[{"id":3678169,"web_url":"http://patchwork.ozlabs.org/comment/3678169/","msgid":"","list_archive_url":null,"date":"2026-04-16T13:28:15","subject":"Re: [PATCH] c++: Reject splicing of function parameters [PR123783]","submitter":{"id":14370,"url":"http://patchwork.ozlabs.org/api/people/14370/","name":"Marek Polacek","email":"polacek@redhat.com"},"content":"On Thu, Apr 16, 2026 at 12:28:07PM +0200, Jakub Jelinek wrote:\n> Hi!\n> \n> When discussing the PR last night, I've realized that we don't reject\n> splicing of function parameters, even when\n> http://eel.is/c++draft/expr.prim.splice#2\n> doesn't list function parameters anywhere and so\n> http://eel.is/c++draft/expr.prim.splice#2.7 should apply.\n> Note, this is something that can't be tested in check_splice_expr,\n> because REFLECT_EXPR_KIND isn't propagated to it and whether some\n> reflection is a reflection of a function parameter or variable_of thereof\n> only differentiates in that kind, otherwise it is PARM_DECL in both\n> cases. So, the following patch diagnoses it in splice function.\n> And for the case where using [:variable_of(e):] could make it valid\n> a message suggesting that is emitted too.\n> In mangle3.C I had to drop one test, it is now rejected and the\n> test tests mangling, so better shouldn't have errors in it.\n> For the dep14.C case, I've applied variable_of on it so that we get\n> the previous behavior. And in func2 there it was actually (I think\n> mistakenly) asking for paramete of func rather than func2, fixed that too.\n> \n> Tested on x86_64-linux, ok for trunk if it passes full bootstrap/regtest?\n> \n> 2026-04-16 Jakub Jelinek \n> \n> \tPR c++/123783\n> \t* reflect.cc (splice): Reject splicing of function parameters.\n> \n> \t* g++.dg/reflect/mangle3.C: Remove f5()\n> \tcase.\n> \t* g++.dg/reflect/dep14.C: Include .\n> \t(func): Apply variable_of to each vector member.\n> \t(func2): Likewise. Use ^^func2 instead of ^^func.\n> \t* g++.dg/reflect/splice12.C: New test.\n> \n> --- gcc/cp/reflect.cc.jj\t2026-04-15 08:52:04.297374316 +0200\n> +++ gcc/cp/reflect.cc\t2026-04-16 11:39:30.763707538 +0200\n> @@ -8510,6 +8510,18 @@ splice (tree refl)\n> return error_mark_node;\n> }\n> \n> + if (REFLECT_EXPR_KIND (refl) == REFLECT_PARM)\n> + {\n> + auto_diagnostic_group d;\n> + error_at (cp_expr_loc_or_input_loc (refl),\n> +\t\t\"cannot use a function parameter reflection in a splice \"\n> +\t\t\"expression\");\n> + if (DECL_CONTEXT (REFLECT_EXPR_HANDLE (refl)) == current_function_decl)\n> +\tinform (cp_expr_loc_or_input_loc (refl),\n> +\t\t\"apply % on it before splicing\");\n> + return error_mark_node;\n> + }\n> +\n\nLooks ok. Optionally: \n- add a comment saying why this isn't in check_splice_expr\n- we could also have a location_t for the cp_expr_loc_or_input_loc\n since it's used 3x in the function now\n- could add %qD in the error message since that's what check_splice_expr\n does\n\n> /* We are bringing some entity from the unevaluated expressions world\n> to possibly outside of that, mark it used. */\n> if (!mark_used (REFLECT_EXPR_HANDLE (refl)))\n> --- gcc/testsuite/g++.dg/reflect/mangle3.C.jj\t2026-04-15 09:57:58.778962876 +0200\n> +++ gcc/testsuite/g++.dg/reflect/mangle3.C\t2026-04-16 11:47:13.739937787 +0200\n> @@ -175,8 +175,6 @@ g (int p)\n> // { dg-final { scan-assembler \"_Z2f5ILDmvlLi42EEEDtDST_EEv\" } }\n> f5();\n> // { dg-final { scan-assembler \"_Z2f5ILDmobixL_Z3arrEL\\[ilx]1EEEDtDST_EEv\" } }\n> - f5();\n> -// { dg-final { scan-assembler \"_Z2f5ILDmpa_1giEEDtDST_EEv\" } }\n> f5();\n> // { dg-final { scan-assembler \"_Z2f5ILDmba_1SEEDtDST_EEv\" } }\n> f6(Y{42});\n> --- gcc/testsuite/g++.dg/reflect/dep14.C.jj\t2026-04-15 21:28:31.118011956 +0200\n> +++ gcc/testsuite/g++.dg/reflect/dep14.C\t2026-04-16 12:10:13.009767880 +0200\n> @@ -3,6 +3,7 @@\n> // { dg-additional-options \"-freflection\" }\n> \n> #include \n> +#include \n> \n> namespace __impl {\n> template\n> @@ -30,7 +31,7 @@ int func(int counter, float factor) {\n> std::array args;\n> \n> std::size_t i = 0;\n> - [:expand(parameters_of(^^func)):] >> [&i, &counter, &factor, &args]\n> + [:expand(parameters_of(^^func) | std::views::transform(std::meta::variable_of)):] >> [&i, &counter, &factor, &args]\n> {\n> args[i++] = &[:e:]; // { dg-error \"use of local variable\" }\n> };\n> @@ -38,10 +39,10 @@ int func(int counter, float factor) {\n> }\n> \n> int func2(int counter, float factor) {\n> - std::array args;\n> + std::array args;\n> \n> std::size_t i = 0;\n> - [:expand(parameters_of(^^func)):] >> [&]\n> + [:expand(parameters_of(^^func2) | std::views::transform(std::meta::variable_of)):] >> [&]\n> {\n> args[i++] = &[:e:]; // { dg-error \"use of local variable|trying to capture \\[^\\n\\r]* in instantiation of generic lambda\" }\n> };\n> --- gcc/testsuite/g++.dg/reflect/splice12.C.jj\t2026-04-16 11:53:29.075638834 +0200\n> +++ gcc/testsuite/g++.dg/reflect/splice12.C\t2026-04-16 12:05:05.248940576 +0200\n> @@ -0,0 +1,21 @@\n> +// PR c++/123783\n> +// { dg-do compile { target c++26 } }\n> +// { dg-additional-options \"-freflection\" }\n> +\n> +#include \n> +\n> +void\n> +bar (int a)\n> +{\n> +}\n> +\n> +void\n> +foo (int a)\n> +{\n> + auto b = &[: ^^a :];\n> + auto c = &[: variable_of (parameters_of (^^foo)[0]) :];\n> + auto d = &[: parameters_of (^^foo)[0] :];\t\t\t// { dg-error \"cannot use a function parameter reflection in a splice expression\" }\n> +\t\t\t\t\t\t\t\t// { dg-message \"apply 'std::meta::variable_of' on it before splicing\" \"\" { target *-*-* } .-1 }\n> + auto e = &[: parameters_of (^^bar)[0] :];\t\t\t// { dg-error \"cannot use a function parameter reflection in a splice expression\" }\n> + auto f = [] { auto g = &[: parameters_of (^^foo)[0] :]; };\t// { dg-error \"cannot use a function parameter reflection in a splice expression\" }\n> +}\n> \n> \tJakub\n> \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=QWDwOLbf;\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=QWDwOLbf","sourceware.org; dmarc=pass (p=quarantine dis=none)\n header.from=redhat.com","sourceware.org; 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