_wrapper or std::ref_view
> > > > > > > because they
> > > > > > > have pointer members instead of references, but perhaps loosening
> > > > > > > the check
> > > > > > > to include that case would make sense?
> > > > > >
> > > > > > Sorry, I don't understand what you mean by loosening the check. I
> > > > > > could
> > > > > > hardcode std::reference_wrapper and std::ref_view but I don't think
> > > > > > that's
> > > > > > what you meant.
> > > > >
> > > > > Indeed that's not what I meant, but as I was saying in our meeting I
> > > > > think
> > > > > it's worth doing; the compiler has various tweaks to handle specific
> > > > > standard-library classes better.
> > > > Okay, done in the patch below. Except that I'm not including a test for
> > > > std::ranges::ref_view because I don't really know how that works.
> > > >
> > > > > > Surely I cannot _not_ warn for any class that contains a T*.
> > > > >
> > > > > I was thinking if a constructor takes a T& and the class has a T*
> > > > > that would
> > > > > be close enough, though this also wouldn't handle the standard library
> > > > > classes so the benefit is questionable.
> > > > >
> > > > > > Here's the patch so that we have some actual code to discuss...
> > > > > > Thanks.
> > > > > >
> > > > > > -- >8 --
> > > > > > Here, -Wdangling-reference triggers where it probably shouldn't,
> > > > > > causing
> > > > > > some grief. The code in question uses a reference wrapper with a
> > > > > > member
> > > > > > function returning a reference to a subobject of a non-temporary
> > > > > > object:
> > > > > >
> > > > > > const Plane & meta = fm.planes().inner();
> > > > > >
> > > > > > I've tried a few approaches, e.g., checking that the member
> > > > > > function's
> > > > > > return type is the same as the type of the enclosing class (which is
> > > > > > the case for member functions returning *this), but that then breaks
> > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > >
> > > > > > Perhaps we want to look at the member function's enclosing class
> > > > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > > > member and a constructor taking the same reference type) and don't
> > > > > > warn if so, supposing that the member function returns a reference
> > > > > > to a non-temporary object.
> > > > > >
> > > > > > It's ugly, but better than asking users to add #pragmas into their
> > > > > > code.
> > > > > >
> > > > > > PR c++/107532
> > > > > >
> > > > > > gcc/cp/ChangeLog:
> > > > > >
> > > > > > * call.cc (do_warn_dangling_reference): Don't warn when the
> > > > > > member function comes from a reference wrapper class.
> > > > >
> > > > > Let's factor the new code out into e.g. reference_like_class_p
> > > >
> > > > Done. Thanks,
> > > >
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > >
> > > > -- >8 --
> > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > some grief. The code in question uses a reference wrapper with a member
> > > > function returning a reference to a subobject of a non-temporary object:
> > > >
> > > > const Plane & meta = fm.planes().inner();
> > > >
> > > > I've tried a few approaches, e.g., checking that the member function's
> > > > return type is the same as the type of the enclosing class (which is
> > > > the case for member functions returning *this), but that then breaks
> > > > Wdangling-reference4.C with std::optional<std::string>.
> > > >
> > > > Perhaps we want to look at the member function's enclosing class
> > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > member and a constructor taking the same reference type, or is
> > > > std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
> > > > supposing that the member function returns a reference to a
> > > > non-temporary
> > > > object.
> > > >
> > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > >
> > > > PR c++/107532
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > > * call.cc (reference_like_class_p): New.
> > > > (do_warn_dangling_reference): Don't warn when the member
> > > > function comes
> > > > from a reference_like_class_p.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > * g++.dg/warn/Wdangling-reference8.C: New test.
> > > > * g++.dg/warn/Wdangling-reference9.C: New test.
