Bug 100974

Summary: [C++23] Implement if consteval
Product: gcc Reporter: Jason Merrill <jason>
Component: c++Assignee: Jakub Jelinek <jakub>
Severity: normal CC: jakub, mpolacek, webrown.cpp
Priority: P3    
Version: 12.0   
Target Milestone: ---   
Host: Target:
Build: Known to work:
Known to fail: Last reconfirmed: 2021-06-08 00:00:00
Bug Depends on:    
Bug Blocks: 98940    
Attachments: gcc12-pr100974.patch

Description Jason Merrill 2021-06-08 17:43:41 UTC
Comment 1 Jakub Jelinek 2021-06-09 11:58:47 UTC
I have this roughly implemented, adding testcases now.
Comment 2 Jakub Jelinek 2021-06-09 14:29:09 UTC
Created attachment 50971 [details]

Untested implementation.
Comment 3 CVS Commits 2021-06-11 13:53:04 UTC
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:


commit r12-1390-g117c64266405e244da4dae3ae7b60905af63b955
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jun 11 15:50:34 2021 +0200

    c++: Add C++23 consteval if support - P1938R3 [PR100974]
    The following patch implements consteval if support.
    There is a new IF_STMT_CONSTEVAL_P flag on IF_STMT and IF_COND is
    boolean_false_node to match the non-manifestly constant evaluation
    behavior, while constexpr evaluation special-cases it.  Perhaps cleaner
    would be to set the condition to __builtin_is_constant_evaluated () call
    but we need the IF_STMT_CONSTEVAL_P flag anyway and the IL would be larger.
    And I'm not changing the libstdc++ side, where perhaps we could change
    std::is_constant_evaluated definition for
     #ifdef __cpp_if_consteval
    case to if consteval { return true; } else { return false; }
    but we need to keep it defined to __builtin_is_constant_evaluated ()
    for C++20 or older.
    2021-06-11  Jakub Jelinek  <jakub@redhat.com>
            PR c++/100974
            * c-cppbuiltin.c (c_cpp_builtins): Predefine __cpp_if_consteval for
            -std=c++2b for P1938R3 consteval if support.
            * cp-tree.h (struct saved_scope): Add consteval_if_p
            member.  Formatting fix for the discarded_stmt comment.
            (in_consteval_if_p, IF_STMT_CONSTEVAL_P): Define.
            * parser.c (cp_parser_lambda_expression): Temporarily disable
            in_consteval_if_p when parsing lambda body.
            (cp_parser_selection_statement): Parse consteval if.
            * decl.c (struct named_label_entry): Add in_consteval_if member.
            (level_for_consteval_if): New function.
            (poplevel_named_label_1, check_previous_goto_1, check_goto): Handle
            consteval if.
            * constexpr.c (cxx_eval_builtin_function_call): Clarify in comment
            why CP_BUILT_IN_IS_CONSTANT_EVALUATED needs to *non_constant_p
            for !ctx->manifestly_const_eval.
            (cxx_eval_conditional_expression): For IF_STMT_CONSTEVAL_P evaluate
            condition as if it was __builtin_is_constant_evaluated call.
            (potential_constant_expression_1): For IF_STMT_CONSTEVAL_P always
            recurse on both branches.
            * cp-gimplify.c (genericize_if_stmt): Genericize IF_STMT_CONSTEVAL_P
            as the else branch.
            * pt.c (tsubst_expr) <case IF_STMT>: Copy IF_STMT_CONSTEVAL_P.
            Temporarily set in_consteval_if_p when recursing on
            IF_STMT_CONSTEVAL_P then branch.
            (tsubst_lambda_expr): Temporarily disable
            in_consteval_if_p when instantiating lambda body.
            * call.c (immediate_invocation_p): Return false when
            * g++.dg/cpp23/consteval-if1.C: New test.
            * g++.dg/cpp23/consteval-if2.C: New test.
            * g++.dg/cpp23/consteval-if3.C: New test.
            * g++.dg/cpp23/consteval-if4.C: New test.
            * g++.dg/cpp23/consteval-if5.C: New test.
            * g++.dg/cpp23/consteval-if6.C: New test.
            * g++.dg/cpp23/consteval-if7.C: New test.
            * g++.dg/cpp23/consteval-if8.C: New test.
            * g++.dg/cpp23/consteval-if9.C: New test.
            * g++.dg/cpp23/consteval-if10.C: New test.
            * g++.dg/cpp23/feat-cxx2b.C: Add __cpp_if_consteval tests.
Comment 4 Jakub Jelinek 2021-06-11 14:02:05 UTC
Implemented for GCC 12.