c++: Implement C++26 P0609R3 - Attributes for Structured Bindings [PR114456]
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Commit Message
Hi!
The following patch implements the P0609R3 paper; we build the
VAR_DECLs for the structured binding identifiers early, so all we need
IMHO is just to parse the attributed identifier list and pass the attributes
to the VAR_DECL creation.
The paper mentions maybe_unused and gnu::nonstring attributes as examples
where they can be useful. Not sure about either of them.
For maybe_unused, the thing is that both GCC and clang already don't
diagnose maybe unused for the structured binding identifiers, because it
would be a false positive too often; and there is no easy way to find out
if a structured binding has been written with the P0609R3 paper in mind or
not (maybe we could turn it on if in the structured binding is any
attribute, even if just [[]] and record that as a flag on the whole
underlying decl, so that we'd diagnose
auto [a, b, c[[]]] = d;
// use a, c but not b
but not
auto [e, f, g] = d;
// use a, c but not b
). For gnu::nonstring, the issue is that we currently don't allow the
attribute on references to char * or references to char[], just on
char */char[]. I've filed a PR for that.
The first testcase in the patch tests it on [[]] and [[maybe_unused]],
just whether it is parsed properly, second on gnu::deprecated, which
works. Haven't used deprecated attribute because the paper said that
attribute is for further investigation.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2024-04-29 Jakub Jelinek <jakub@redhat.com>
PR c++/114456
gcc/c-family/
* c-cppbuiltin.cc (c_cpp_builtins): Predefine
__cpp_structured_bindings for C++26 to 202403L rather than
201606L.
gcc/cp/
* parser.cc (cp_parser_decomposition_declaration): Implement C++26
P0609R3 - Attributes for Structured Bindings. Parse attributed
identifier lists for structured binding declarations, pass the
attributes to start_decl.
gcc/testsuite/
* g++.dg/cpp26/decomp1.C: New test.
* g++.dg/cpp26/decomp2.C: New test.
* g++.dg/cpp26/feat-cxx26.C (__cpp_structured_bindings): Expect
202403 rather than 201606.
Jakub
Comments
On 4/29/24 00:11, Jakub Jelinek wrote:
> Hi!
>
> The following patch implements the P0609R3 paper; we build the
> VAR_DECLs for the structured binding identifiers early, so all we need
> IMHO is just to parse the attributed identifier list and pass the attributes
> to the VAR_DECL creation.
>
> The paper mentions maybe_unused and gnu::nonstring attributes as examples
> where they can be useful. Not sure about either of them.
> For maybe_unused, the thing is that both GCC and clang already don't
> diagnose maybe unused for the structured binding identifiers, because it
> would be a false positive too often; and there is no easy way to find out
> if a structured binding has been written with the P0609R3 paper in mind or
> not (maybe we could turn it on if in the structured binding is any
> attribute, even if just [[]] and record that as a flag on the whole
> underlying decl, so that we'd diagnose
> auto [a, b, c[[]]] = d;
> // use a, c but not b
> but not
> auto [e, f, g] = d;
> // use a, c but not b
> ). For gnu::nonstring, the issue is that we currently don't allow the
> attribute on references to char * or references to char[], just on
> char */char[]. I've filed a PR for that.
>
> The first testcase in the patch tests it on [[]] and [[maybe_unused]],
> just whether it is parsed properly, second on gnu::deprecated, which
> works. Haven't used deprecated attribute because the paper said that
> attribute is for further investigation.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
> 2024-04-29 Jakub Jelinek <jakub@redhat.com>
>
> PR c++/114456
> gcc/c-family/
> * c-cppbuiltin.cc (c_cpp_builtins): Predefine
> __cpp_structured_bindings for C++26 to 202403L rather than
> 201606L.
> gcc/cp/
> * parser.cc (cp_parser_decomposition_declaration): Implement C++26
> P0609R3 - Attributes for Structured Bindings. Parse attributed
> identifier lists for structured binding declarations, pass the
> attributes to start_decl.