> > > > ---
> > > > gcc/cp/call.cc | 48 ++++++++++++
> > > > .../g++.dg/warn/Wdangling-reference8.C | 77
> > > > +++++++++++++++++++
> > > > .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
> > > > 3 files changed, 146 insertions(+)
> > > > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> > > > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> > > >
> > > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > > index 991730713e6..672722998ee 100644
> > > > --- a/gcc/cp/call.cc
> > > > +++ b/gcc/cp/call.cc
> > > > @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> > > > return true;
> > > > }
> > > > +/* Return true if a class CTYPE is either std::reference_wrapper or
> > > > + std::ref_view, or a reference wrapper class. We consider a class
> > > > + a reference wrapper class if it has a reference member and a
> > > > + constructor taking the same reference type. */
> > > > +
> > > > +static bool
> > > > +reference_like_class_p (tree ctype)
> > > > +{
> > > > + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> > > > + if (decl_in_std_namespace_p (tdecl))
> > > > + {
> > > > + tree name = DECL_NAME (tdecl);
> > > > + return (name
> > > > + && (id_equal (name, "reference_wrapper")
> > > > + || id_equal (name, "ref_view")));
> > > > + }
> > > > + for (tree fields = TYPE_FIELDS (ctype);
> > > > + fields;
> > > > + fields = DECL_CHAIN (fields))
> > > > + {
> > > > + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> > > > + continue;
> > > > + tree type = TREE_TYPE (fields);
> > > > + if (!TYPE_REF_P (type))
> > > > + continue;
> > > > + /* OK, the field is a reference member. Do we have a constructor
> > > > + taking its type? */
> > > > + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> > > > + {
> > > > + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> > > > + if (args
> > > > + && same_type_p (TREE_VALUE (args), type)
> > > > + && TREE_CHAIN (args) == void_list_node)
> > > > + return true;
> > > > + }
> > > > + }
> > > > + return false;
> > > > +}
> > > > +
> > > > /* Helper for maybe_warn_dangling_reference to find a problematic
> > > > CALL_EXPR
> > > > that initializes the LHS (and at least one of its arguments
> > > > represents
> > > > a temporary, as outlined in maybe_warn_dangling_reference), or
> > > > NULL_TREE
> > > > @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
> > > > if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> > > > return NULL_TREE;
> > > > + /* An attempt to reduce the number of -Wdangling-reference
> > > > + false positives concerning reference wrappers (c++/107532).
> > > > + Here we suppose that a member function of such a reference
> > > > + wrapper class returns a reference to a non-temporary object.
> > > > */
> > > > + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > > > + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> > > > + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
> > >
> > > Ah, in this case I was thinking rather than return we would want to look
> > > through to the initializer of the reference wrapper, and warn if that's a
> > > temporary, so we can catch the *2 cases in your tests.
> > >
> > > So, treating ref-like classes as much like references as we can. Some of
> > > your v1 patch ought to be useful in implementing this, but only looking
> > > through one call at a time, not all of them like that patch.
> >
> > Maybe this one, then? I still have to loop through the calls though; EXPR
> > in
> > do_warn_dangling_reference can be e.g.
> >
> > Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes
> > ((const struct FrameMetadata *) fm)>)
> >
> > or
> >
> > Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes
> > (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>)
> >
> > and we want to warn only about the latter, but that means that I need to
> > look into the nested call 'planes' to see if the initializer was a
> > temporary.
>
> Right, but I was thinking we want to recurse like a few lines above, rather
> than loop.
Ah yes, I can do that if I introduce a parameter that tells us
if we're processing an argument or not. I think I'm finally
more or less satisfied with the patch, thanks.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
This patch adjusts do_warn_dangling_reference so that we look through
reference wrapper classes (meaning, has a reference member and a
constructor taking the same reference type, or is std::reference_wrapper
or std::ranges::ref_view) and don't warn for them, supposing that the
member function returns a reference to a non-temporary object.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Add new bool parameter. See through
reference_like_class_p.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
* g++.dg/warn/Wdangling-reference9.C: New test.
