{"id":2222011,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2222011/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/adkUGggIiITbC-cp@tucnak/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/1.1/projects/17/?format=json","name":"GNU Compiler Collection","link_name":"gcc","list_id":"gcc-patches.gcc.gnu.org","list_email":"gcc-patches@gcc.gnu.org","web_url":null,"scm_url":null,"webscm_url":null},"msgid":"","date":"2026-04-10T15:15:38","name":"c++, libstdc++: Implement LWG4483 - Multidimensional arrays are not supported by meta::reflect_constant_array and related functions","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"35791807f0ebfd9692fb6526f9e21d89e1307567","submitter":{"id":671,"url":"http://patchwork.ozlabs.org/api/1.1/people/671/?format=json","name":"Jakub Jelinek","email":"jakub@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/adkUGggIiITbC-cp@tucnak/mbox/","series":[{"id":499480,"url":"http://patchwork.ozlabs.org/api/1.1/series/499480/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=499480","date":"2026-04-10T15:15:38","name":"c++, libstdc++: Implement LWG4483 - Multidimensional arrays are not supported by meta::reflect_constant_array and related functions","version":1,"mbox":"http://patchwork.ozlabs.org/series/499480/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2222011/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2222011/checks/","tags":{},"headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=YfX3nHBv;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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charset=us-ascii","Content-Disposition":"inline","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Reply-To":"Jakub Jelinek ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"Hi!\n\nThe following patch attempts to implement LWG4483. As written in the\napproved resolution, some checks are done on strip_array_types (valuet)\nrather than on valuet and one is skipped. Tomasz on IRC reasoned why\ninput_range should otherwise already ensure we see similar type, so just\nfor the possibility of fuzzed the patch adds some verification.\nAnd as the array cases are contiguous, instead of actually recursing (for\nwhich we'd need to have lvalue of the *it rather than prvalue) it just\nwalks the CONSTRUCTORs for the arrays and handles the elts in there\nrecursively.\nThe changes do exactly what the LWG4483 change says to do.\n\nTested on x86_64-linux, ok for trunk?\n\n2026-04-10 Jakub Jelinek \n\n\t* reflect.cc (adjust_array_elt): New function.\n\t(get_range_elts): Implement LWG4483 - Multidimensional arrays are not\n\tsupported by meta::reflect_constant_array and related functions.\n\tHandle ARRAY_TYPE valuet. Don't unshare_expr in the class valuet case,\n\tget_template_param_object will unshare.\n\n\t* g++.dg/reflect/reflect_constant_array9.C: New test.\n\t* g++.dg/reflect/reflect_constant_array10.C: New test.\n\t* g++.dg/reflect/reflect_constant_array11.C: New test.\n\t* g++.dg/reflect/define_static_array6.C: New test.\n\t* g++.dg/reflect/define_static_object2.C: Uncomment older tests and\n\tfix them, add tests for unions.\n\n\t* include/std/meta (define_static_object): Adjust for LWG4483 changes\n\t- handle unions and arrays differently.