On Tue, Jul 12, 2022 at 04:15:00PM -0400, Jason Merrill wrote:
> On 7/12/22 16:10, Jason Merrill wrote:
> > On 7/8/22 13:41, Marek Polacek wrote:
> > > This patch implements C++23 P2255R2, which adds two new type traits to
> > > detect reference binding to a temporary. They can be used to detect code
> > > like
> > >
> > > std::tuple<const std::string&> t("meow");
> > >
> > > which is incorrect because it always creates a dangling reference,
> > > because
> > > the std::string temporary is created inside the selected constructor of
> > > std::tuple, and not outside it.
> > >
> > > There are two new compiler builtins,
> > > __reference_constructs_from_temporary
> > > and __reference_converts_from_temporary. The former is used to simulate
> > > direct- and the latter copy-initialization context. But I had a
> > > hard time
> > > finding a test where there's actually a difference. Under DR 2267, both
> > > of these are invalid:
> > >
> > > struct A { } a;
> > > struct B { explicit B(const A&); };
> > > const B &b1{a};
> > > const B &b2(a);
> > >
> > > so I had to peruse [over.match.ref], and eventually realized that the
> > > difference can be seen here:
> > >
> > > struct G {
> > > operator int(); // #1
> > > explicit operator int&&(); // #2
> > > };
> > >
> > > int&& r1(G{}); // use #2 (no temporary)
> > > int&& r2 = G{}; // use #1 (a temporary is created to be bound to int&&)
> > >
> > > The implementation itself was rather straightforward because we already
> > > have conv_binds_ref_to_prvalue. The main function here is
> > > reference_from_temporary. The renaming to ref_conv_binds_to_temporary_p
> > > is because previously the function didn't distinguish between an invalid
> > > conversion and one that binds to a prvalue.
> > >
> > > The patch also adds the relevant class and variable templates to
> > > <type_traits>.
> > >
> > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > >
> > > PR c++/104477
> > >
> > > gcc/c-family/ChangeLog:
> > >
> > > * c-common.cc (c_common_reswords): Add
> > > __reference_constructs_from_temporary and
> > > __reference_converts_from_temporary.
> > > * c-common.h (enum rid): Add RID_REF_CONSTRUCTS_FROM_TEMPORARY and
> > > RID_REF_CONVERTS_FROM_TEMPORARY.
> > >
> > > gcc/cp/ChangeLog:
> > >
> > > * call.cc (ref_conv_binds_directly_p): Rename to ...
> > > (ref_conv_binds_to_temporary_p): ... this. Add a new bool
> > > parameter. Return true only if the conversion is valid and
> > > conv_binds_ref_to_prvalue returns true.
> > > * constraint.cc (diagnose_trait_expr): Handle
> > > CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and
> > > CPTK_REF_CONVERTS_FROM_TEMPORARY.
> > > * cp-tree.h (enum cp_trait_kind): Add
> > > CPTK_REF_CONSTRUCTS_FROM_TEMPORARY
> > > and CPTK_REF_CONVERTS_FROM_TEMPORARY.
> > > (ref_conv_binds_directly_p): Rename to ...
> > > (ref_conv_binds_to_temporary_p): ... this.
> > > (reference_from_temporary): Declare.
> > > * cxx-pretty-print.cc (pp_cxx_trait_expression): Handle
> > > CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and
> > > CPTK_REF_CONVERTS_FROM_TEMPORARY.
> > > * method.cc (reference_from_temporary): New.
> > > * parser.cc (cp_parser_primary_expression): Handle
> > > RID_REF_CONSTRUCTS_FROM_TEMPORARY and
> > > RID_REF_CONVERTS_FROM_TEMPORARY.
> > > (cp_parser_trait_expr): Likewise.
> > > (warn_for_range_copy): Adjust to call ref_conv_binds_to_temporary_p.
> > > * semantics.cc (trait_expr_value): Handle
> > > CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and
> > > CPTK_REF_CONVERTS_FROM_TEMPORARY.
> > > (finish_trait_expr): Likewise.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/std/type_traits (reference_constructs_from_temporary,
> > > reference_converts_from_temporary): New class templates.
> > > (reference_constructs_from_temporary_v,
> > > reference_converts_from_temporary_v): New variable templates.
> > > (__cpp_lib_reference_from_temporary): Define for C++23.
> > > * include/std/version (__cpp_lib_reference_from_temporary):
> > > Define for
> > > C++23.
> > > * testsuite/20_util/variable_templates_for_traits.cc: Test
> > > reference_constructs_from_temporary_v and
> > > reference_converts_from_temporary_v.
> > > * testsuite/20_util/reference_from_temporary/value.cc: New test.
