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:
> > > > > > 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. Fixed
> > > > > > by
> > > > > > the conv_is_prvalue hunk.
> > > > > >
> > > > > > That broke a bunch of tests. I've distilled the issue into a
> > > > > > simple test
> > > > > > in elision4.C. Essentially, we have
> > > > > >
> > > > > > struct B { /* ... */ };
> > > > > > struct D : B { };
> > > > > > B b = D();
> > > > > >
> > > > > > and we set force_elide in build_over_call, but we're unable to
> > > > > > actually
> > > > > > elide the B::B(B&&) call, and crash on gcc_assert (!force_elide);.
> > > > > >
> > > > > > <https://en.cppreference.com/w/cpp/language/copy_elision> says that
> > > > > > copy
> > > > > > elision "can only apply when the object being initialized is known
> > > > > > not to be
> > > > > > a potentially-overlapping subobject". So I suppose we shouldn't
> > > > > > force_elide
> > > > > > the B::B(B&&) call. I don't belive the CWG 2327 code was added to
> > > > > > handle
> > > > > > derived-to-base conversions, at that time conv_binds_ref_to_prvalue
> > > > > > wasn't
> > > > > > checking ck_base at all.
> > > > > >
> > > > > > Does that make sense? If so...
> > > > > >
> > > > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > > > >
> > > > > > PR c++/107085
> > > > > >
> > > > > > gcc/cp/ChangeLog:
> > > > > >
> > > > > > * call.cc (conv_is_prvalue): Return true if the base subobject
> > > > > > is part
> > > > > > of a temporary object.
> > > > >
> > > > > No, the base subobject of a prvalue is an xvalue.
> > > >
> > > > Ah, so this is just like T().m where T() is a prvalue but the whole
> > > > thing
> > > > is an xvalue. Duly noted.
> > >
> > > Exactly.
> > >
> > > > > I think the problem is that an expression being a prvalue is a subset
> > > > > of
> > > > > binding a reference to a temporary, and we shouldn't try to express
> > > > > both of
> > > > > those using the same function: you need a separate
> > > > > conv_binds_ref_to_temporary.
> > > >
> > > > Ack, so how about this? Thanks,
> > > >
> > > > 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): Use it.
> > > >
> > > > 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 | 23
> > > > ++++++++++++++++++-
> > > > gcc/testsuite/g++.dg/cpp0x/elision4.C | 15 ++++++++++++
> > > > .../reference_constructs_from_temporary1.C | 2 +-
> > > > .../ext/reference_converts_from_temporary1.C | 2 +-
> > > > 4 files changed, 39 insertions(+), 3 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..715a83f5a69 100644
> > > > --- 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.
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...
> Incidentally, ref_conv_binds_directly should also probably be reversed to
> ref_conv_binds_to_temporary since you can "bind directly" to an xvalue that
> refers to a temporary.
Good point, I at least did that.
-- >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 | 33 +++++++++++++++----
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, 49 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..5a3d509f6c8 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -9210,15 +9210,36 @@ 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. */
+ return (c->kind == ck_base && conv_is_prvalue (next_conversion (c)));
+}
+
+/* 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 +9251,7 @@ 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 90e06f914fb..bceedab5cd6 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: 50c35c691517291dbb77b1661761bc59950ba101
--
2.37.3