\n\n\n\tJakub","diff":"--- gcc/cp/reflect.cc.jj\t2026-04-10 08:45:20.716811803 +0200\n+++ gcc/cp/reflect.cc\t2026-04-10 15:32:30.747654790 +0200\n@@ -394,6 +394,47 @@ replace_parm_r (tree *tp, int *walk_subt\n static tree throw_exception (location_t, const constexpr_ctx *, const char *,\n \t\t\t tree, bool *, tree *);\n \n+/* Helper function for get_range_elts, handle adjustment of ARRAY_TYPE elts\n+ of a retvec. */\n+\n+static tree\n+adjust_array_elt (location_t loc, const constexpr_ctx *ctx, tree valuet,\n+\t\t tree expr, tree fun, bool *non_constant_p, tree *jump_target)\n+{\n+ if (TREE_CODE (valuet) == ARRAY_TYPE)\n+ {\n+ if (TREE_CODE (expr) != CONSTRUCTOR\n+\t || TREE_CODE (TREE_TYPE (expr)) != ARRAY_TYPE)\n+\treturn throw_exception (loc, ctx, \"reflect_constant_array failed\",\n+\t\t\t\tfun, non_constant_p, jump_target);\n+ unsigned int i;\n+ tree val;\n+ FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (expr), i, val)\n+\t{\n+\t CONSTRUCTOR_ELT (expr, i)->value\n+\t = adjust_array_elt (loc, ctx, TREE_TYPE (valuet), val, fun,\n+\t\t\t\tnon_constant_p, jump_target);\n+\t if (*jump_target || *non_constant_p)\n+\t return NULL_TREE;\n+\t}\n+ return expr;\n+ }\n+ else if (INTEGRAL_TYPE_P (valuet))\n+ {\n+ if (TREE_CODE (expr) == INTEGER_CST)\n+\treturn expr;\n+ return throw_exception (loc, ctx, \"array element not a constant integer\",\n+\t\t\t fun, non_constant_p, jump_target);\n+ }\n+ expr = convert_reflect_constant_arg (valuet, expr);\n+ if (expr == error_mark_node)\n+ return throw_exception (loc, ctx, \"reflect_constant failed\",\n+\t\t\t fun, non_constant_p, jump_target);\n+ if (VAR_P (expr))\n+ expr = DECL_INITIAL (expr);\n+ return expr;\n+}\n+\n /* Kinds for get_range_elts. */\n \n enum get_range_elts_kind {\n@@ -513,23 +554,24 @@ get_range_elts (location_t loc, const co\n \t}\n if (kind == REFLECT_CONSTANT_ARRAY)\n \t{\n-\t if (!structural_type_p (valuet))\n+\t tree valuete = strip_array_types (valuet);\n+\t if (!structural_type_p (valuete))\n \t {\n \t if (!cxx_constexpr_quiet_p (ctx))\n \t\t{\n \t\t auto_diagnostic_group d;\n \t\t error_at (loc, \"% argument with \"\n \t\t\t\t \"%qT which is not a structural type\", inst);\n-\t\t structural_type_p (valuet, true);\n+\t\t structural_type_p (valuete, true);\n \t\t}\n \t *non_constant_p = true;\n \t return NULL_TREE;\n \t }\n \t TREE_VEC_ELT (args, 0)\n-\t = build_stub_type (valuet,\n-\t\t\t cp_type_quals (valuet) | TYPE_QUAL_CONST,\n+\t = build_stub_type (valuete,\n+\t\t\t cp_type_quals (valuete) | TYPE_QUAL_CONST,\n \t\t\t false);\n-\t if (!is_xible (INIT_EXPR, valuet, args))\n+\t if (!is_xible (INIT_EXPR, valuete, args))\n \t {\n \t if (!cxx_constexpr_quiet_p (ctx))\n \t\terror_at (loc, \"% argument with %qT \"\n@@ -551,7 +593,23 @@ get_range_elts (location_t loc, const co\n \t }\n \t tree referencet = TYPE_MAIN_VARIANT (instr);\n \t TREE_VEC_ELT (args, 0) = referencet;\n-\t if (!is_xible (INIT_EXPR, valuet, args))\n+\t if (valuete != valuet)\n+\t {\n+\t tree rt = referencet;\n+\t if (TYPE_REF_P (rt))\n+\t\trt = TREE_TYPE (rt);\n+\t if (!same_type_ignoring_top_level_qualifiers_p (valuet, rt))\n+\t\t{\n+\t\t if (!cxx_constexpr_quiet_p (ctx))\n+\t\t error_at (loc, \"% argument with \"\n+\t\t\t\t \"%qT which is not compatible with %qT \"\n+\t\t\t\t \"%\",\n+\t\t\t inst, referencet);\n+\t\t *non_constant_p = true;\n+\t\t return NULL_TREE;\n+\t\t}\n+\t }\n+\t else if (!is_xible (INIT_EXPR, valuet, args))\n \t {\n \t if (!cxx_constexpr_quiet_p (ctx))\n \t\terror_at (loc, \"% argument with %qT \"\n@@ -595,12 +653,22 @@ get_range_elts (location_t loc, const co\n \telse\n \t {\n \t gcc_assert (kind == REFLECT_CONSTANT_ARRAY);\n+\t if (TREE_CODE (valuet) == ARRAY_TYPE)\n+\t {\n+\t\tretvec[i]\n+\t\t = adjust_array_elt (loc, ctx, valuet,\n+\t\t\t\t unshare_expr (retvec[i]), fun,\n+\t\t\t\t non_constant_p, jump_target);\n+\t\tif (*jump_target || *non_constant_p)\n+\t\t return NULL_TREE;\n+\t\tcontinue;\n+\t }\n \t tree