> > > * testsuite/20_util/reference_from_temporary/value2.cc: New test.
> > > * testsuite/20_util/reference_from_temporary/version.cc: New test.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > > * g++.dg/ext/reference_constructs_from_temporary1.C: New test.
> > > * g++.dg/ext/reference_converts_from_temporary1.C: New test.
> > > ---
> > > gcc/c-family/c-common.cc | 4 +
> > > gcc/c-family/c-common.h | 2 +
> > > gcc/cp/call.cc | 14 +-
> > > gcc/cp/constraint.cc | 8 +
> > > gcc/cp/cp-tree.h | 7 +-
> > > gcc/cp/cxx-pretty-print.cc | 6 +
> > > gcc/cp/method.cc | 28 +++
> > > gcc/cp/parser.cc | 14 +-
> > > gcc/cp/semantics.cc | 8 +
> > > .../reference_constructs_from_temporary1.C | 214 ++++++++++++++++++
> > > .../ext/reference_converts_from_temporary1.C | 214 ++++++++++++++++++
> > > libstdc++-v3/include/std/type_traits | 39 ++++
> > > libstdc++-v3/include/std/version | 5 +-
> > > .../20_util/reference_from_temporary/value.cc | 110 +++++++++
> > > .../reference_from_temporary/value2.cc | 28 +++
> > > .../reference_from_temporary/version.cc | 27 +++
> > > .../20_util/variable_templates_for_traits.cc | 14 ++
> > > 17 files changed, 730 insertions(+), 12 deletions(-)
> > > create mode 100644
> > > gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
> > > create mode 100644
> > > gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
> > > create mode 100644
> > > libstdc++-v3/testsuite/20_util/reference_from_temporary/value.cc
> > > create mode 100644
> > > libstdc++-v3/testsuite/20_util/reference_from_temporary/value2.cc
> > > create mode 100644
> > > libstdc++-v3/testsuite/20_util/reference_from_temporary/version.cc
> > >
> > > diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
> > > index 1b8e73f7bc5..655c3aefee6 100644
> > > --- a/gcc/c-family/c-common.cc
> > > +++ b/gcc/c-family/c-common.cc
> > > @@ -537,6 +537,10 @@ const struct c_common_resword c_common_reswords[] =
> > > { "__is_constructible", RID_IS_CONSTRUCTIBLE, D_CXXONLY },
> > > { "__is_nothrow_assignable", RID_IS_NOTHROW_ASSIGNABLE, D_CXXONLY },
> > > { "__is_nothrow_constructible", RID_IS_NOTHROW_CONSTRUCTIBLE,
> > > D_CXXONLY },
> > > + { "__reference_constructs_from_temporary",
> > > RID_REF_CONSTRUCTS_FROM_TEMPORARY,
> > > + D_CXXONLY },
> > > + { "__reference_converts_from_temporary",
> > > RID_REF_CONVERTS_FROM_TEMPORARY,
> > > + D_CXXONLY },
> > > /* C++ transactional memory. */
> > > { "synchronized", RID_SYNCHRONIZED, D_CXX_OBJC | D_TRANSMEM },
> > > diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> > > index c0900848965..f9064393b4e 100644
> > > --- a/gcc/c-family/c-common.h
> > > +++ b/gcc/c-family/c-common.h
> > > @@ -184,6 +184,8 @@ enum rid
> > > RID_IS_UNION, RID_UNDERLYING_TYPE,
> > > RID_IS_ASSIGNABLE, RID_IS_CONSTRUCTIBLE,
> > > RID_IS_NOTHROW_ASSIGNABLE, RID_IS_NOTHROW_CONSTRUCTIBLE,
> > > + RID_REF_CONSTRUCTS_FROM_TEMPORARY,
> > > + RID_REF_CONVERTS_FROM_TEMPORARY,
> > > /* C++11 */
> > > RID_CONSTEXPR, RID_DECLTYPE, RID_NOEXCEPT, RID_NULLPTR,
> > > RID_STATIC_ASSERT,
> > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > index fc98552fda2..1ba209f61f1 100644
> > > --- a/gcc/cp/call.cc
> > > +++ b/gcc/cp/call.cc
> > > @@ -9109,21 +9109,23 @@ conv_binds_ref_to_prvalue (conversion *c)
> > > return conv_is_prvalue (next_conversion (c));
> > > }
> > > -/* True iff converting EXPR to a reference type TYPE does not involve
> > > - creating a temporary. */
> > > +/* True iff converting EXPR to a reference type TYPE binds the
> > > reference to
> > > + a temporary. DIRECT_INIT_P says whether the conversion should
> > > be done
> > > + in direct- or copy-initialization context. */
> > > bool
> > > -ref_conv_binds_directly_p (tree type, tree expr)
> > > +ref_conv_binds_to_temporary_p (tree type, tree expr,
> > > + bool direct_init_p /*= false*/)
> > > {
> > > gcc_assert (TYPE_REF_P (type));
> > > /* Get the high-water mark for the CONVERSION_OBSTACK. */
> > > void *p = conversion_obstack_alloc (0);