> gcc/testsuite/
> * g++.dg/cpp26/decomp1.C: New test.
> * g++.dg/cpp26/decomp2.C: New test.
> * g++.dg/cpp26/feat-cxx26.C (__cpp_structured_bindings): Expect
> 202403 rather than 201606.
>
> --- gcc/cp/parser.cc.jj 2024-04-26 11:42:24.653016208 +0200
> +++ gcc/cp/parser.cc 2024-04-26 13:59:17.791482874 +0200
> @@ -16075,13 +16075,37 @@ cp_parser_decomposition_declaration (cp_
>
> /* Parse the identifier-list. */
> auto_vec<cp_expr, 10> v;
> + bool attr_diagnosed = false;
> + int first_attr = -1;
> + unsigned int cnt = 0;
> if (!cp_lexer_next_token_is (parser->lexer, CPP_CLOSE_SQUARE))
> while (true)
> {
> cp_expr e = cp_parser_identifier (parser);
> if (e.get_value () == error_mark_node)
> break;
> + tree attr = NULL_TREE;
> + if (cp_next_tokens_can_be_std_attribute_p (parser))
> + {
> + if (cxx_dialect >= cxx17 && cxx_dialect < cxx26 && !attr_diagnosed)
> + {
> + pedwarn (cp_lexer_peek_token (parser->lexer)->location,
> + OPT_Wc__26_extensions,
> + "structured bindings with attributed identifiers "
> + "only available with %<-std=c++2c%> or "
> + "%<-std=gnu++2c%>");
> + attr_diagnosed = true;
> + }
> + attr = cp_parser_std_attribute_spec_seq (parser);
> + if (attr == error_mark_node)
> + attr = NULL_TREE;
> + if (attr && first_attr == -1)
> + first_attr = v.length ();
> + }
> v.safe_push (e);
> + ++cnt;
> + if (first_attr != -1)
> + v.safe_push (attr);
> if (!cp_lexer_next_token_is (parser->lexer, CPP_COMMA))
> break;
> cp_lexer_consume_token (parser->lexer);
> @@ -16139,8 +16163,11 @@ cp_parser_decomposition_declaration (cp_
> declarator->id_loc = e.get_location ();
> }
> tree elt_pushed_scope;
> + tree attr = NULL_TREE;
> + if (first_attr != -1 && i >= (unsigned) first_attr)
> + attr = v[++i].get_value ();
> tree decl2 = start_decl (declarator, &decl_specs, SD_DECOMPOSITION,
> - NULL_TREE, NULL_TREE, &elt_pushed_scope);
> + NULL_TREE, attr, &elt_pushed_scope);
> if (decl2 == error_mark_node)
> decl = error_mark_node;
> else if (decl != error_mark_node && DECL_CHAIN (decl2) != prev)
> @@ -16183,7 +16210,7 @@ cp_parser_decomposition_declaration (cp_
>
> if (decl != error_mark_node)
> {
> - cp_decomp decomp = { prev, v.length () };
> + cp_decomp decomp = { prev, cnt };
> cp_finish_decl (decl, initializer, non_constant_p, NULL_TREE,
> (is_direct_init ? LOOKUP_NORMAL : LOOKUP_IMPLICIT),
> &decomp);
> @@ -16193,7 +16220,7 @@ cp_parser_decomposition_declaration (cp_
> else if (decl != error_mark_node)
> {
> *maybe_range_for_decl = prev;
> - cp_decomp decomp = { prev, v.length () };
> + cp_decomp decomp = { prev, cnt };
> /* Ensure DECL_VALUE_EXPR is created for all the decls but
> the underlying DECL. */
> cp_finish_decomp (decl, &decomp);
> --- gcc/c-family/c-cppbuiltin.cc.jj 2024-04-26 11:42:24.621016651 +0200
> +++ gcc/c-family/c-cppbuiltin.cc 2024-04-26 13:42:50.559184222 +0200
> @@ -1044,7 +1044,8 @@ c_cpp_builtins (cpp_reader *pfile)
> /* Old macro, superseded by
> __cpp_nontype_template_parameter_auto. */
> cpp_define (pfile, "__cpp_template_auto=201606L");
> - cpp_define (pfile, "__cpp_structured_bindings=201606L");
> + if (cxx_dialect <= cxx23)
> + cpp_define (pfile, "__cpp_structured_bindings=201606L");
> cpp_define (pfile, "__cpp_variadic_using=201611L");
> cpp_define (pfile, "__cpp_guaranteed_copy_elision=201606L");
