[Bug c++/92687] decltype of a structured binding to a tuple component is a reference type inside a template function

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92687

--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:867cbadb912ab75d0eaf919a3f992595e508482b

commit r14-9258-g867cbadb912ab75d0eaf919a3f992595e508482b
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Mar 1 16:59:08 2024 +0100

    c++: Fix up decltype of non-dependent structured binding decl in template
[PR92687]

    finish_decltype_type uses DECL_HAS_VALUE_EXPR_P (expr) check for
    DECL_DECOMPOSITION_P (expr) to determine if it is
    array/struct/vector/complex etc. subobject proxy case vs. structured
    binding using std::tuple_{size,element}.
    For non-templates or when templates are already instantiated, that works
    correctly, finalized DECL_DECOMPOSITION_P non-base vars indeed have
    DECL_VALUE_EXPR in the former case and don't have it in the latter.
    It works fine for dependent structured bindings as well, cp_finish_decomp
in
    that case creates DECLTYPE_TYPE tree and defers the handling until
    instantiation.
    As the testcase shows, this doesn't work for the non-dependent structured
    binding case in templates, because DECL_HAS_VALUE_EXPR_P is set in that
case
    always; cp_finish_decomp ends with:
      if (processing_template_decl)
        {
          for (unsigned int i = 0; i < count; i++)
            if (!DECL_HAS_VALUE_EXPR_P (v[i]))
              {
                tree a = build_nt (ARRAY_REF, decl, size_int (i),
                                   NULL_TREE, NULL_TREE);
                SET_DECL_VALUE_EXPR (v[i], a);
                DECL_HAS_VALUE_EXPR_P (v[i]) = 1;
              }
        }
    and those artificial ARRAY_REFs are used in various places during
    instantiation to find out what base the DECL_DECOMPOSITION_P VAR_DECLs
    have and their positions.

    The following patch fixes that by changing lookup_decomp_type, such that
    it doesn't ICE when called on a DECL_DECOMPOSITION_P var which isn't in a
    hash table, but returns NULL_TREE in that case, and for
processing_template_decl
    asserts DECL_HAS_VALUE_EXPR_P is non-NULL and just calls
lookup_decomp_type.
    If it returns non-NULL, it is a structured binding using tuple and its
result
    is returned, otherwise it falls through to returning unlowered_expr_type
(expr)
    because it is an array, structure etc. subobject proxy.
    For !processing_template_decl it keeps doing what it did before,
    DECL_HAS_VALUE_EXPR_P meaning it is an array/structure etc. subobject
proxy,
    otherwise the tuple case.

    2024-03-01  Jakub Jelinek  <jakub@redhat.com>

            PR c++/92687
            * decl.cc (lookup_decomp_type): Return NULL_TREE if
decomp_type_table
            doesn't have entry for V.
            * semantics.cc (finish_decltype_type): If ptds.saved, assert
            DECL_HAS_VALUE_EXPR_P is true and decide on tuple vs. non-tuple
based
            on if lookup_decomp_type is NULL or not.

            * g++.dg/cpp1z/decomp59.C: New test.