[PATCH v4] c++: fixes for derived-to-base reference binding [PR107085]

[Marek Polacek <polacek@redhat.com>]

On Thu, Oct 06, 2022 at 06:03:57PM -0400, Jason Merrill wrote:
> On 10/6/22 17:43, Marek Polacek wrote:
> > On Thu, Oct 06, 2022 at 02:00:40PM -0400, Jason Merrill wrote:
> > > On 10/6/22 13:51, Marek Polacek wrote:
> > > > On Thu, Oct 06, 2022 at 10:58:44AM -0400, Jason Merrill wrote:
> > > > > On 10/6/22 10:49, Marek Polacek wrote:
> > > > > > On Wed, Oct 05, 2022 at 08:25:29PM -0400, Jason Merrill wrote:
> > > > > > > On 10/5/22 17:27, Marek Polacek wrote:
> > > > > > --- a/gcc/cp/call.cc
> > > > > > +++ b/gcc/cp/call.cc
> > > > > > @@ -9210,6 +9210,27 @@ conv_binds_ref_to_prvalue (conversion *c)
> > > > > >       return conv_is_prvalue (next_conversion (c));
> > > > > >     }
> > > > > > +/* 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.  */
> > > > > > +
> > > > > > +static bool
> > > > > > +conv_binds_ref_to_temporary (conversion *c)
> > > > > > +{
> > > > > > +  if (conv_binds_ref_to_prvalue (c))
> > > > > > +    return true;
> > > > > > +  if (c->kind != ck_ref_bind)
> > > > > > +    return false;
> > > > > > +  c = next_conversion (c);
> > > > > > +  /* This is the case for
> > > > > > +       struct Base {};
> > > > > > +       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.  */
> > > > > > +  return (c->kind == ck_base && conv_is_prvalue (next_conversion (c)));
> > > > > 
> > > > > I think you also want to check for the case of c->u.expr being a
> > > > > COMPONENT_REF/ARRAY_REF around a TARGET_EXPR, as you mentioned.
> > > > 
> > > > I see.  So this would be achieved using e.g.
> > > > 
> > > > struct B { };
> > > > struct D : B { };
> > > > struct C {
> > > >     D d;
> > > > };
> > > > 
> > > > const B& b = C{}.d;
> > > 
> > > Yes.
> > > 
> > > > Except I'm not sure how to trigger this via the built-in, which takes two types.
> > > > Am I missing something obvious?
> > > 
> > > Indeed, it can't be triggered by the built-in.  But I see
> > > ref_conv_binds_directly is also called from warn_for_range_copy, which ought
> > > to be able to trigger it.
> > 
> > Even there, I think, it's not possible to trigger it with, say (d is an
> > array of D):
> > 
> >       for (const B &b : C{}.d)
> >         // ...
> > 
> > because warn_for_range_copy gets *__for_begin as the EXPR, which means that
> > we won't get to the TARGET_EXPR.  It's like
> > 
> >    auto&& __for_begin = C{}.d;
> >    const B &b = *__for_begin;
> > 
> > and the conversion warn_for_range_copy sees is D -> const B -> const B&, the
> > original .u.expr is *__for_begin.
> 
> Ah, right, and so the C{} temporary gets lifetime-extended and there's no
> problem to warn about.
> 
> > I could add some checking assert to conv_binds_ref_to_temporary to see if
> > we ever encounter a COMPONENT_REF/ARRAY_REF around a TARGET_EXPR...
> 
> Or try to handle it properly even if we can't exercise the code yet.

Okay, done.
 
> I'm thinking again about warning for e.g.
> 
> const B& b = frotz(C{}.d);
> 
> where frotz passes through its reference argument, which becomes a dangling
> reference.  Handling the C{}.d case seems relevant to such a warning.
> 
> I wonder how many false positives there would be for warning about this for
> any function that both takes and returns a reference?

I suppose we'd have to ask if any (or all?) of the parameters is/are
a reference, to handle std::max?

> Do you want to try that, or shall I?

So this is like the first example in https://gcc.gnu.org/PR106393, which
is assigned to...me.  Let me take a stab at the simpler example.  If I
can't make any progress within, say, a week, I'll unassign so as not to
block it.