expr = convert_reflect_constant_arg (valuet, retvec[i]);\n \t if (expr == error_mark_node)\n \t return throw_exception (loc, ctx, \"reflect_constant failed\",\n \t\t\t\t fun, non_constant_p, jump_target);\n \t if (VAR_P (expr))\n-\t expr = unshare_expr (DECL_INITIAL (expr));\n+\t expr = DECL_INITIAL (expr);\n \t retvec[i] = expr;\n \t }\n }\n--- gcc/testsuite/g++.dg/reflect/reflect_constant_array9.C.jj\t2026-04-10 14:58:28.892432782 +0200\n+++ gcc/testsuite/g++.dg/reflect/reflect_constant_array9.C\t2026-04-10 15:46:45.523823103 +0200\n@@ -0,0 +1,28 @@\n+// LWG4483 - Multidimensional arrays are not supported by\n+// meta::reflect_constant_array and related functions.\n+// { dg-do compile { target c++26 } }\n+// { dg-additional-options \"-freflection\" }\n+\n+#include \n+\n+consteval auto\n+foo ()\n+{\n+ int a[3][3][3];\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\ta[i][j][k] = i + j + k;\n+ return std::meta::reflect_constant_array (a);\n+}\n+\n+static_assert (foo () == foo ());\n+static_assert (type_of (foo ()) == ^^const int [3][3][3]);\n+constexpr auto &m = [: foo () :];\n+consteval {\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\tif (m[i][j][k] != i + j + k)\n+\t throw 1;\n+}\n--- gcc/testsuite/g++.dg/reflect/reflect_constant_array10.C.jj\t2026-04-10 16:43:43.192287885 +0200\n+++ gcc/testsuite/g++.dg/reflect/reflect_constant_array10.C\t2026-04-10 16:57:20.060192312 +0200\n@@ -0,0 +1,53 @@\n+// LWG4483 - Multidimensional arrays are not supported by\n+// meta::reflect_constant_array and related functions.\n+// { dg-do compile { target c++26 } }\n+// { dg-additional-options \"-freflection\" }\n+\n+#include \n+#include \n+\n+consteval auto\n+foo ()\n+{\n+ int a[3][3][3];\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\ta[i][j][k] = i + 3 * j + 9 * k;\n+ std::span b = a;\n+ return std::meta::reflect_constant_array (b);\n+}\n+\n+static_assert (foo () == foo ());\n+static_assert (type_of (foo ()) == ^^const int [3][3][3]);\n+constexpr auto &m = [: foo () :];\n+consteval {\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\tif (m[i][j][k] != i + 3 * j + 9 * k)\n+\t throw 1;\n+}\n+\n+consteval auto\n+bar ()\n+{\n+ int a[3][3][3];\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\ta[i][j][k] = i + 3 * j + 9 * k;\n+ std::span b = a;\n+ return std::meta::reflect_constant_array (b | std::views::reverse);\n+}\n+\n+static_assert (bar () == bar ());\n+static_assert (type_of (bar ()) == ^^const int [3][3][3]);\n+constexpr auto &n = [: bar () :];\n+consteval {\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\tif (n[i][j][k] != (2 - i) + 3 * j + 9 * k)\n+\t throw 1;\n+}\n--- gcc/testsuite/g++.dg/reflect/reflect_constant_array11.C.jj\t2026-04-10 17:00:44.179663685 +0200\n+++ gcc/testsuite/g++.dg/reflect/reflect_constant_array11.C\t2026-04-10 17:05:50.683365136 +0200\n@@ -0,0 +1,18 @@\n+// { dg-do compile { target c++26 } }\n+// { dg-additional-options \"-freflection\" }\n+// Test std::meta::reflect_constant_string.\n+\n+#include \n+#include \n+#include \n+\n+using namespace std::meta;\n+\n+struct A { int a, b; mutable int c; };\n+constexpr A aa[2] = { { 1, 2, 3 }, { 4, 5, 6 } };\n+constexpr auto a = reflect_constant_array (aa);\n+// { dg-error \"'reflect_constant_array' argument with 'std::ranges::range_value_t' \\\\\\{aka 'A'\\\\\\} which is not a structural type\" \"\" { target *-*-* } .-1 }\n+struct B { constexpr B (int x, int y) : a (x), b (y) {} constexpr ~B () {} B (const B &) = delete; int a, b; };\n+constexpr B b[2][2] = { { { 1, 2 }, { 2, 3 } }, { { 3, 4 }, { 4, 5 } } };\n+constexpr auto c = reflect_constant_array (b);\n+// { dg-error \"'reflect_constant_array' argument with 'std::ranges::range_value_t' \\\\\\{aka 'B \\\\\\[2\\\\\\]'\\\\\\} which is not copy constructible\" \"\" { target *-*-* } .