Here ever since r10-7313-gb599bf9d6d1e18, reduced_constant_expression_p
in C++11/14 is rejecting the marked sub-aggregate initializer (of type S)
W w = {.D.2445={.s={.D.2387={.m=0}, .b=0}}};
^
ultimately because said initializer has CONSTRUCTOR_NO_CLEARING set,
hence the function must verify that all fields of S are initialized.
And before C++17 it doesn't expect to see base class fields (since
next_initializable_field skips over them), so the presence thereof
causes r_c_e_p to return false.
The reason r10-7313-gb599bf9d6d1e18 causes this is because in that
commit we began using CONSTRUCTOR_NO_CLEARING to precisely track whether
we're in middle of activating a union member. This ends up affecting
clear_no_implicit_zero, which recurses into sub-aggregate initializers
only if the outer initializer has CONSTRUCTOR_NO_CLEARING set. After
that commit, the outer union initializer above no longer has the flag
set at this point and so clear_no_implicit_zero no longer recurses into
the marked inner initializer.
But arguably r_c_e_p should be able to accept the marked initializer
regardless of whether CONSTRUCTOR_NO_CLEARING is set. The primary bug
therefore seems to be that r_c_e_p relies on next_initializable_field
which skips over base class fields in C++11/14. To fix this, this patch
introduces a new helper function next_subobject_field which is like
next_initializable_field except that it never skips base class fields,
and makes r_c_e_p use it. This patch then renames next_initializable_field
to next_aggregate_field (and makes it skip over vptr fields again).
NB: This minimal backport of r13-211-g0c7bce0ac184c0 for 12.2 just adds
next_subobject_field and makes reduced_constant_expression_p use it.
PR c++/105491
gcc/cp/ChangeLog:
* constexpr.cc (reduced_constant_expression_p): Use
next_subobject_field instead.
* cp-tree.h (next_subobject_field): Declare.
* decl.cc (next_subobject_field): Define.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-union7.C: New test.
* g++.dg/cpp0x/constexpr-union7a.C: New test.
* g++.dg/cpp2a/constinit17.C: New test.
(cherry picked from commit
0c7bce0ac184c057bacad9c8e615ce82923835fd)
return field;
}
+/* FIELD is an element of TYPE_FIELDS or NULL. In the former case, the value
+ returned is the next FIELD_DECL (possibly FIELD itself) that corresponds
+ to a subobject. If there are no more such fields, the return value will be
+ NULL. */
+
+tree
+next_subobject_field (tree field)
+{
+ while (field
+ && (TREE_CODE (field) != FIELD_DECL
+ || DECL_UNNAMED_BIT_FIELD (field)
+ || (DECL_ARTIFICIAL (field)
+ && !DECL_FIELD_IS_BASE (field)
+ && !DECL_VIRTUAL_P (field))))
+ field = DECL_CHAIN (field);
+
+ return field;
+}
+
/* Return true for [dcl.init.list] direct-list-initialization from
single element of enumeration with a fixed underlying type. */