> > > + const int flags = direct_init_p ? LOOKUP_NORMAL : LOOKUP_IMPLICIT;
> > > conversion *conv = implicit_conversion (type, TREE_TYPE (expr), expr,
> > > - /*c_cast_p=*/false,
> > > - LOOKUP_IMPLICIT, tf_none);
> > > - bool ret = conv && !conv->bad_p && !conv_binds_ref_to_prvalue (conv);
> > > + /*c_cast_p=*/false, flags, tf_none);
> > > + bool ret = conv && !conv->bad_p && conv_binds_ref_to_prvalue (conv);
> > > /* Free all the conversions we allocated. */
> > > obstack_free (&conversion_obstack, p);
> > > diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> > > index 591155cee22..648cc9d176d 100644
> > > --- a/gcc/cp/constraint.cc
> > > +++ b/gcc/cp/constraint.cc
> > > @@ -3687,6 +3687,14 @@ diagnose_trait_expr (tree expr, tree args)
> > > case CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS:
> > > inform (loc, " %qT does not have unique object
> > > representations", t1);
> > > break;
> > > + case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
> > > + inform (loc, " %qT is not a reference that binds to a temporary "
> > > + "object of type %qT (direct-initialization)", t1, t2);
> > > + break;
> > > + case CPTK_REF_CONVERTS_FROM_TEMPORARY:
> > > + inform (loc, " %qT is not a reference that binds to a temporary "
> > > + "object of type %qT (copy-initialization)", t1, t2);
> > > + break;
> > > case CPTK_BASES:
> > > case CPTK_DIRECT_BASES:
> > > case CPTK_UNDERLYING_TYPE:
> > > diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> > > index 2fde4f83b41..c3bed31e455 100644
> > > --- a/gcc/cp/cp-tree.h
> > > +++ b/gcc/cp/cp-tree.h
> > > @@ -1397,7 +1397,9 @@ enum cp_trait_kind
> > > CPTK_IS_ASSIGNABLE,
> > > CPTK_IS_CONSTRUCTIBLE,
> > > CPTK_IS_NOTHROW_ASSIGNABLE,
> > > - CPTK_IS_NOTHROW_CONSTRUCTIBLE
> > > + CPTK_IS_NOTHROW_CONSTRUCTIBLE,
> > > + CPTK_REF_CONSTRUCTS_FROM_TEMPORARY,
> > > + CPTK_REF_CONVERTS_FROM_TEMPORARY
> > > };
> > > /* The types that we are processing. */
> > > @@ -6520,7 +6522,7 @@ 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 bool ref_conv_binds_directly_p (tree, tree);
> > > +extern bool ref_conv_binds_to_temporary_p (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> **,
> > > @@ -7105,6 +7107,7 @@ extern tree forward_parm (tree);
> > > extern bool is_trivially_xible (enum tree_code, tree, tree);
> > > extern bool is_nothrow_xible (enum tree_code, tree, tree);
> > > extern bool is_xible (enum tree_code, tree, tree);
> > > +extern bool reference_from_temporary (tree, tree, bool);
> > > extern tree get_defaulted_eh_spec (tree, tsubst_flags_t =
> > > tf_warning_or_error);
> > > extern bool maybe_explain_implicit_delete (tree);
> > > extern void explain_implicit_non_constexpr (tree);
> > > diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
> > > index 7e4db2e413b..44590830a61 100644
> > > --- a/gcc/cp/cxx-pretty-print.cc
> > > +++ b/gcc/cp/cxx-pretty-print.cc
> > > @@ -2696,6 +2696,12 @@ pp_cxx_trait_expression (cxx_pretty_printer
> > > *pp, tree t)
> > > case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
> > > pp_cxx_ws_string (pp, "__is_nothrow_constructible");
> > > break;
> > > + case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
> > > + pp_cxx_ws_string (pp, "__reference_constructs_from_temporary");
> > > + break;
> > > + case CPTK_REF_CONVERTS_FROM_TEMPORARY:
> > > + pp_cxx_ws_string (pp, "__reference_converts_from_temporary");
> > > + break;
> > > default:
> > > gcc_unreachable ();
> > > diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
> > > index 0dffd648b0b..dd9715b6725 100644
> > > --- a/gcc/cp/method.cc
> > > +++ b/gcc/cp/method.cc
> > > @@ -2211,6 +2211,34 @@ is_xible (enum tree_code code, tree to, tree from)
> > > return !!expr;
> > > }
> > > +/* Return true iff conjunction_v<is_reference<T>,
> > > is_constructible<T, U>> is
> > > + true, and the initialization
> > > + T t(VAL<U>); // DIRECT_INIT_P
> > > + or
> > > + T t = VAL<U>; // !DIRECT_INIT_P
> > > + binds t to a temporary object whose lifetime is extended.
