Re: [PATCH] c++: can't eval PTRMEM_CST in incomplete class [PR107574]

[Jason Merrill <jason@redhat.com>]

On 2/2/23 19:28, Marek Polacek wrote:
> Here we're attempting to evaluate a PTRMEM_CST in a class that hasn't
> been completed yet, but that doesn't work:
> 
>          /* We can't lower this until the class is complete.  */
>          if (!COMPLETE_TYPE_P (DECL_CONTEXT (member)))
>            return cst;
> 
> and then this unlowered PTRMEM_CST is used as EXPR in
> 
>      tree op1 = build_nop (ptrdiff_type_node, expr);
> 
> and we crash in a subsequent cp_fold_convert which gets type=ptrdiff_type_node,
> expr=PTRMEM_CST and does
> 
>    else if (TREE_CODE (expr) == PTRMEM_CST
>             && same_type_p (TYPE_PTRMEM_CLASS_TYPE (type),
>                             PTRMEM_CST_CLASS (expr)))
> 
> where TYPE_PTRMEM_CLASS_TYPE (type) is going to crash since the type
> is ptrdiff_type_node.  We could just add a TYPE_PTRMEM_P check before
> accessing TYPE_PTRMEM_CLASS_TYPE but I think it's nicer to explain why
> we couldn't evaluate the expression.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/107574
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (cxx_eval_constant_expression): Emit an error when
> 	a PTRMEM_CST cannot be evaluated.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/ptrmem-cst1.C: New test.
> ---
>   gcc/cp/constexpr.cc                      |  9 +++++++++
>   gcc/testsuite/g++.dg/cpp0x/ptrmem-cst1.C | 11 +++++++++++
>   2 files changed, 20 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/ptrmem-cst1.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 5b31f9c27d1..2c03988b097 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -7691,6 +7691,15 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
>   	    if (!same_type_ignoring_top_level_qualifiers_p (type, TREE_TYPE (op))
>   		&& !can_convert_qual (type, op))
>   	      op = cplus_expand_constant (op);
> +	    if (TREE_CODE (op) == PTRMEM_CST && !TYPE_PTRMEM_P (type))
> +	      {
> +		if (!ctx->quiet)
> +		  error_at (loc, "%qE is not a constant expression when the "
> +			    "class %qT is still incomplete", op,
> +			    PTRMEM_CST_CLASS (op));
> +		*non_constant_p = true;
> +		return t;
> +	      }

Hmm, maybe handle this a few lines higher, in this existing if:

>         if (TREE_CODE (op) == PTRMEM_CST && !TYPE_PTRMEM_P (type))
>           op = cplus_expand_constant (op);

?  OK with that change.