c++: ICE with ()-init and TARGET_EXPR eliding [PR116424]

c++: ICE with ()-init and TARGET_EXPR eliding [PR116424]

Here we crash on a cp_gimplify_expr/TARGET_EXPR assert:

      gcc_checking_assert (!TARGET_EXPR_ELIDING_P (*expr_p)
                           || !TREE_ADDRESSABLE (TREE_TYPE (*expr_p)));

We cannot elide the TARGET_EXPR because we're taking its address.

It is set as eliding in massage_init_elt.  I've tried to not set
TARGET_EXPR_ELIDING_P when the context is not direct-initialization.
That didn't work: even when it's not direct-initialization now, it
can become one later, for instance, after split_nonconstant_init.
One problem is that replace_placeholders_for_class_temp_r will replace
placeholders in non-eliding TARGET_EXPRs with the slot, but if we then
elide the TARGET_EXPR, we end up with a "stray" VAR_DECL and crash.
(Only some TARGET_EXPRs are handled by replace_decl.)

I thought I'd have to go back to
<https://gcc.gnu.org/pipermail/gcc-patches/2024-May/651163.html> but
then I realized that this problem occurrs only with ()-init but not
{}-init.  With {}-init, there is no problem, because we are clearing
TARGET_EXPR_ELIDING_P in process_init_constructor_record:

       /* We can't actually elide the temporary when initializing a
          potentially-overlapping field from a function that returns by
          value.  */
       if (ce->index
           && TREE_CODE (next) == TARGET_EXPR
           && unsafe_copy_elision_p (ce->index, next))
         TARGET_EXPR_ELIDING_P (next) = false;

But that does not happen for ()-init because we have no ce->index.
()-init doesn't allow brace elision so we don't really reshape them.

But I can just move the clearing a few lines down and then it handles
both ()-init and {}-init.

	PR c++/116424

gcc/cp/ChangeLog:

	* typeck2.cc (process_init_constructor_record): Move the clearing of
	TARGET_EXPR_ELIDING_P down.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/paren-init38.C: New test.