{"id":2215606,"url":"http://patchwork.ozlabs.org/api/patches/2215606/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/acLrujgOt2w8gd5_@redhat.com/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/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,"list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"","list_archive_url":null,"date":"2026-03-24T19:53:30","name":"[v2] c++/reflection: ICE with substitute and undeduced auto [PR123613]","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"b6ac09e132916693a25f6251e422e66ad612ff01","submitter":{"id":14370,"url":"http://patchwork.ozlabs.org/api/people/14370/?format=json","name":"Marek Polacek","email":"polacek@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/acLrujgOt2w8gd5_@redhat.com/mbox/","series":[{"id":497341,"url":"http://patchwork.ozlabs.org/api/series/497341/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=497341","date":"2026-03-24T19:53:30","name":"[v2] c++/reflection: ICE with substitute and undeduced auto [PR123613]","version":2,"mbox":"http://patchwork.ozlabs.org/series/497341/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2215606/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2215606/checks/","tags":{},"related":[],"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=JzSSYXyJ;\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 ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"On Fri, Mar 20, 2026 at 06:19:18PM -0400, Jason Merrill wrote:\n> On 3/19/26 2:01 PM, Marek Polacek wrote:\n> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?\n> > \n> > -- >8 --\n> > In substitute7.C we have\n> > \n> > template \n> > constexpr auto construct_from = T{Vs...}; // #1\n> > struct Inner {};\n> > struct Outer { Inner m; };\n> > constexpr auto r = substitute(^^construct_from,\n> > { ^^Outer, ^^construct_from });\n> > \n> > which crashes because the auto in #1 hadn't been deduced when we\n> > called eval_can_substitute -> lookup_template_variable when processing\n> > the substitute call.\n> \n> Hmm, why wasn't it already deduced when we took the reflection? Likewise for\n> the resolve_, actually.\n\nIt seems like I can do the instantiation earlier, in get_reflection. We\nalready perform the resolve_ there, so eval_can_substitute probably does\nnot need to do it again.\n \nThus, v2:\n\nBootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?\n\n-- >8 --\nIn substitute7.C we have\n\n template \n constexpr auto construct_from = T{Vs...}; // #1\n struct Inner {};\n struct Outer { Inner m; };\n constexpr auto r = substitute(^^construct_from,\n { ^^Outer, ^^construct_from });\n\nwhich crashes because the auto in #1 hadn't been deduced when we\ncalled eval_can_substitute -> lookup_template_variable when processing\nthe substitute call. We can call mark_used to resolve this, because\nit has:\n\n if (undeduced_auto_decl (decl))\n maybe_instantiate_decl (decl);\n\nand to make the json-parser.C test work, we have to do something\nsimilar in eval_substitute, otherwise we crash due to another\nundeduced auto.\n\n\tPR c++/123613\n\ngcc/cp/ChangeLog:\n\n\t* reflect.cc (get_reflection): Call mark_used.\n\t(eval_can_substitute): Don't resolve_nondeduced_context here.\n\t(eval_substitute): Call lookup_and_finish_template_variable instead\n\tof lookup_template_variable and finish_template_variable.\n\ngcc/testsuite/ChangeLog:\n\n\t* g++.dg/reflect/json-parser.C: New test.\n\t* g++.dg/reflect/substitute7.C: New test.\n\t* g++.dg/reflect/test.json: New test.