[{"id":3687072,"web_url":"http://patchwork.ozlabs.org/comment/3687072/","msgid":"","list_archive_url":null,"date":"2026-05-06T13:20:24","subject":"Re: [PATCH] c++: deferred parsing of default arguments [PR50479]","submitter":{"id":14370,"url":"http://patchwork.ozlabs.org/api/people/14370/","name":"Marek Polacek","email":"polacek@redhat.com"},"content":"On Tue, May 05, 2026 at 09:19:12PM -0400, Marek Polacek wrote:\n> @@ -37088,11 +37132,13 @@ cp_parser_save_default_args (cp_parser* parser, tree decl)\n> /* DEFAULT_ARG contains the saved tokens for the initializer of DECL,\n> which is either a FIELD_DECL or PARM_DECL. Parse it and return\n> the result. For a PARM_DECL, PARMTYPE is the corresponding type\n> - from the parameter-type-list. */\n> + from the parameter-type-list. If adjust_lambda_scope_p is true,\n\nThis should be \"ADJUST_LAMBDA_SCOPE_P\", fixed locally.\n\n> + we do start_/finish_lambda_scope. */\n\nMarek","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=aq82AmJe;\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; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.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=aq82AmJe","sourceware.org; dmarc=pass (p=quarantine dis=none)\n header.from=redhat.com","sourceware.org; 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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"}},{"id":3687356,"web_url":"http://patchwork.ozlabs.org/comment/3687356/","msgid":"","list_archive_url":null,"date":"2026-05-06T20:15:03","subject":"Re: [PATCH] c++: deferred parsing of default arguments [PR50479]","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 5/5/26 9:19 PM, Marek Polacek wrote:\n> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?\n> \n> Additionally tested with the \"Optimization only\" checks disabled\n> to see if it isn't hiding any errors.\n> \n> -- >8 --\n> In\n> \n> void fn (int i = sizeof (i)) {}\n> \n> the i in sizeof should refer to the parameter ([basic.scope.pdecl]) and\n> since the second i is not evaluated, the code is valid per\n> [dcl.fct.default]/9: A parameter shall not appear as a potentially\n> evaluated expression in a default argument.\n> \n> To make this work, we have to defer parsing of the default argument\n> until we've done grokdeclarator + pushdecl on i.\n\nHow about doing that by moving grokdeclarator/pushdecl into \ncp_parser_parameter_declaration rather than mess with DEFERRED_PARSE?\n\n> Unfortunately,\n> lambdas complicated this patch somewhat. _late_parse_one_default_arg\n> always does start_lambda_scope which here would set lambda_scope.scope\n> to the parameter which doesn't yet have a DECL_CONTEXT, and we'd crash.\n> But for the default argument in\n> \n> inline void g(int n) {\n> int bef(int i = []{ return 1; }());\n> }\n> \n> the scope should be 'g' anyway, not 'bef'. Thus the adjust_lambda_scope_p\n> parameter to preserve the existing behavior.\n> \n> \tPR c++/50479\n> \tPR c++/62244\n> \n> gcc/cp/ChangeLog:\n> \n> \t* parser.cc (defining_nonlambda_class_type_p): New.\n> \t(cp_parser_parameter_declaration_list): Maybe perform deferred\n> \tparsing of default arguments.\n> \t(cp_parser_parameter_declaration): Defer parsing of named\n> \tparameters.\n> \t(cp_parser_noexcept_specification_opt): Use\n> \tdefining_nonlambda_class_type_p.\n> \t(cp_parser_late_parse_one_default_arg): New parameter. Use it to\n> \tavoid the calls to start_/finish_lambda_scope.\n> \n> gcc/testsuite/ChangeLog:\n> \n> \t* g++.dg/reflect/parm1.C: Uncomment code.\n> \t* g++.dg/parse/defarg22.C: New test.\n> \t* g++.dg/parse/defarg23.C: New test.\n> \t* g++.dg/parse/defarg24.C: New test.\n> \t* g++.dg/parse/defarg25.C: New test.\n> \t* g++.dg/parse/defarg26.C: New test.