Re: [PATCH] c++: constexpr union member access folding [PR114709]

[Jason Merrill <jason@redhat.com>]

On 4/22/24 15:18, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> for trunk/13/12?

OK with a rationale comment.

> -- >8 --
> 
> The object/offset canonicalization performed in cxx_fold_indirect_ref
> is undesirable for union member accesses because it loses information
> about the member being accessed which we may later need to diagnose an
> inactive-member access.  So this patch restricts the canonicalization
> accordingly.
> 
> 	PR c++/114709
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (cxx_fold_indirect_ref): Restrict object/offset
> 	canonicalization to RECORD_TYPE member accesses.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/constexpr-union8.C: New test.
> ---
>   gcc/cp/constexpr.cc                           | 1 +
>   gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C | 8 ++++++++
>   2 files changed, 9 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index fcc847d85df..941a478e889 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -5797,6 +5797,7 @@ cxx_fold_indirect_ref (const constexpr_ctx *ctx, location_t loc, tree type,
>        more folding opportunities.  */
>     auto canonicalize_obj_off = [] (tree& obj, tree& off) {
>       while (TREE_CODE (obj) == COMPONENT_REF
> +	   && TREE_CODE (TREE_TYPE (TREE_OPERAND (obj, 0))) == RECORD_TYPE
>   	   && (tree_int_cst_sign_bit (off) || integer_zerop (off)))
>         {
>   	tree field = TREE_OPERAND (obj, 1);
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C
> new file mode 100644
> index 00000000000..34c264944b6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C
> @@ -0,0 +1,8 @@
> +// PR c++/114709
> +// { dg-do compile { target c++11 } }
> +
> +struct T1 { int a, b; };
> +struct T2 { int c; double d; };
> +union U { T1 t1; T2 t2; };
> +
> +constexpr int v = U{{1,2}}.t2.*&T2::c; // { dg-error "accessing 'U::t2'" }