Created attachment 58563 [details]
gcc15-pr115744-wip.patch

Untested WIP patch.

(In reply to Jakub Jelinek from comment #1)
> Untested WIP patch.

In the conversion lval change, I'd think we want vc_discard for conversion to void, and to pass through whatever lval for VIEW_CONVERT_EXPR.

Created attachment 58580 [details]
gcc15-pr115744.patch

With the PR115754 change in, this seems mostly about the FTMs and making placement new constexpr.
The only thing that doesn't work is the
new (p + 1) int[]{2, 3};      // error (in this paper)
case from the paper xfailed in the last testcase, the IL has there
<<< Unknown tree: expr_stmt
  (void) (TARGET_EXPR <D.2640, (void *) TARGET_EXPR <D.2641, VIEW_CONVERT_EXPR<int *>(b) + 4>>, TARGET_EXPR <D.2642, operator new [] (8, NON_LVALUE_EXPR <D.2640>)>,   int * D.2643;
  <<< Unknown tree: expr_stmt
    (void) (D.2643 = (int *) D.2642) >>>;
and so there is nothing that would trigger some error during constexpr evaluation.
Shouldn't build_new_1 for maybe_constexpr_fn (current_function_decl) in the array_p case add some extra cast to the array type it is looking for rather
than just the ultimate element type?