> > > + VAL<T> is defined in [meta.unary.prop]:
> > > + -- If T is a reference or function type, VAL<T> is an expression
> > > with the
> > > + same type and value category as declval<T>().
> > > + -- Otherwise, VAL<T> is a prvalue that initially has type T. */
> > > +
> > > +bool
> > > +reference_from_temporary (tree to, tree from, bool direct_init_p)
> > > +{
> > > + /* Check is_reference<T>. */
> > > + if (!TYPE_REF_P (to))
> > > + return false;
> > > + /* Check is_constructible<T, U>.
> > > + ??? This check doesn't seem to be necessary; if T isn't
> > > constructible
> > > + from U, we won't be able to create a conversion. */
> > > + if (!is_xible (INIT_EXPR, to, build_tree_list (NULL_TREE, from)))
> > > + return false;
> >
> > I agree with the comment, did you try leaving this out? If it stays I'd
> > think it needs to consider 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_to_temporary_p (to, val, direct_init_p);
> > > +}
> > > +
> > > /* Categorize various special_function_kinds. */
> > > #define SFK_CTOR_P(sfk) \
> > > ((sfk) >= sfk_constructor && (sfk) <= sfk_move_constructor)
> > > diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> > > index bf9ea3685f8..edee94bda13 100644
> > > --- a/gcc/cp/parser.cc
> > > +++ b/gcc/cp/parser.cc
> > > @@ -5917,6 +5917,8 @@ cp_parser_primary_expression (cp_parser *parser,
> > > case RID_IS_CONSTRUCTIBLE:
> > > case RID_IS_NOTHROW_ASSIGNABLE:
> > > case RID_IS_NOTHROW_CONSTRUCTIBLE:
> > > + case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
> > > + case RID_REF_CONVERTS_FROM_TEMPORARY:
> > > return cp_parser_trait_expr (parser, token->keyword);
> > > // C++ concepts
> > > @@ -10988,6 +10990,14 @@ cp_parser_trait_expr (cp_parser* parser,
> > > enum rid keyword)
> > > kind = CPTK_IS_NOTHROW_CONSTRUCTIBLE;
> > > variadic = true;
> > > break;
> > > + case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
> > > + kind = CPTK_REF_CONSTRUCTS_FROM_TEMPORARY;
> > > + binary = true;
> > > + break;
> > > + case RID_REF_CONVERTS_FROM_TEMPORARY:
> > > + kind = CPTK_REF_CONVERTS_FROM_TEMPORARY;
> > > + binary = true;
> > > + break;
> > > default:
> > > gcc_unreachable ();
> > > }
> > > @@ -13811,7 +13821,7 @@ warn_for_range_copy (tree decl, tree expr)
> > > if (TYPE_REF_P (type))
> > > {
> > > - if (glvalue_p (expr) && !ref_conv_binds_directly_p (type, expr))
> > > + if (glvalue_p (expr) && ref_conv_binds_to_temporary_p (type,
> > > expr))
> > > {
> > > auto_diagnostic_group d;
> > > if (warning_at (loc, OPT_Wrange_loop_construct,
> > > @@ -13842,7 +13852,7 @@ warn_for_range_copy (tree decl, tree expr)
> > > tree rtype = cp_build_reference_type (type, /*rval*/false);
> > > /* If we could initialize the reference directly, it wouldn't
> > > involve any
> > > copies. */
> > > - if (!ref_conv_binds_directly_p (rtype, expr))
> > > + if (ref_conv_binds_to_temporary_p (rtype, expr))
> > > return;
> >
> > I think this case wants the old handling of invalid conversions you
> > mentioned in your intro; we don't want to suggest changing to a
> > reference if that's ill-formed.
> >
> > In passing we might change the comment to "If we can initialize a
> > reference directly, suggest that to avoid the copy." and move it above
> > the rtype declaration.
>
> Hmm, and I suspect we get false positives when expr is a prvalue, so
> initializing a non-reference variable of the same cv-unqualified type
> involves no extra copy?
I couldn't provoke a false positive here. Note that
ref_conv_binds_to_temporary_p
always gets a reference so copy elision isn't applicable here.
Marek
