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 the conv_binds_ref_to_prvalue function. The main function here is
ref_xes_from_temporary.
I've changed the return type of ref_conv_binds_directly to tristate, because
previously the function didn't distinguish between an invalid conversion and
one that binds to a prvalue. Since it no longer returns a bool, I removed
the _p suffix.
The patch also adds the relevant class and variable templates to <type_traits>.
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_directly): ... this. Add a new bool parameter. Change
the return type to tristate.
* constraint.cc (diagnose_trait_expr): Handle
CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and CPTK_REF_CONVERTS_FROM_TEMPORARY.
* cp-tree.h: Include "tristate.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_directly): ... this.
(ref_xes_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 (ref_xes_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_directly.
* 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.
Marek Polacek
9a15d3beac