https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229
--- Comment #12 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-11 branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:24ee44c37a28eae80430a1ad86ffb0923446db2d commit r11-9430-g24ee44c37a28eae80430a1ad86ffb0923446db2d Author: Marek Polacek <pola...@redhat.com> Date: Fri Dec 10 15:38:35 2021 -0500 c++: Allow constexpr decltype(auto) [PR102229] My r11-2202 was trying to enforce [dcl.type.auto.deduct]/4, which says "If the placeholder-type-specifier is of the form type-constraint[opt] decltype(auto), T shall be the placeholder alone." But this made us reject 'constexpr decltype(auto)', which, after clarification from CWG, should be valid. [dcl.type.auto.deduct]/4 is supposed to be a syntactic constraint, not semantic, so it's OK that the constexpr marks the object as const. As a consequence, checking TYPE_QUALS in do_auto_deduction is too late, and we have a FIXME there anyway. So in this patch I'm attempting to detect 'const decltype(auto)' earlier. If I'm going to use TYPE_QUALS, it needs to happen before we mark the object as const due to constexpr, that is, before grokdeclarator's /* A `constexpr' specifier used in an object declaration declares the object as `const'. */ if (constexpr_p && innermost_code != cdk_function) ... Constrained decltype(auto) was a little problem, hence the TYPENAME check. But in a typename context you can't use decltype(auto) anyway, I think. PR c++/102229 gcc/cp/ChangeLog: * decl.c (check_decltype_auto): New. (grokdeclarator): Call it. * pt.c (do_auto_deduction): Don't check decltype(auto) here. gcc/testsuite/ChangeLog: * g++.dg/cpp1y/decltype-auto5.C: New test. (cherry picked from commit 06d5dcef72542baf49ac245cfde2ad7ecef0916b)
