CWG 2707 and CWG 2913: trailing requires-clause on a deduction guide

[stevensudit-corvid]

Issue Summary

Core issues 2707 and 2913 allow a requires-clause at the end of a deduction guide, after the simple-template-id. The front end does not parse that clause yet and reports a missing semicolon. Both issues are recent defect reports, so I am reporting this as something not implemented yet.

gcc and clang accept the code. The workaround for cpfe is to write the same constraint in the template head.

Reproducing Source Code

template<class T> concept A = true;
template<class T> struct w {};
template<class T> w(T) -> w<T> requires A<T>;

Command-line Options

cpfe --c++23 repro.cpp

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

The diagnostic, at 158320e (version 7.0):

"repro.cpp", line 3: error: expected a ";"
  template<class T> w(T) -> w<T> requires A<T>;
                                 ^

--c++20 and --c++26 give the same error, and so do the GNU, Clang and Microsoft modes.

The grammar changed twice. Core issue 2707, accepted as a defect report in March 2024, added the optional requires-clause before the arrow. Core issue 2913, accepted as a defect report in November 2024, moved it after the simple-template-id, to match function declarations. The current working draft has:

deduction-guide:
    explicit-specifier(opt) simple-template-name ( parameter-declaration-clause ) -> simple-template-id requires-clause(opt) ;

The template-head spelling of the same guide compiles:

template<class T> requires A<T> w(T) -> w<T>;

In the source, a guide's declarator is scanned by a direct call to function_declarator: templates.c:20809 in scan_template_declaration for a guide template, and decls.c:21759 in scan_deduction_guide for a non-template guide. The trailing requires-clause of a function is scanned at the end of declarator (declarator.c:9147), which the guide path does not go through, so the requires keyword is left for the caller's semicolon check. deduction_guides_are_redeclarations (symbol_tbl.c:7306) already compares the trailing requires-clauses of two guides.

Reproducing (Standard) Configuration(s)

linux-gcc-release

Reproducing (Non-standard) Configuration

No response

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