\n> ---\n> gcc/cp/parser.cc | 86 +++++++++++++++++++++------\n> gcc/testsuite/g++.dg/parse/defarg22.C | 16 +++++\n> gcc/testsuite/g++.dg/parse/defarg23.C | 36 +++++++++++\n> gcc/testsuite/g++.dg/parse/defarg24.C | 12 ++++\n> gcc/testsuite/g++.dg/parse/defarg25.C | 22 +++++++\n> gcc/testsuite/g++.dg/parse/defarg26.C | 18 ++++++\n> gcc/testsuite/g++.dg/reflect/parm1.C | 5 +-\n> 7 files changed, 173 insertions(+), 22 deletions(-)\n> create mode 100644 gcc/testsuite/g++.dg/parse/defarg22.C\n> create mode 100644 gcc/testsuite/g++.dg/parse/defarg23.C\n> create mode 100644 gcc/testsuite/g++.dg/parse/defarg24.C\n> create mode 100644 gcc/testsuite/g++.dg/parse/defarg25.C\n> create mode 100644 gcc/testsuite/g++.dg/parse/defarg26.C\n> \n> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc\n> index d128de771b5..9c8cd7bdc90 100644\n> --- a/gcc/cp/parser.cc\n> +++ b/gcc/cp/parser.cc\n> @@ -3104,7 +3104,7 @@ static void cp_parser_save_default_args\n> static void cp_parser_late_parsing_for_member\n> (cp_parser *, tree);\n> static tree cp_parser_late_parse_one_default_arg\n> - (cp_parser *, tree, tree, tree);\n> + (cp_parser *, tree, tree, tree, bool = true);\n> static void cp_parser_late_parsing_nsdmi\n> (cp_parser *, tree);\n> static void cp_parser_late_parsing_default_args\n> @@ -28126,6 +28126,17 @@ function_being_declared_is_template_p (cp_parser* parser)\n> \t (current_class_type));\n> }\n> \n> +/* Return true if we are defining a class which is not a lambda closure\n> + type. */\n> +\n> +static bool\n> +defining_nonlambda_class_type_p ()\n> +{\n> + return (at_class_scope_p ()\n> +\t && TYPE_BEING_DEFINED (current_class_type)\n> +\t && !LAMBDA_TYPE_P (current_class_type));\n> +}\n> +\n> /* Parse a parameter-declaration-clause.\n> \n> parameter-declaration-clause:\n> @@ -28370,6 +28381,32 @@ cp_parser_parameter_declaration_list (cp_parser* parser,\n> \t retrofit_lang_decl (decl);\n> \t DECL_PARM_INDEX (decl) = ++index;\n> \t DECL_PARM_LEVEL (decl) = function_parm_depth ();\n> +\n> +\t if (tree &def = parameter->default_argument)\n> +\t if (TREE_CODE (def) == DEFERRED_PARSE\n> +\t\t/* If we're defining a class, let _class_specifier\n> +\t\t -> _late_parsing_default_args re-parse the default\n> +\t\t argument. */\n> +\t\t&& !defining_nonlambda_class_type_p ())\n> +\t {\n> +\t\tauto lvf\n> +\t\t = make_temp_override (parser->local_variables_forbidden_p);\n> +\t\tparser->local_variables_forbidden_p\n> +\t\t = LOCAL_VARS_AND_THIS_FORBIDDEN;\n> +\t\tpush_unparsed_function_queues (parser);\n> +\t\t/* We used to not defer parsing for default arguments outside\n> +\t\t classes, and just called _default_argument. In order to\n> +\t\t preserve mangling of lambdas (see e.g. in lambda-mangle.C),\n> +\t\t we tell this function to omit the call to start_lambda_scope\n> +\t\t otherwise the lambda scope would be the current PARM_DECL\n> +\t\t which doesn't have a context yet. Even if it had a context,\n> +\t\t it would be wrong for block-extern function declarations\n> +\t\t where the context should be the enclosing function body. */\n> +\t\tdef = cp_parser_late_parse_one_default_arg (parser, decl, def,\n> +\t\t\t\t\t\t\t TREE_TYPE (decl),\n> +\t\t\t\t\t\t\t /*lscope=*/false);\n> +\t\tpop_unparsed_function_queues (parser);\n> +\t }\n> \t}\n> \n> /* Add the new parameter to the list. */\n> @@ -28710,21 +28747,30 @@ cp_parser_parameter_declaration (cp_parser *parser,\n> eq_token = token;\n> if (declarator)\n> \tdeclarator->init_loc = token->location;\n> - /* If we are defining a class, then the tokens that make up the\n> -\t default argument must be saved and processed later. */\n> - if (!template_parm_p && at_class_scope_p ()\n> -\t && TYPE_BEING_DEFINED (current_class_type)\n> -\t && !LAMBDA_TYPE_P (current_class_type))\n> -\tdefault_argument = cp_parser_cache_defarg (parser, /*nsdmi=*/false);\n> -\n> /* A constrained-type-specifier may declare a type\n> \t template-parameter. */\n> - else if (declares_constrained_type_template_parameter (type))\n> + if (declares_constrained_type_template_parameter (type))\n> default_argument\n> = cp_parser_default_type_template_argument (parser);\n> -\n> - /* Outside of a class definition, we can just parse the\n> -\t assignment-expression. */\n> + /* If we are defining a class, then the tokens that make up the\n> +\t default argument must be saved and processed later. We also\n> +\t have to defer parsing for named parameters to correctly handle\n> +\t void fn (int i = sizeof (i));\n> +\t which is OK. */\n> + else if (!template_parm_p\n> +\t && (defining_nonlambda_class_type_p ()\n> +\t\t || (get_id_declarator (declarator)\n> +\t\t /* Optimization only: if we see \"int i = 42\",\n> +\t\t\t we don't have to defer parsing. */\n> +\t\t && !((cp_lexer_nth_token_is (parser->lexer, 2,\n> +\t\t\t\t\t\t CPP_NUMBER)\n> +\t\t\t || cp_lexer_nth_token_is (parser->lexer, 2,\n> +\t\t\t\t\t\t CPP_KEYWORD))\n> +\t\t\t && (cp_lexer_nth_token_is (parser->lexer, 3,\n> +\t\t\t\t\t\t CPP_CLOSE_PAREN)\n> +\t\t\t\t|| cp_lexer_nth_token_is (parser->lexer, 3,\n> +\t\t\t\t\t\t\t CPP_COMMA))))))\n> +\tdefault_argument = cp_parser_cache_defarg (parser, /*nsdmi=*/false);\n> else\n> \tdefault_argument\n> \t = cp_parser_default_argument (parser, template_parm_p);\n> @@ -32177,9 +32223,7 @@ cp_parser_noexcept_specification_opt (cp_parser* parser,\n> \t && !((cp_lexer_nth_token_is (parser->lexer, 3, CPP_NUMBER)\n> \t\t|| cp_lexer_nth_token_is (parser->lexer, 3, CPP_KEYWORD))\n> \t && cp_lexer_nth_token_is (parser->lexer, 4, CPP_CLOSE_PAREN))\n> -\t && at_class_scope_p ()\n> -\t && TYPE_BEING_DEFINED (current_class_type)\n> -\t && !LAMBDA_TYPE_P (current_class_type))\n> +\t && defining_nonlambda_class_type_p ())\n> \treturn cp_parser_save_noexcept (parser);\n> \n> cp_lexer_consume_token (parser->lexer);\n> @@ -37088,11 +37132,13 @@ cp_parser_save_default_args (cp_parser* parser, tree decl)\n> /* DEFAULT_ARG contains the saved tokens for the initializer of DECL,\n> which is either a FIELD_DECL or PARM_DECL. Parse it and return\n> the result. For a PARM_DECL, PARMTYPE is the corresponding type\n> - from the parameter-type-list. */\n> + from the parameter-type-list. If adjust_lambda_scope_p is true,\n> + we do start_/finish_lambda_scope. */\n> \n> static tree\n> cp_parser_late_parse_one_default_arg (cp_parser *parser, tree decl,\n> -\t\t\t\t tree default_arg, tree parmtype)\n> +\t\t\t\t tree default_arg, tree parmtype,\n> +\t\t\t\t bool adjust_lambda_scope_p/*=true*/)\n> {\n> cp_token_cache *tokens;\n> tree parsed_arg;\n> @@ -37105,14 +37151,16 @@ cp_parser_late_parse_one_default_arg (cp_parser *parser, tree decl,\n> tokens = DEFPARSE_TOKENS (default_arg);\n> cp_parser_push_lexer_for_tokens (parser, tokens);\n> \n> - start_lambda_scope (decl);\n> + if (adjust_lambda_scope_p)\n> + start_lambda_scope (decl);\n> \n> /* Parse the default argument. */\n> parsed_arg = cp_parser_initializer (parser);\n> if (BRACE_ENCLOSED_INITIALIZER_P (parsed_arg))\n> maybe_warn_cpp0x (CPP0X_INITIALIZER_LISTS);\n> \n> - finish_lambda_scope ();\n> + if (adjust_lambda_scope_p)\n> + finish_lambda_scope ();\n> \n> if (parsed_arg == error_mark_node)\n> cp_parser_skip_to_end_of_statement (parser);\n> diff --git a/gcc/testsuite/g++.dg/parse/defarg22.C b/gcc/testsuite/g++.dg/parse/defarg22.C\n> new file mode 100644\n> index 00000000000..f393b95a591\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/parse/defarg22.C\n> @@ -0,0 +1,16 @@\n> +// PR c++/50479\n> +// { dg-do compile }\n> +\n> +void fn1 (int i = sizeof (i)) {}\n> +void fn2 (int i, int = sizeof (i)) {}\n> +\n> +void\n> +g ()\n> +{\n> + void fn4 (int i = sizeof (i));\n> + void fn5 (int i, int = sizeof (i));\n> + fn1 ();\n> + fn2 (42);\n> + fn4 ();\n> + fn5 (42);\n> +}\n> diff --git a/gcc/testsuite/g++.dg/parse/defarg23.C b/gcc/testsuite/g++.dg/parse/defarg23.C\n> new file mode 100644\n> index 00000000000..0aeb1b33394\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/parse/defarg23.C\n> @@ -0,0 +1,36 @@\n> +// PR c++/50479\n> +// { dg-do compile { target c++14 } }\n> +\n> +inline void\n> +fn1 ()\n> +{\n> + int f (int i = []{ return 1; }());\n> +}\n> +\n> +auto l = [](int i = sizeof (i)) { };\n> +void fn2 (int i = []{ return 1; }()) {}\n> +void fn3 (int x = []{ decltype(x) y{}; return y; }()) {}\n> +void fn6 (int i = sizeof (i), decltype (i) = sizeof (i)) {}\n> +\n> +struct S {\n> + int mfn1 (int i = sizeof (i)) { return i; }\n> + int mfn2 (int i, int = sizeof (i)) { return i; }\n> + int lm = [](int i6 = sizeof (i6)) { return i6; }();\n> + int mfn3 (int i = []{ return 1; }()) { return i; }\n> + int i = mfn1 () + mfn2 (42) + mfn3 () + lm;\n> +};\n> +\n> +void\n> +g ()\n> +{\n> + auto ll = [](int i5 = sizeof (i5)) { };\n> + void fn4 (int i = []{ return 1; }());\n> + void fn5 (int x = []{ decltype(x) y{}; return y; }());\n> + fn1 ();\n> + fn2 ();\n> + fn3 ();\n> + fn4 ();\n> + fn5 ();\n> + fn6 ();\n> + l ();\n> +}\n> diff --git a/gcc/testsuite/g++.dg/parse/defarg24.C b/gcc/testsuite/g++.dg/parse/defarg24.C\n> new file mode 100644\n> index 00000000000..48b6f1f2a1c\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/parse/defarg24.C\n> @@ -0,0 +1,12 @@\n> +// PR c++/50479\n> +// { dg-do compile { target c++11 } }\n> +\n> +void fn1 (int x = []{ return x; }()) {}\t // { dg-error \"use of parameter outside function body\" }\n> +void fn2 (int x, int = []{ return x; }()) {} // { dg-error \"use of parameter outside function body\" }\n> +\n> +void\n> +g ()\n> +{\n> + void fn3 (int z = []{ return z; }());\t // { dg-error \"use of parameter outside function body\" }\n> + void fn4 (int z, int = []{ return z; }()); // { dg-error \"use of parameter outside function body\" }\n> +}\n> diff --git a/gcc/testsuite/g++.dg/parse/defarg25.C b/gcc/testsuite/g++.dg/parse/defarg25.C\n> new file mode 100644\n> index 00000000000..95989885dfa\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/parse/defarg25.C\n> @@ -0,0 +1,22 @@\n> +// PR c++/62244\n> +// { dg-do compile { target c++11 } }\n> +\n> +int a;\n> +void f (int a = a); // { dg-error \"parameter .a. may not appear in this context\" }\n> +\n> +int foo(int x = (decltype(x)(42))) { return 0; }\n> +\n> +static int value;\n> +\n> +int\n> +bar (int &value = value) // { dg-error \"parameter .value. may not appear in this context\" }\n> +{\n> + return value;\n> +}\n> +\n> +void\n> +g ()\n> +{\n> + bar ();\n> +}\n> +\n> diff --git a/gcc/testsuite/g++.dg/parse/defarg26.C b/gcc/testsuite/g++.dg/parse/defarg26.C\n> new file mode 100644\n> index 00000000000..170f7f5c0ec\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/parse/defarg26.C\n> @@ -0,0 +1,18 @@\n> +// PR c++/50479\n> +// { dg-do compile { target c++11 } }\n> +\n> +template\n> +struct A;\n> +\n> +template<>\n> +struct A {\n> + static const int value = 1;\n> +};\n> +\n> +long foo;\n> +\n> +void bar(char foo = A::value) {} // { dg-error \"incomplete type\" }\n> +\n> +int main() {\n> + bar();\n> +}\n> diff --git a/gcc/testsuite/g++.dg/reflect/parm1.C b/gcc/testsuite/g++.dg/reflect/parm1.C\n> index 8416be5b0db..c844c1dff3f 100644\n> --- a/gcc/testsuite/g++.dg/reflect/parm1.C\n> +++ b/gcc/testsuite/g++.dg/reflect/parm1.C\n> @@ -4,9 +4,8 @@\n> \n> #include \n> \n> -// FIXME PR62244\n> -//consteval int fn(decltype(^^::) x = ^^x) { return 0; }\n> -//constexpr int x = fn ();\n> +consteval int fn(decltype(^^::) x = ^^x) { return 0; }\n> +constexpr int x = fn ();\n> \n> consteval auto\n> ref (std::meta::info r)\n> \n> base-commit: 3a9643ffe7fea815aab8afa122a4bf12fbc5b54e","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=iiC6QzB1;\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; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.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=iiC6QzB1","sourceware.org; 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