> cpp_define (pfile, "__cpp_nontype_template_parameter_auto=201606L");
> @@ -1090,6 +1091,7 @@ c_cpp_builtins (cpp_reader *pfile)
> cpp_define (pfile, "__cpp_constexpr=202306L");
> cpp_define (pfile, "__cpp_static_assert=202306L");
> cpp_define (pfile, "__cpp_placeholder_variables=202306L");
> + cpp_define (pfile, "__cpp_structured_bindings=202403L");
> }
> if (flag_concepts)
> {
> --- gcc/testsuite/g++.dg/cpp26/decomp1.C.jj 2024-04-26 13:42:50.559184222 +0200
> +++ gcc/testsuite/g++.dg/cpp26/decomp1.C 2024-04-26 14:24:59.276090634 +0200
> @@ -0,0 +1,33 @@
> +// { dg-do compile { target c++11 } }
> +// { dg-options "" }
> +
> +namespace std {
> + template<typename T> struct tuple_size;
> + template<int, typename> struct tuple_element;
> +}
> +
> +struct A {
> + int i;
> + template <int I> int& get() { return i; }
> +};
> +
> +template<> struct std::tuple_size<A> { static const int value = 3; };
> +template<int I> struct std::tuple_element<I,A> { using type = int; };
> +
> +struct B {
> + int i, j;
> + long long k, l;
> +} z[6];
> +
> +void
> +foo (A &a, B &b)
> +{
> + auto [ c [[]], d, e [[maybe_unused]] ] = a; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
> + // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
> + auto [ f, h [[maybe_unused]] [[]], i [[]], j ] = b; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
> + // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
> + for (auto [ k, l [[maybe_unused]], m, n [[]]] : z) // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
> + ; // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
> + auto &[o[[]][[]][[]], p[[]], q[[]]] = a; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
> + ; // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
> +}
> --- gcc/testsuite/g++.dg/cpp26/decomp2.C.jj 2024-04-26 14:21:31.454974172 +0200
> +++ gcc/testsuite/g++.dg/cpp26/decomp2.C 2024-04-26 14:28:11.665421202 +0200
> @@ -0,0 +1,46 @@
> +// { dg-do compile { target c++11 } }
> +// { dg-options "" }
> +
> +namespace std {
> + template<typename T> struct tuple_size;
> + template<int, typename> struct tuple_element;
> +}
> +
> +struct A {
> + int i;
> + template <int I> int& get() { return i; }
> +};
> +
> +template<> struct std::tuple_size<A> { static const int value = 3; };
> +template<int I> struct std::tuple_element<I,A> { using type = int; };
> +
> +struct B {
> + int i, j;
> + long long k, l;
> +} z[6];
> +
> +void
> +foo (A &a, B &b)
> +{
> + auto [ c [[]], d, e [[gnu::deprecated]] ] = a; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
> + // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
> + // { dg-message "declared here" "" { target *-*-* } .-2 }
> + ++c;
> + ++d;
> + ++e; // { dg-warning "'e' is deprecated" }
> + auto [ f, h [[gnu::deprecated]] [[]], i [[]], j ] = b;// { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
> + // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
> + // { dg-message "declared here" "" { target *-*-* } .-2 }
> + ++f;
> + ++h; // { dg-warning "'h' is deprecated" }
> + ++i;
> + ++j;
> + for (auto [ k, l [[gnu::deprecated]], m, n [[]]] : z) // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
> + { // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
