tree: Fix up wrong-code with certain C++ default arguments [PR123818]

The following testcase is miscompiled since r0-69852-g4038c495f (at least
if one can somehow arrange in C++98 to have AGGR_INIT_EXPR or any other
FE specific trees nested in constructor elts for CONSTRUCTOR nested inside
of default argument, if not, then since C++11 support that allows that has
been implemented).
The problem is that we unfortunately store default arguments in TREE_PURPOSE
of TYPE_ARG_TYPES nodes of the FUNCTION/METHOD_TYPE and those are shared,
with type_hash_canon used to unify them.  The default arguments aren't
considered in type_hash_canon_hash at all, but the equality hook considers
them in type_list_equal by calling simple_cst_equal on those default
arguments.  That function is a tri-state, returns 0 for surely unequal,
1 for equal and -1 for "I don't know", usually because there are FE trees
etc. (though, admittedly it is unclear why such distinction is done, as
e.g. for VAR_DECLs etc. it sees in there it returns 0 rather than -1).
Anyway, the r0-69852-g4038c495f change changed CONSTRUCTOR_ELTS from
I think a tree list of elements to a vector and changed the simple_cst_equal
implementation of CONSTRUCTOR from just recursing on CONSTRUCTOR_ELTS (which
I think would just return -1 because I don't see simple_cst_equal having
TREE_LIST nor TREE_VEC handling) to something that thinks simple_cst_equal
returns a bool rather than tri-state, plus has a comment whether it should
handle indexes too.
So, in particular on the testcase which has in default arguments with
CONSTRUCTOR inside it with AGGR_INIT_EXPR as the single element (but
could be as well in multiple elements), the recursive call returns -1
for "I don't know" on the element and the function considers it the same
as if it returned 1, they are equal.

The following patch fixes it by calling the recursive non tail-recursive
simple_cst_equal calls like everywhere else in the function, by returning
what it returned if it returned <= 0 and otherwise continuing.
Plus given the comment I've also implemented checking the index.
The special case for FIELD_DECL is so that if both indexes are FIELD_DECLs
and they are different, we don't return -1 but 0.

2026-02-04  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123818
	* tree.cc (simple_cst_equal) <case CONSTRUCTOR>: Return -1 if some
	recursive call returns -1.  Also compare indexes.

	* g++.dg/cpp0x/pr123818.C: New test.

0 files changed, 0 insertions, 0 deletions