Meanwhile, here's the updated patch:

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

-- >8 --
This PR reports that

  struct Base {};
  struct Derived : Base {};
  static_assert(__reference_constructs_from_temporary(Base const&, Derived));

doesn't pass, which it should: it's just like

  const Base& b(Derived{});

where we bind 'b' to the Base subobject of a temporary object of type
Derived.  The ck_base conversion didn't have ->need_temporary_p set because
we didn't need to create a temporary object just for the base, but the whole
object is a temporary so we're still binding to a temporary.  Since the
Base subobject is an xvalue, a new function is introduced.

	PR c++/107085

gcc/cp/ChangeLog:

	* call.cc (conv_binds_ref_to_temporary): New.
	(ref_conv_binds_directly): Rename to...
	(ref_conv_binds_to_temporary): ...this.  Use
	conv_binds_ref_to_temporary.
	* cp-tree.h (ref_conv_binds_directly): Rename to...
	(ref_conv_binds_to_temporary): ...this.
	* method.cc (ref_xes_from_temporary): Use ref_conv_binds_to_temporary.
	* parser.cc (warn_for_range_copy): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/reference_constructs_from_temporary1.C: Adjust expected
	result.
	* g++.dg/ext/reference_converts_from_temporary1.C: Likewise.
	* g++.dg/cpp0x/elision4.C: New test.
---
 gcc/cp/call.cc                                | 45 ++++++++++++++++---
 gcc/cp/cp-tree.h                              |  2 +-
 gcc/cp/method.cc                              |  2 +-
 gcc/cp/parser.cc                              |  5 ++-
 gcc/testsuite/g++.dg/cpp0x/elision4.C         | 15 +++++++
 .../reference_constructs_from_temporary1.C    |  2 +-
 .../ext/reference_converts_from_temporary1.C  |  2 +-
 7 files changed, 61 insertions(+), 12 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/elision4.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index bd04a1d309a..833cd014b7d 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -9210,15 +9210,48 @@ conv_binds_ref_to_prvalue (conversion *c)
   return conv_is_prvalue (next_conversion (c));
 }
 
-/* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE does
-   not involve creating a temporary.  Return tristate::TS_FALSE if converting
-   EXPR to a reference type TYPE binds the reference to a temporary.  If the
-   conversion is invalid or bad, return tristate::TS_UNKNOWN.  DIRECT_INIT_P
+/* 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.  */
+
+static bool
+conv_binds_ref_to_temporary (conversion *c)
+{
+  if (conv_binds_ref_to_prvalue (c))
+    return true;
+  if (c->kind != ck_ref_bind)
+    return false;
+  c = next_conversion (c);
+  /* This is the case for
+       struct Base {};
+       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)
+    {
+      if (TREE_CODE (c->u.expr) == TARGET_EXPR)
+	return true;
+      if ((TREE_CODE (c->u.expr) == COMPONENT_REF
+	   || TREE_CODE (c->u.expr) == ARRAY_REF)
+	  && TREE_CODE (TREE_OPERAND (c->u.expr, 0)) == TARGET_EXPR)
+	return true;
+    }
+  return false;
+}
+
+/* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE binds
+   the reference to a temporary.  Return tristate::TS_FALSE if converting
+   EXPR to a reference type TYPE doesn't bind the reference to a temporary.  If
+   the conversion is invalid or bad, return tristate::TS_UNKNOWN.  DIRECT_INIT_P
    says whether the conversion should be done in direct- or copy-initialization
    context.  */
 
 tristate
-ref_conv_binds_directly (tree type, tree expr, bool direct_init_p /*= false*/)
+ref_conv_binds_to_temporary (tree type, tree expr, bool direct_init_p/*=false*/)
 {
   gcc_assert (TYPE_REF_P (type));
 
@@ -9230,7 +9263,7 @@ ref_conv_binds_directly (tree type, tree expr, bool direct_init_p /*= false*/)
 					  /*c_cast_p=*/false, flags, tf_none);
   tristate ret (tristate::TS_UNKNOWN);
   if (conv && !conv->bad_p)
-    ret = tristate (!conv_binds_ref_to_prvalue (conv));
+    ret = tristate (conv_binds_ref_to_temporary (conv));
 