---
gcc/cp/call.cc | 97 +++++++++++++++----
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 ++++
3 files changed, 178 insertions(+), 17 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f7c5d9da94b..2a8edc2e7e2 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
return true;
}
+/* Return true if a class CTYPE is either std::reference_wrapper or
+ std::ref_view, or a reference wrapper class. We consider a class
+ a reference wrapper class if it has a reference member and a
+ constructor taking the same reference type. */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+ if (decl_in_std_namespace_p (tdecl))
+ {
+ tree name = DECL_NAME (tdecl);
+ return (name
+ && (id_equal (name, "reference_wrapper")
+ || id_equal (name, "ref_view")));
+ }
+ for (tree fields = TYPE_FIELDS (ctype);
+ fields;
+ fields = DECL_CHAIN (fields))
+ {
+ if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+ continue;
+ tree type = TREE_TYPE (fields);
+ if (!TYPE_REF_P (type))
+ continue;
+ /* OK, the field is a reference member. Do we have a constructor
+ taking its type? */
+ for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
+ {
+ tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+ if (args
+ && same_type_p (TREE_VALUE (args), type)
+ && TREE_CHAIN (args) == void_list_node)
+ return true;
+ }
+ }
+ return false;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
const int& y = (f(1), 42); // NULL_TREE
const int& z = f(f(1)); // f(f(1))
- EXPR is the initializer. */
+ EXPR is the initializer. If ARG_P is true, we're processing an argument
+ to a function; the point is to distinguish between, for example,
+
+ Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
+
+ where we shouldn't warn, and
+
+ Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
+
+ where we should warn (Ref is a reference_like_class_p so we see through
+ it. */
static tree
-do_warn_dangling_reference (tree expr)
+do_warn_dangling_reference (tree expr, bool arg_p)
{
STRIP_NOPS (expr);
+ if (TREE_CODE (expr) == ADDR_EXPR)
+ expr = TREE_OPERAND (expr, 0);
+
+ if (arg_p && expr_represents_temporary_p (expr))
+ {
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Here we suppose that a member function of such a reference
+ wrapper class returns a reference to a non-temporary object. */
+ tree e = expr;
+ while (handled_component_p (e))
+ e = TREE_OPERAND (e, 0);
+ e = TREE_TYPE (e);
+ if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
+ return expr;
+ }
+
switch (TREE_CODE (expr))
{
case CALL_EXPR:
@@ -13829,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
std::pair<const int&, const int&> v = std::minmax(1, 2);
which also creates a dangling reference, because std::minmax
returns std::pair<const T&, const T&>(b, a). */
- if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
+ if (!arg_p
+ && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
return NULL_TREE;
/* Here we're looking to see if any of the arguments is a temporary
@@ -13842,14 +13909,10 @@ do_warn_dangling_reference (tree expr)
if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
&& !TYPE_REF_P (TREE_TYPE (arg)))
continue;
- /* It could also be another call taking a temporary and returning
- it and initializing this reference parameter. */
- if (do_warn_dangling_reference (arg))
- return expr;
- STRIP_NOPS (arg);
- if (TREE_CODE (arg) == ADDR_EXPR)
- arg = TREE_OPERAND (arg, 0);
- if (expr_represents_temporary_p (arg))
+ /* Recurse to see if the argument is a temporary. It could also
+ be another call taking a temporary and returning it and
+ initializing this reference parameter. */
+ if (do_warn_dangling_reference (arg, /*arg_p=*/true))
return expr;
/* Don't warn about member function like:
std::any a(...);
@@ -13866,15 +13929,15 @@ do_warn_dangling_reference (tree expr)
return NULL_TREE;
}
case COMPOUND_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
case COND_EXPR:
- if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
+ if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
return t;
- return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
case PAREN_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
case TARGET_EXPR:
- return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
+ return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
default:
return NULL_TREE;
}
@@ -13917,7 +13980,7 @@ maybe_warn_dangling_reference (const_tree decl, tree
init)
= make_temp_override (global_dc->dc_warn_system_headers,
(!in_system_header_at (DECL_SOURCE_LOCATION (decl))
|| global_dc->dc_warn_system_headers));
- if (tree call = do_warn_dangling_reference (init))
+ if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
{
auto_diagnostic_group d;
if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..330de1fd05d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning
"dangling reference" }
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling
reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling
reference" }
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference"
}
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..9ad83f7365e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+ std::reference_wrapper<const X> wrapit() const { return x; }
+ X x;
+};
+
+void
+g (const S& s)
+{
+ const auto& a1 = s.wrapit().get();
+ (void) a1;
+ const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
+ (void) a2;
+}
base-commit: f661c0bb6371f355966a67b5ce71398e80792948
--
2.39.1
