C++ PATCH for c++/90825 - endless recursion when evaluating sizeof

[Jason Merrill]

On 6/11/19 7:47 AM, Jakub Jelinek wrote:
> On Mon, Jun 10, 2019 at 09:59:46PM -0400, Marek Polacek wrote:
>> This test segvs since r269078, this hunk in particular:
>>
>> @@ -4581,8 +4713,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
>>         break;
>>
>>       case SIZEOF_EXPR:
>> -      r = fold_sizeof_expr (t);
>> -      VERIFY_CONSTANT (r);
>> +      r = cxx_eval_constant_expression (ctx, fold_sizeof_expr (t), lval,
>> +                   non_constant_p, overflow_p,
>> +                   jump_target);
>>         break;
>>
>> In a template, fold_sizeof_expr will just create a new SIZEOF_EXPR, that is the
>> same, but not identical; see cxx_sizeof_expr.  Then cxx_eval_constant_expression
> 
> Not always, if it calls cxx_sizeof_expr, it will, but if it calls
> cxx_sizeof_or_alignof_type it will only if the type is dependent or VLA.
> 
> So, I'd think you should call cxx_eval_constant_expression if TREE_CODE (r)
> != SIZEOF_EXPR, otherwise probably *non_constant_p = true; is in order,
> maybe together with gcc_assert (ctx->quiet); ?  I'd hope that if we really
> require a constant expression we evaluate it in !processing_template_decl
> contexts.

Makes sense.  Also, cxx_sizeof_expr should probably only return a 
SIZEOF_EXPR if the operand is instantiation-dependent.

Jason