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.

0 files changed, 0 insertions, 0 deletions