> + // { dg-message "declared here" "" { target *-*-* } .-2 }
> + ++k;
> + ++l; // { dg-warning "'l' is deprecated" }
> + ++m;
> + ++n;
> + }
> +}
> --- gcc/testsuite/g++.dg/cpp26/feat-cxx26.C.jj 2024-04-26 11:42:24.675015903 +0200
> +++ gcc/testsuite/g++.dg/cpp26/feat-cxx26.C 2024-04-26 13:42:50.559184222 +0200
> @@ -394,8 +394,8 @@
>
> #ifndef __cpp_structured_bindings
> # error "__cpp_structured_bindings"
> -#elif __cpp_structured_bindings != 201606
> -# error "__cpp_structured_bindings != 201606"
> +#elif __cpp_structured_bindings != 202403
> +# error "__cpp_structured_bindings != 202403"
> #endif
>
> #ifndef __cpp_template_template_args
>
> Jakub
>
@@ -16075,13 +16075,37 @@ cp_parser_decomposition_declaration (cp_
/* Parse the identifier-list. */
auto_vec<cp_expr, 10> v;
+ bool attr_diagnosed = false;
+ int first_attr = -1;
+ unsigned int cnt = 0;
if (!cp_lexer_next_token_is (parser->lexer, CPP_CLOSE_SQUARE))
while (true)
{
cp_expr e = cp_parser_identifier (parser);
if (e.get_value () == error_mark_node)
break;
+ tree attr = NULL_TREE;
+ if (cp_next_tokens_can_be_std_attribute_p (parser))
+ {
+ if (cxx_dialect >= cxx17 && cxx_dialect < cxx26 && !attr_diagnosed)
+ {
+ pedwarn (cp_lexer_peek_token (parser->lexer)->location,
+ OPT_Wc__26_extensions,
+ "structured bindings with attributed identifiers "
+ "only available with %<-std=c++2c%> or "
+ "%<-std=gnu++2c%>");
+ attr_diagnosed = true;
+ }
+ attr = cp_parser_std_attribute_spec_seq (parser);
+ if (attr == error_mark_node)
+ attr = NULL_TREE;
+ if (attr && first_attr == -1)
+ first_attr = v.length ();
+ }
v.safe_push (e);
+ ++cnt;
+ if (first_attr != -1)
+ v.safe_push (attr);
if (!cp_lexer_next_token_is (parser->lexer, CPP_COMMA))
break;
cp_lexer_consume_token (parser->lexer);
@@ -16139,8 +16163,11 @@ cp_parser_decomposition_declaration (cp_
declarator->id_loc = e.get_location ();
}
tree elt_pushed_scope;
+ tree attr = NULL_TREE;
+ if (first_attr != -1 && i >= (unsigned) first_attr)
+ attr = v[++i].get_value ();
tree decl2 = start_decl (declarator, &decl_specs, SD_DECOMPOSITION,
- NULL_TREE, NULL_TREE, &elt_pushed_scope);
+ NULL_TREE, attr, &elt_pushed_scope);
if (decl2 == error_mark_node)
decl = error_mark_node;
else if (decl != error_mark_node && DECL_CHAIN (decl2) != prev)
@@ -16183,7 +16210,7 @@ cp_parser_decomposition_declaration (cp_
if (decl != error_mark_node)
{
- cp_decomp decomp = { prev, v.length () };
+ cp_decomp decomp = { prev, cnt };
cp_finish_decl (decl, initializer, non_constant_p, NULL_TREE,
(is_direct_init ? LOOKUP_NORMAL : LOOKUP_IMPLICIT),
&decomp);
@@ -16193,7 +16220,7 @@ cp_parser_decomposition_declaration (cp_
else if (decl != error_mark_node)
{
*maybe_range_for_decl = prev;
- cp_decomp decomp = { prev, v.length () };
+ cp_decomp decomp = { prev, cnt };
/* Ensure DECL_VALUE_EXPR is created for all the decls but
the underlying DECL. */
cp_finish_decomp (decl, &decomp);
@@ -1044,7 +1044,8 @@ c_cpp_builtins (cpp_reader *pfile)
/* Old macro, superseded by
__cpp_nontype_template_parameter_auto. */
cpp_define (pfile, "__cpp_template_auto=201606L");
- cpp_define (pfile, "__cpp_structured_bindings=201606L");
+ if (cxx_dialect <= cxx23)
+ cpp_define (pfile, "__cpp_structured_bindings=201606L");
cpp_define (pfile, "__cpp_variadic_using=201611L");
