[{"id":3669338,"web_url":"http://patchwork.ozlabs.org/comment/3669338/","msgid":"<61dd8ddd-21d5-4da0-9949-08acf979e116@redhat.com>","list_archive_url":null,"date":"2026-03-25T21:33:33","subject":"Re: [PATCH v2] c++/reflection: ICE with substitute and undeduced auto\n [PR123613]","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 3/24/26 7:53 PM, Marek Polacek wrote:\n> 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> \n> It seems like I can do the instantiation earlier, in get_reflection. We\n> already perform the resolve_ there, so eval_can_substitute probably does\n> not need to do it again.\n> \n> Thus, v2:\n> \n> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?\n\nOK.\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. We can call mark_used to resolve this, because\n> it has:\n> \n> if (undeduced_auto_decl (decl))\n> maybe_instantiate_decl (decl);\n> \n> and to make the json-parser.C test work, we have to do something\n> similar in eval_substitute, otherwise we crash due to another\n> undeduced auto.\n> \n> \tPR c++/123613\n> \n> gcc/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> \n> gcc/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> diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc\n> index 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))\n> diff --git a/gcc/testsuite/g++.dg/reflect/json-parser.C b/gcc/testsuite/g++.dg/reflect/json-parser.C\n> new file mode 100644\n> index 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> +}\n> diff --git a/gcc/testsuite/g++.dg/reflect/substitute7.C b/gcc/testsuite/g++.dg/reflect/substitute7.C\n> new file mode 100644\n> index 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 });\n> diff --git a/gcc/testsuite/g++.dg/reflect/test.json b/gcc/testsuite/g++.dg/reflect/test.json\n> new file mode 100644\n> index 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> \n> base-commit: 5cd3889135d77bf951e4ffe169868b453c36257d","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=g3CMKjcE;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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