[Bug c++/99130] variable template in unevaluated context is instantiated when should not be

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99130

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:4db3cb781c355341fa041e6b5bbbfc495c6a0fdb

commit r13-2540-g4db3cb781c355341fa041e6b5bbbfc495c6a0fdb
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Sep 8 09:45:45 2022 -0400

    c++: unnecessary instantiation of constexpr var [PR99130]

    Here the use of 'value' from within an unevaluated context causes us
    to overeagerly instantiate it, via maybe_instantiate_decl called from
    mark_used, despite the use occurring in a context that doesn't require
    a definition.

    This seems to only affect constexpr variable specializations, though
    we used to have the same issue for constexpr function specializations
    until r6-1309-g81371eff9bc7ef made us delay their instantiation until
    necessary during constexpr evaluation.

    This patch expands upon the r6-1309 fix to make mark_used avoid
    unnecessarily instantiating constexpr variable specializations too,
    by pulling out from maybe_instantiate_decl the condition

      (decl_maybe_constant_var_p (decl)
       || (TREE_CODE (decl) == FUNCTION_DECL
           && DECL_OMP_DECLARE_REDUCTION_P (decl))
       || undeduced_auto_decl (decl))

    into each of its three callers (including mark_used), removing the
    problematic first test from mark_used, and simplifying accordingly.
    The net result is that only mark_used is changed because the other two
    callers, resolve_address_of_overloaded_function and decl_constant_var_p,
    already guard the call appropriately.  (This relaxation of mark_used
    seems to be safe because during constexpr evaluation we already take
    care to instantiate a constexpr variable as necessary via
    decl_constant_value etc).

            PR c++/99130

    gcc/cp/ChangeLog:

            * decl2.cc (maybe_instantiate_decl): Adjust function comment.
            Check VAR_OR_FUNCTION_DECL_P.  Pull out the disjunction into ...
            (mark_used): ... here, removing the decl_maybe_constant_var_p
            part of it.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/constexpr-decltype5.C: New test.