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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id s15-20020a05620a29cf00b006fbbffd08f9sm548959qkp.87.2022.11.17.06.42.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 17 Nov 2022 06:42:19 -0800 (PST) Message-ID: <740b5e1e-7143-c291-5594-af937867fbc3@redhat.com> Date: Thu, 17 Nov 2022 09:42:18 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: [PATCH] c++, v3: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions To: Jakub Jelinek Cc: Jonathan Wakely , gcc-patches@gcc.gnu.org, Marek Polacek References: <016f168b-f143-baff-5f71-c48d4611ae11@redhat.com> From: Jason Merrill In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-6.4 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 11/17/22 04:13, Jakub Jelinek wrote: > On Wed, Nov 16, 2022 at 03:26:32PM -0500, Jason Merrill wrote: >> On 11/16/22 09:46, Jakub Jelinek wrote: >>> On Wed, Nov 16, 2022 at 09:33:27AM -0500, Jason Merrill wrote: >>>>> and at that point I fear decl_maybe_constant_var_p will not work >>>>> properly. Shall this hunk be moved somewhere else (cp_finish_decl?) >>>>> where we can already call it, or do the above in start_decl for >>>>> cxx_dialect < cxx20 and add a cxx_dialect == cxx20 hunk in cp_finish_decl? >>>> >>>> Hmm, I'd expect decl_maybe_constant_var_p to work fine at this point. >>> >>> For static constexpr vars sure, but what about >>> static const where start_decl doesn't know the initializer? >>> Sure, decl_maybe_constant_var_p will not crash in that case, but >>> it will return true even if the static const var doesn't have >>> a constant initializer. Sure, we'd catch that later on when actually >>> trying to constexpr evaluate the function and hitting there the >>> spots added for C++23 in potential_constant_expression*/cxx_eval_*, >>> but it would mean that we don't reject it when nothing calls the functions. >>> >>> I meant something like: >>> constexpr int bar (int x) { if (x) throw 1; return 0; } >>> constexpr int foo () { static const int a = bar (1); return 0; } >>> with -std=c++20 IMHO shouldn't be accepted, while in C++23 it should. >> >> I'd expect us to reject that in C++20 in potential_constant_expression, but >> it's a fair point; it is awkward that P2242 wasn't also accepted as a DR. I pointed out that inconsistency on the reflectors, and it seems that EWG didn't actually intend P2647 to be a DR, either; it was a CWG error to propose it as such. But let's go ahead and build on the work you've done. >> Moving the check from start_decl to cp_finish_decl makes sense to me. > > So like this? I had to outline the check from start_decl to a function > because it is needed in cp_finish_decl in two different places (the > processing_template_decl path and at the end and it can't be done before the > processing_template_decl spot, because the initializer isn't finalized at > that point for !procesing_template_decl). Also, decl_maybe_constant_var_p > doesn't do what is needed when !processing_template_decl, but I think > we want decl_maybe_constant_var_p in templates so that we don't instantiate > anything. > > Lightly tested so far, ok for trunk if it passes full bootstrap/regtest? > > 2022-11-17 Jakub Jelinek > > gcc/c-family/ > * c-cppbuiltin.cc (c_cpp_builtins): Bump __cpp_constexpr > value from 202207L to 202211L. > gcc/cp/ > * constexpr.cc (cxx_eval_constant_expression): Implement C++23 > P2647R1 - Permitting static constexpr variables in constexpr functions. > Allow decl_constant_var_p static or thread_local vars for > C++20 and later. > (potential_constant_expression_1): For C++20 or later, allow > static or thread_local decl_maybe_constant_var_p vars, for > !processing_template_decl only decl_constant_var_p vars. > * decl.cc (diagnose_static_in_constexpr): New function. > (start_decl): Use it for C++17 or earlier. > (cp_finish_decl): Call it for C++20. > gcc/testsuite/ > * g++.dg/cpp23/constexpr-nonlit17.C: New test. > * g++.dg/cpp23/constexpr-nonlit18.C: New test. > * g++.dg/cpp23/feat-cxx2b.C: Adjust expected __cpp_constexpr > value. > * g++.dg/ext/stmtexpr19.C: Don't expect an error for C++20 or later. > > --- gcc/c-family/c-cppbuiltin.cc.jj 2022-11-17 09:00:42.106249011 +0100 > +++ gcc/c-family/c-cppbuiltin.cc 2022-11-17 09:01:49.286320527 +0100 > @@ -1074,7 +1074,7 @@ c_cpp_builtins (cpp_reader *pfile) > /* Set feature test macros for C++23. */ > cpp_define (pfile, "__cpp_size_t_suffix=202011L"); > cpp_define (pfile, "__cpp_if_consteval=202106L"); > - cpp_define (pfile, "__cpp_constexpr=202207L"); > + cpp_define (pfile, "__cpp_constexpr=202211L"); > cpp_define (pfile, "__cpp_multidimensional_subscript=202211L"); > cpp_define (pfile, "__cpp_named_character_escapes=202207L"); > cpp_define (pfile, "__cpp_static_call_operator=202207L"); > --- gcc/cp/constexpr.cc.jj 2022-11-17 08:48:30.530357181 +0100 > +++ gcc/cp/constexpr.cc 2022-11-17 09:56:50.479522863 +0100 > @@ -7098,7 +7098,8 @@ cxx_eval_constant_expression (const cons > && (TREE_STATIC (r) > || (CP_DECL_THREAD_LOCAL_P (r) && !DECL_REALLY_EXTERN (r))) > /* Allow __FUNCTION__ etc. */ > - && !DECL_ARTIFICIAL (r)) > + && !DECL_ARTIFICIAL (r) > + && (cxx_dialect < cxx20 || !decl_constant_var_p (r))) I don't think we need to check cxx_dialect here since diagnose_static_in_constexpr will have already complained. > { > if (!ctx->quiet) > { > @@ -9588,7 +9589,12 @@ potential_constant_expression_1 (tree t, > tmp = DECL_EXPR_DECL (t); > if (VAR_P (tmp) && !DECL_ARTIFICIAL (tmp)) > { > - if (CP_DECL_THREAD_LOCAL_P (tmp) && !DECL_REALLY_EXTERN (tmp)) > + if (CP_DECL_THREAD_LOCAL_P (tmp) > + && !DECL_REALLY_EXTERN (tmp) > + && (cxx_dialect < cxx20 > + || (processing_template_decl > + ? !decl_maybe_constant_var_p (tmp) > + : !decl_constant_var_p (tmp)))) Or here. > { > if (flags & tf_error) > constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p, > @@ -9596,7 +9602,11 @@ potential_constant_expression_1 (tree t, > "% context", tmp); > return false; > } > - else if (TREE_STATIC (tmp)) > + else if (TREE_STATIC (tmp) > + && (cxx_dialect < cxx20 > + || (processing_template_decl > + ? !decl_maybe_constant_var_p (tmp) > + : !decl_constant_var_p (tmp)))) > { > if (flags & tf_error) > constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p, > --- gcc/cp/decl.cc.jj 2022-11-16 14:44:43.692339668 +0100 > +++ gcc/cp/decl.cc 2022-11-17 09:54:30.556482833 +0100 > @@ -5600,6 +5600,41 @@ groktypename (cp_decl_specifier_seq *typ > return type; > } > > +/* For C++17 and older diagnose static or thread_local decls in constexpr > + or consteval functions. For C++20 similarly, except if they are > + usable in constant expressions. */ > + > +static void > +diagnose_static_in_constexpr (tree decl) > +{ > + if (current_function_decl && VAR_P (decl) > + && DECL_DECLARED_CONSTEXPR_P (current_function_decl) > + && cxx_dialect < cxx23 > + && (cxx_dialect < cxx20 > + || (processing_template_decl > + ? !decl_maybe_constant_var_p (decl) > + : !decl_constant_var_p (decl)))) For (maybe) constant variables let's make this error a pedwarn in C++20 and below. > + { > + bool ok = false; > + if (CP_DECL_THREAD_LOCAL_P (decl) && !DECL_REALLY_EXTERN (decl)) > + error_at (DECL_SOURCE_LOCATION (decl), > + "%qD defined % in %qs function only " > + "available with %<-std=c++2b%> or %<-std=gnu++2b%>", decl, > + DECL_IMMEDIATE_FUNCTION_P (current_function_decl) > + ? "consteval" : "constexpr"); > + else if (TREE_STATIC (decl)) > + error_at (DECL_SOURCE_LOCATION (decl), > + "%qD defined % in %qs function only available " > + "with %<-std=c++2b%> or %<-std=gnu++2b%>", decl, > + DECL_IMMEDIATE_FUNCTION_P (current_function_decl) > + ? "consteval" : "constexpr"); > + else > + ok = true; > + if (!ok) > + cp_function_chain->invalid_constexpr = true; > + } > +} > + > /* Process a DECLARATOR for a function-scope or namespace-scope > variable or function declaration. > (Function definitions go through start_function; class member > @@ -5860,28 +5895,8 @@ start_decl (const cp_declarator *declara > DECL_THIS_STATIC (decl) = 1; > } > > - if (current_function_decl && VAR_P (decl) > - && DECL_DECLARED_CONSTEXPR_P (current_function_decl) > - && cxx_dialect < cxx23) > - { > - bool ok = false; > - if (CP_DECL_THREAD_LOCAL_P (decl) && !DECL_REALLY_EXTERN (decl)) > - error_at (DECL_SOURCE_LOCATION (decl), > - "%qD defined % in %qs function only " > - "available with %<-std=c++2b%> or %<-std=gnu++2b%>", decl, > - DECL_IMMEDIATE_FUNCTION_P (current_function_decl) > - ? "consteval" : "constexpr"); > - else if (TREE_STATIC (decl)) > - error_at (DECL_SOURCE_LOCATION (decl), > - "%qD defined % in %qs function only available " > - "with %<-std=c++2b%> or %<-std=gnu++2b%>", decl, > - DECL_IMMEDIATE_FUNCTION_P (current_function_decl) > - ? "consteval" : "constexpr"); > - else > - ok = true; > - if (!ok) > - cp_function_chain->invalid_constexpr = true; > - } > + if (cxx_dialect < cxx20) > + diagnose_static_in_constexpr (decl); Can we drop this call (and make the ones in cp_finish_decl unconditional)? > > if (!processing_template_decl && VAR_P (decl)) > start_decl_1 (decl, initialized); > @@ -8424,6 +8439,10 @@ cp_finish_decl (tree decl, tree init, bo > set_user_assembler_name (decl, asmspec); > DECL_HARD_REGISTER (decl) = 1; > } > + > + if (cxx_dialect == cxx20) > + diagnose_static_in_constexpr (decl); > + > return; > } > > @@ -8749,6 +8768,9 @@ cp_finish_decl (tree decl, tree init, bo > && !DECL_HARD_REGISTER (decl)) > targetm.lower_local_decl_alignment (decl); > > + if (cxx_dialect == cxx20) > + diagnose_static_in_constexpr (decl); > + > invoke_plugin_callbacks (PLUGIN_FINISH_DECL, decl); > } > > --- gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C.jj 2022-11-17 09:00:42.108248984 +0100 > +++ gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C 2022-11-17 09:00:42.108248984 +0100 > @@ -0,0 +1,12 @@ > +// P2647R1 - Permitting static constexpr variables in constexpr functions > +// { dg-do compile { target c++20 } } > + > +constexpr char > +test () > +{ > + static const int x = 5; > + static constexpr char c[] = "Hello World"; > + return *(c + x); > +} > + > +static_assert (test () == ' '); > --- gcc/testsuite/g++.dg/cpp23/constexpr-nonlit18.C.jj 2022-11-17 09:29:45.776136195 +0100 > +++ gcc/testsuite/g++.dg/cpp23/constexpr-nonlit18.C 2022-11-17 10:04:32.894045579 +0100 > @@ -0,0 +1,49 @@ > +// P2647R1 - Permitting static constexpr variables in constexpr functions > +// { dg-do compile { target c++14 } } > + > +constexpr int > +f1 (int x) > +{ > + if (x) > + throw 1; > + return 0; > +} > + > +constexpr int > +f2 () > +{ > + static const int a = f1 (1); // { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } } > + return 0; > +} > + > +constexpr int > +f3 () > +{ > + static const int a = 5; // { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++17_down } } > + return 0; > +} > + > +constexpr int > +f4 () // { dg-message "declared here" "" { target c++20_down } } > +{ // { dg-message "is not usable as a 'constexpr' function because:" "" { target c++23 } .-1 } > + static const int a = f1 (1); // { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } } > + return 0; // { dg-error "'a' defined 'static' in 'constexpr' context" "" { target c++23 } .-1 } > +} > + > +constexpr int a4 = f4 (); // { dg-error "called in a constant expression" } > + > +constexpr int > +f5 () > +{ > + static const int a = f1 (0); // { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++17_down } } > + return 0; > +} > + > +constexpr int > +f6 () // { dg-message "declared here" "" { target c++17_down } } > +{ > + static const int a = f1 (0); // { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++17_down } } > + return 0; > +} > + > +constexpr int a6 = f6 (); // { dg-error "called in a constant expression" "" { target c++17_down } } > --- gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C.jj 2022-11-17 08:48:30.561356753 +0100 > +++ gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C 2022-11-17 09:00:42.108248984 +0100 > @@ -134,8 +134,8 @@ > > #ifndef __cpp_constexpr > # error "__cpp_constexpr" > -#elif __cpp_constexpr != 202207 > -# error "__cpp_constexpr != 202207" > +#elif __cpp_constexpr != 202211 > +# error "__cpp_constexpr != 202211" > #endif > > #ifndef __cpp_decltype_auto > --- gcc/testsuite/g++.dg/ext/stmtexpr19.C.jj 2022-11-17 08:48:02.730741221 +0100 > +++ gcc/testsuite/g++.dg/ext/stmtexpr19.C 2022-11-17 09:00:42.109248970 +0100 > @@ -8,7 +8,7 @@ const test* setup() > { > static constexpr test atest = > { > - ({ static const int inner = 123; &inner; }) // { dg-error "static" } > + ({ static const int inner = 123; &inner; }) // { dg-error "static" "" { target c++17_down } } > }; > > return &atest; > > > Jakub >