cpp_define (pfile, "__cpp_guaranteed_copy_elision=201606L");
cpp_define (pfile, "__cpp_nontype_template_parameter_auto=201606L");
@@ -1090,6 +1091,7 @@ c_cpp_builtins (cpp_reader *pfile)
cpp_define (pfile, "__cpp_constexpr=202306L");
cpp_define (pfile, "__cpp_static_assert=202306L");
cpp_define (pfile, "__cpp_placeholder_variables=202306L");
+ cpp_define (pfile, "__cpp_structured_bindings=202403L");
}
if (flag_concepts)
{
@@ -0,0 +1,33 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+namespace std {
+ template<typename T> struct tuple_size;
+ template<int, typename> struct tuple_element;
+}
+
+struct A {
+ int i;
+ template <int I> int& get() { return i; }
+};
+
+template<> struct std::tuple_size<A> { static const int value = 3; };
+template<int I> struct std::tuple_element<I,A> { using type = int; };
+
+struct B {
+ int i, j;
+ long long k, l;
+} z[6];
+
+void
+foo (A &a, B &b)
+{
+ auto [ c [[]], d, e [[maybe_unused]] ] = a; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
+ // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+ auto [ f, h [[maybe_unused]] [[]], i [[]], j ] = b; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
+ // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+ for (auto [ k, l [[maybe_unused]], m, n [[]]] : z) // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
+ ; // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+ auto &[o[[]][[]][[]], p[[]], q[[]]] = a; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
+ ; // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+}
@@ -0,0 +1,46 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+namespace std {
+ template<typename T> struct tuple_size;
+ template<int, typename> struct tuple_element;
+}
+
+struct A {
+ int i;
+ template <int I> int& get() { return i; }
+};
+
+template<> struct std::tuple_size<A> { static const int value = 3; };
+template<int I> struct std::tuple_element<I,A> { using type = int; };
+
+struct B {
+ int i, j;
+ long long k, l;
+} z[6];
+
+void
+foo (A &a, B &b)
+{
+ auto [ c [[]], d, e [[gnu::deprecated]] ] = a; // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
+ // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+ // { dg-message "declared here" "" { target *-*-* } .-2 }
+ ++c;
+ ++d;
+ ++e; // { dg-warning "'e' is deprecated" }
+ auto [ f, h [[gnu::deprecated]] [[]], i [[]], j ] = b;// { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
+ // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+ // { dg-message "declared here" "" { target *-*-* } .-2 }
+ ++f;
+ ++h; // { dg-warning "'h' is deprecated" }
+ ++i;
+ ++j;
+ for (auto [ k, l [[gnu::deprecated]], m, n [[]]] : z) // { dg-warning "structured bindings with attributed identifiers only available with" "" { target { c++17 && c++23_down } } }
+ { // { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+ // { dg-message "declared here" "" { target *-*-* } .-2 }
+ ++k;
+ ++l; // { dg-warning "'l' is deprecated" }
+ ++m;
+ ++n;
+ }
+}
@@ -394,8 +394,8 @@
#ifndef __cpp_structured_bindings
# error "__cpp_structured_bindings"
-#elif __cpp_structured_bindings != 201606
-# error "__cpp_structured_bindings != 201606"
+#elif __cpp_structured_bindings != 202403
+# error "__cpp_structured_bindings != 202403"
#endif
#ifndef __cpp_template_template_args