-1 }\n--- gcc/testsuite/g++.dg/reflect/define_static_array6.C.jj\t2026-04-10 15:44:52.229788936 +0200\n+++ gcc/testsuite/g++.dg/reflect/define_static_array6.C\t2026-04-10 15:52:38.099857297 +0200\n@@ -0,0 +1,27 @@\n+// LWG4483 - Multidimensional arrays are not supported by\n+// meta::reflect_constant_array and related functions.\n+// { dg-do compile { target c++26 } }\n+// { dg-additional-options \"-freflection\" }\n+\n+#include \n+\n+consteval auto\n+foo ()\n+{\n+ int a[3][3][3];\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\ta[i][j][k] = i + j + k;\n+ return std::define_static_array (a);\n+}\n+\n+static_assert (foo ().size () == 3);\n+constexpr auto *m = foo ().data ();\n+consteval {\n+ for (int i = 0; i < 3; ++i)\n+ for (int j = 0; j < 3; ++j)\n+ for (int k = 0; k < 3; ++k)\n+\tif (m[i][j][k] != i + j + k)\n+\t throw 1;\n+}\n--- gcc/testsuite/g++.dg/reflect/define_static_object2.C.jj\t2026-03-27 10:17:16.120298331 +0100\n+++ gcc/testsuite/g++.dg/reflect/define_static_object2.C\t2026-04-10 16:09:20.261858550 +0200\n@@ -1,3 +1,5 @@\n+// LWG4483 - Multidimensional arrays are not supported by\n+// meta::reflect_constant_array and related functions.\n // { dg-do compile { target c++26 } }\n // { dg-additional-options \"-freflection\" }\n // Test std::define_static_object.\n@@ -5,14 +7,19 @@\n #include \n \n constexpr int arr[]{1, 2, 3};\n-// LWG4483 use extract(reflect_constant_array())\n-// constexpr const int(*ptr)[3] = std::define_static_object(arr);\n-// static_assert( *ptr == std::define_static_array(arr).data() );\n-// static_assert( ptr = &std::meta::constant_of(arr) );\n+constexpr const int (*ptr)[3] = std::define_static_object (arr);\n+static_assert (*ptr == std::define_static_array (arr).data ());\n+static_assert (ptr == &[: std::meta::constant_of (^^arr) :]);\n \n constexpr int marr[3][3]{1, 2, 3};\n-// LWG4483 array are not structural so this fail\n-// constexpr const int(*mptr)[3][3] = std::define_static_object(marr);\n-// static_assert( *mptr == std::define_static_array(marr).data() );\n-// static_assert( mptr = &std::meta::constant_of(marr) );\n+constexpr const int (*mptr)[3][3] = std::define_static_object (marr);\n+static_assert (*mptr == std::define_static_array (marr).data ());\n+static_assert (mptr == &[: std::meta::constant_of (^^marr) :]);\n \n+union U { int a; long long b; };\n+constexpr U u = { .a = 42 };\n+constexpr const U *up = std::define_static_object (u);\n+static_assert (up->a == 42);\n+constexpr U v = { .b = 43LL };\n+constexpr const U *vp = std::define_static_object (v);\n+static_assert (vp->b == 43LL);\n--- libstdc++-v3/include/std/meta.jj\t2026-04-08 07:49:13.477647992 +0200\n+++ libstdc++-v3/include/std/meta\t2026-04-10 15:57:25.423013150 +0200\n@@ -682,11 +682,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n define_static_object(_Tp&& __t)\n {\n using _Up = remove_cvref_t<_Tp>;\n- if constexpr (meta::is_class_type(^^_Up))\n+ if constexpr (meta::is_class_type(^^_Up) || meta::is_union_type(^^_Up))\n \t{\n \t auto __cst = meta::reflect_constant(std::forward<_Tp>(__t));\n \t return std::addressof(meta::extract(__cst));\n \t}\n+ else if constexpr (meta::is_array_type(^^_Up))\n+\t{\n+\t auto __cst = meta::reflect_constant_array(std::forward<_Tp>(__t));\n+\t return std::addressof(meta::extract(__cst));\n+\t}\n else\n \treturn std::define_static_array(span(std::addressof(__t), 1)).data();\n }\n","prefixes":[]}