   /* Free all the conversions we allocated.  */
   obstack_free (&conversion_obstack, p);
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 8bc1c2dc7fd..469eb2fdb25 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6534,7 +6534,7 @@ extern bool sufficient_parms_p			(const_tree);
 extern tree type_decays_to			(tree);
 extern tree extract_call_expr			(tree);
 extern tree build_trivial_dtor_call		(tree, bool = false);
-extern tristate ref_conv_binds_directly		(tree, tree, bool = false);
+extern tristate ref_conv_binds_to_temporary	(tree, tree, bool = false);
 extern tree build_user_type_conversion		(tree, tree, int,
 						 tsubst_flags_t);
 extern tree build_new_function_call		(tree, vec<tree, va_gc> **,
diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 55af5c43c18..622e1b9802e 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2233,7 +2233,7 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
   tree val = build_stub_object (from);
   if (!TYPE_REF_P (from) && TREE_CODE (from) != FUNCTION_TYPE)
     val = CLASS_TYPE_P (from) ? force_rvalue (val, tf_none) : rvalue (val);
-  return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
+  return ref_conv_binds_to_temporary (to, val, direct_init_p).is_true ();
 }
 
 /* Worker for is_{,nothrow_}convertible.  Attempt to perform an implicit
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 555476e42e7..dc3d17c416c 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -13731,7 +13731,8 @@ warn_for_range_copy (tree decl, tree expr)
 
   if (TYPE_REF_P (type))
     {
-      if (glvalue_p (expr) && ref_conv_binds_directly (type, expr).is_false ())
+      if (glvalue_p (expr)
+	  && ref_conv_binds_to_temporary (type, expr).is_true ())
 	{
 	  auto_diagnostic_group d;
 	  if (warning_at (loc, OPT_Wrange_loop_construct,
@@ -13762,7 +13763,7 @@ warn_for_range_copy (tree decl, tree expr)
   /* If we can initialize a reference directly, suggest that to avoid the
      copy.  */
   tree rtype = cp_build_reference_type (type, /*rval*/false);
-  if (ref_conv_binds_directly (rtype, expr).is_true ())
+  if (ref_conv_binds_to_temporary (rtype, expr).is_false ())
     {
       auto_diagnostic_group d;
       if (warning_at (loc, OPT_Wrange_loop_construct,
diff --git a/gcc/testsuite/g++.dg/cpp0x/elision4.C b/gcc/testsuite/g++.dg/cpp0x/elision4.C
new file mode 100644
index 00000000000..3cc2e3afa5d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/elision4.C
@@ -0,0 +1,15 @@
+// PR c++/107085
+// { dg-do compile { target c++11 } }
+
+struct X {
+  X();
+  X(X&&);
+};
+struct Z : X {};
+X x1 = Z();
+X x2 = X(Z());
+
+struct B { };
+struct D : B { };
+B b1 = D();
+B b2 = B(D());
diff --git a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
index 76de905a35d..5354b1dc4e6 100644
--- a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
+++ b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
@@ -201,7 +201,7 @@ SA(!__reference_constructs_from_temporary(const int&, H));
 SA(!__reference_constructs_from_temporary(int&&, G2));
 SA(!__reference_constructs_from_temporary(const int&, H2));
 
-SA(!__reference_constructs_from_temporary(const Base&, Der));
+SA(__reference_constructs_from_temporary(const Base&, Der));
 
 // This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
 SA(!__reference_constructs_from_temporary(int&&, id<int[3]>));
diff --git a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
index 90196c38742..e6c159e9b00 100644
--- a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
+++ b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
@@ -201,7 +201,7 @@ SA( __reference_converts_from_temporary(const int&, H));
 SA(!__reference_converts_from_temporary(int&&, G2));
 SA(!__reference_converts_from_temporary(const int&, H2));
 
-SA(!__reference_converts_from_temporary(const Base&, Der));
+SA(__reference_converts_from_temporary(const Base&, Der));
 
 // This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
 SA(!__reference_converts_from_temporary(int&&, id<int[3]>));

base-commit: f8ba88b6a811ca9bb4b8411d3f65c329fb480ee1
-- 
2.37.3