Re: [PATCH] c++, v2: Reject UB in static_cast downcasts during constant evaluation

On 7/27/26 5:02 PM, Jakub Jelinek wrote:
> On Fri, Jul 24, 2026 at 02:02:25PM -0400, Jason Merrill wrote:
>> Hmm, yes, because e.g. in constexpr-base2a.C the A subobject is at a
>> non-zero offset so there isn't a B at that address.
>>
>> This seems to indicate that we're representing the conversion badly.
>>
>> This is because the cp_fold_convert in build_static_cast_1 turns
>> NOP_EXPR (B&, NOP_EXPR (B*, POINTER_PLUS_EXPR (NOP_EXPR (A*, a), -4)))
>> into
>> POINTER_PLUS_EXPR (NOP_EXPR (B&, a), -4)
>> so the NOP_EXPR is now an invalid downcast that the patch diagnoses.
> 
> Here is an updated patch, which
> 1) has slightly different wording mostly as you suggested,
>     I have just used the pointed or referenced type rather than a pointer
>     (what would be printed in the reference case anyway?)
> 2) in cp_fold_convert I've stopped using fold_convert for the
>     INDIRECT_TYPE_P to INDIRECT_TYPE_P casts so that we avoid the
>     Convert (T1)(X p+ Y) into ((T1)X p+ Y), for pointer type, when ...
>     problematic fold-const.cc optimization, instead it optimizes just
>     some simple cases
> 3) had to move -fsanitize=null checking for references on nullptr in order
>     not to regress one ubsan testcase
> 
> Bootstrapped successfull on both x86_64-linux and i686-linux, but has one
> regression on both:
> FAIL: std/ranges/adaptors/slide/1.cc  -std=gnu++23 (test for excess errors)
> Excess errors:
> /home/jakub/src/gcc/libstdc++-v3/testsuite/std/ranges/adaptors/slide/1.cc:110: error: non-constant condition for static assertion
> /home/jakub/src/gcc/obj74/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/ranges_base.h:1016: error: '(((((int*)(& x)) + 12) - (((int*)(& x)) + 4)) / 4)' is not a constant expression
> and ditto with -std=gnu++29.  I'm afraid something relies on
> some of the fold_convert optimizations, dunno if we should try harder
> during POINTER_DIFF_EXPR folding or what.
What if we disable the fold_unary_loc transformation if the pointers are 
to different RECORD_TYPEs?  There are already a couple other exceptions.

Jason