\n---\n gcc/cp/reflect.cc | 16 +-\n gcc/testsuite/g++.dg/reflect/json-parser.C | 161 +++++++++++++++++++++\n gcc/testsuite/g++.dg/reflect/substitute7.C | 14 ++\n gcc/testsuite/g++.dg/reflect/test.json | 4 +\n 4 files changed, 188 insertions(+), 7 deletions(-)\n create mode 100644 gcc/testsuite/g++.dg/reflect/json-parser.C\n create mode 100644 gcc/testsuite/g++.dg/reflect/substitute7.C\n create mode 100644 gcc/testsuite/g++.dg/reflect/test.json\n\n\nbase-commit: 5cd3889135d77bf951e4ffe169868b453c36257d","diff":"diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc\nindex 0b1d3a36f37..e4313ae5e98 100644\n--- a/gcc/cp/reflect.cc\n+++ b/gcc/cp/reflect.cc\n@@ -212,8 +212,14 @@ get_reflection (location_t loc, tree t, reflect_kind kind/*=REFLECT_UNDEF*/)\n overload resolution for the expression &S with no target shall\n select a unique function; R represents that function. */\n else if (!processing_template_decl && t != unknown_type_node)\n- /* Resolve all TEMPLATE_ID_EXPRs here. */\n- t = resolve_nondeduced_context_or_error (t, tf_warning_or_error);\n+ {\n+ /* Resolve all TEMPLATE_ID_EXPRs here. */\n+ t = resolve_nondeduced_context_or_error (t, tf_warning_or_error);\n+ /* The argument could have a deduced return type, so we need to\n+\t instantiate it now to find out its type. */\n+ if (!mark_used (t))\n+\treturn error_mark_node;\n+ }\n \n /* For injected-class-name, use the main variant so that comparing\n reflections works (cf. compare3.C). */\n@@ -5359,7 +5365,6 @@ eval_can_substitute (location_t loc, const constexpr_ctx *ctx,\n \treturn throw_exception (loc, ctx,\n \t\t\t\t\"invalid argument to can_substitute\",\n \t\t\t\tfun, non_constant_p, jump_target);\n- a = resolve_nondeduced_context (a, tf_warning_or_error);\n a = convert_from_reference (a);\n TREE_VEC_ELT (rvec, i) = a;\n }\n@@ -5432,10 +5437,7 @@ eval_substitute (location_t loc, const constexpr_ctx *ctx,\n return get_reflection_raw (loc, ret, REFLECT_VALUE);\n }\n else if (variable_template_p (r))\n- {\n- ret = lookup_template_variable (r, rvec, tf_none);\n- ret = finish_template_variable (ret, tf_none);\n- }\n+ ret = lookup_and_finish_template_variable (r, rvec, tf_none);\n else\n {\n if (DECL_FUNCTION_TEMPLATE_P (r))\ndiff --git a/gcc/testsuite/g++.dg/reflect/json-parser.C b/gcc/testsuite/g++.dg/reflect/json-parser.C\nnew file mode 100644\nindex 00000000000..6c54bb51704\n--- /dev/null\n+++ b/gcc/testsuite/g++.dg/reflect/json-parser.C\n@@ -0,0 +1,161 @@\n+// PR c++/123613\n+// { dg-do compile { target c++26 } }\n+// { dg-additional-options \"-freflection\" }\n+\n+#include \n+#include \n+#include \n+#include \n+\n+template \n+struct Outer {\n+ struct Inner;\n+ consteval {\n+ define_aggregate(^^Inner, {Ms...});\n+ }\n+};\n+\n+template \n+using Cls = Outer::Inner;\n+\n+template \n+constexpr auto construct_from = T{Vs...};\n+\n+consteval std::meta::info parse_json(std::string_view json) {\n+ auto cursor = json.begin();\n+ auto end = json.end();\n+\n+ auto is_whitespace = [](char c) {\n+ return c == ' ' || c == '\\n' || c == '\\t';\n+ };\n+\n+ auto skip_whitespace = [&] -> void {\n+ while (is_whitespace(*cursor)) cursor++;\n+ };\n+\n+ auto expect_consume = [&] (char c) -> void {\n+ skip_whitespace();\n+ if (cursor == end || *(cursor++) != c) throw \"unexpected character\";\n+ };\n+\n+ auto parse_until = [&](std::vector delims, std::string &out) -> void {\n+ skip_whitespace();\n+ while (cursor != end &&\n+ !std::ranges::any_of(delims, [&](char c) { return c == *cursor; }))\n+ out += *(cursor++);\n+ };\n+\n+ auto parse_delimited = [&](char lhs, std::string &out, char rhs) -> void {\n+ skip_whitespace();\n+ expect_consume(lhs);\n+ parse_until({rhs}, out);\n+ expect_consume(rhs);\n+ };\n+\n+ auto parse_value = [&](std::string &out) -> void {\n+ skip_whitespace();\n+\n+ bool quoted = false;\n+ unsigned depth = 0;\n+ while (true) {\n+ if (cursor == end) throw \"unexpected end of stream\";\n+ if (is_whitespace(*cursor) && !quoted && depth == 0)\n+ break;\n+\n+ if (depth == 0 && (*cursor == ',' || *cursor == '}'))\n+ break;\n+ out += *(cursor++);\n+\n+ if (out.back() == '{')\n+ ++depth;\n+ else if (out.back() == '}')\n+ --depth;\n+ else if (out.back() == '\"') {\n+ if (quoted && depth == 0)\n+ break;\n+ quoted = true;\n+ }\n+ };\n+ };\n+\n+ skip_whitespace();\n+ expect_consume('{');\n+\n+ std::vector members;\n+ std::vector values = {^^void};\n+\n+ using std::meta::reflect_constant, std::meta::reflect_constant_string;\n+ while (cursor != end && *cursor != '}') {\n+ std::string field_name;\n+ std::string value;\n+\n+ std::meta::info parsed_type;\n+\n+ parse_delimited('\"', field_name, '\"');\n+ expect_consume(':');\n+ parse_value(value);\n+\n+ if (value.empty()) throw \"expected value\";\n+ if (cursor == end) throw \"unexpected end of stream\";\n+\n+ if (value[0] == '\"') {\n+ if (value.back() == '}' && value[1] == 'f') throw \"hmm\";\n+ if (value.back() != '\"') throw \"expected end of string\";\n+ std::string_view contents(&value[1], value.size() - 2);\n+\n+ auto dms = data_member_spec(^^char const*, {.name=field_name});\n+ members.push_back(reflect_constant(dms));\n+ values.push_back(reflect_constant_string(contents));\n+ } else if (value[0] >= '0' && value[0] <= '9') {\n+ int contents = [](std::string_view in) {\n+ int out;\n+ std::from_chars(in.data(), in.data() + in.size(), out);\n+ return out;\n+ }(value);\n+\n+ auto dms = data_member_spec(^^int, {.name=field_name});\n+ members.push_back(reflect_constant(dms));\n+ values.push_back(reflect_constant(contents));\n+ } else if (value[0] == '{') {\n+ std::meta::info parsed = parse_json(value);\n+\n+ auto dms = data_member_spec(type_of(parsed), {.name=field_name});\n+ members.push_back(reflect_constant(dms));\n+ values.push_back(parsed);\n+ }\n+ skip_whitespace();\n+ if (cursor != end && *cursor == ',')\n+ ++cursor;\n+ }\n+ if (cursor == end) throw \"hmm\";\n+ expect_consume('}');\n+\n+ values[0] = substitute(^^Cls, members);\n+ return substitute(^^construct_from, values);\n+}\n+\n+struct JSONString {\n+ std::meta::info Rep;\n+ consteval JSONString(const char *Json) : Rep{parse_json(Json)} {}\n+};\n+\n+template \n+consteval auto operator\"\"_json() {\n+ return [:json.Rep:];\n+}\n+\n+template \n+inline constexpr auto json_to_object = [: json.Rep :];\n+\n+constexpr const char data[] = {\n+ #embed \"test.json\"\n+ , 0\n+};\n+\n+int main() {\n+ constexpr auto v = json_to_object;\n+ static_assert(std::string_view(v.outer) == \"text\");\n+ static_assert(v.inner.number == 2996);\n+ static_assert(std::string_view(v.inner.field) == \"yes\");\n+ static_assert(R\"({\"field\": \"yes\", \"number\": 2996})\"_json.number == 2996);\n+}\ndiff --git a/gcc/testsuite/g++.dg/reflect/substitute7.C b/gcc/testsuite/g++.dg/reflect/substitute7.C\nnew file mode 100644\nindex 00000000000..15bacd09371\n--- /dev/null\n+++ b/gcc/testsuite/g++.dg/reflect/substitute7.C\n@@ -0,0 +1,14 @@\n+// PR c++/123613\n+// { dg-do compile { target c++26 } }\n+// { dg-additional-options \"-freflection\" }\n+\n+#include \n+\n+template \n+constexpr auto construct_from = T{Vs...};\n+\n+struct Inner {};\n+struct Outer { Inner m; };\n+\n+constexpr auto r = substitute(^^construct_from,\n+ { ^^Outer, ^^construct_from });\ndiff --git a/gcc/testsuite/g++.dg/reflect/test.json b/gcc/testsuite/g++.dg/reflect/test.json\nnew file mode 100644\nindex 00000000000..b95cd80b88e\n--- /dev/null\n+++ b/gcc/testsuite/g++.dg/reflect/test.json\n@@ -0,0 +1,4 @@\n+{ \n+ \"outer\": \"text\", \n+ \"inner\": { \"field\": \"yes\", \"number\": 2996 }\n+} \n","prefixes":["v2"]}