[PATCH v2] c++: Fix noexcept with unevaluated operand [PR101087]

[Jason Merrill]

On 7/8/21 4:26 PM, Marek Polacek wrote:
> On Thu, Jul 08, 2021 at 09:35:02AM -0400, Marek Polacek wrote:
>> On Thu, Jul 08, 2021 at 09:30:27AM -0400, Jason Merrill wrote:
>>> On 7/7/21 9:40 PM, Marek Polacek wrote:
>>>> It sounds plausible that this assert
>>>>
>>>>     int f();
>>>>     static_assert(noexcept(sizeof(f())));
>>>>
>>>> should pass: sizeof produces a std::size_t and its operand is not
>>>> evaluated, so it can't throw.  noexcept should only evaluate to
>>>> false for potentially evaluated operands.  Therefore I think that
>>>> check_noexcept_r shouldn't walk into operands of sizeof/decltype/
>>>> alignof/typeof.  Only checking cp_unevaluated_operand therein does
>>>> not work, because expr_noexcept_p can be called in an unevaluated
>>>> context, so I resorted to the following cp_evaluated hack.  Does
>>>> that seem acceptable?
>>>
>>> I suppose, but why not check for SIZEOF_EXPR/ALIGNOF_EXPR/NOEXCEPT_EXPR
>>> directly?
>>
>> I thought I would, but then it occurred to me that it might be better to
>> rely on cp_walk_subtrees which ++/--s cp_unevaluated_operand for those
>> codes.  I'd be happy to change the patch to check those codes directly;
>> maybe I'm overthinking things here.
> 
> So here's v2 which checks the codes directly, via a new inline:
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK for trunk and 11, at least.  I lean toward putting it on older 
release branches as well, but it doesn't seem urgent.

> -- >8 --
> It sounds plausible that this assert
> 
>    int f();
>    static_assert(noexcept(sizeof(f())));
> 
> should pass: sizeof produces a std::size_t and its operand is not
> evaluated, so it can't throw.  noexcept should only evaluate to
> false for potentially evaluated operands.  Therefore I think that
> check_noexcept_r shouldn't walk into operands of sizeof/decltype/
> alignof/typeof.
> 
> 	PR c++/101087
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (unevaluated_p): New.
> 	* except.c (check_noexcept_r): Use it.  Don't walk into
> 	unevaluated operands.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/noexcept70.C: New test.
> ---
>   gcc/cp/cp-tree.h                        | 13 +++++++++++++
>   gcc/cp/except.c                         |  9 ++++++---
>   gcc/testsuite/g++.dg/cpp0x/noexcept70.C |  5 +++++
>   3 files changed, 24 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept70.C
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index b4501576b26..d4810c0c986 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -8465,6 +8465,19 @@ is_constrained_auto (const_tree t)
>     return is_auto (t) && PLACEHOLDER_TYPE_CONSTRAINTS_INFO (t);
>   }
>   
> +/* True if CODE, a tree code, denotes a tree whose operand is not evaluated
> +   as per [expr.context], i.e., an operand to sizeof, typeof, decltype, or
> +   alignof.  */
> +
> +inline bool
> +unevaluated_p (tree_code code)
> +{
> +  return (code == DECLTYPE_TYPE
> +	  || code == ALIGNOF_EXPR
> +	  || code == SIZEOF_EXPR
> +	  || code == NOEXCEPT_EXPR);
> +}
> +
>   /* RAII class to push/pop the access scope for T.  */
>   
>   struct push_access_scope_guard
> diff --git a/gcc/cp/except.c b/gcc/cp/except.c
> index a8cea53cf91..a8acbc4b7b2 100644
> --- a/gcc/cp/except.c
> +++ b/gcc/cp/except.c
> @@ -1033,12 +1033,15 @@ check_handlers (tree handlers)
>        expression whose type is a polymorphic class type (10.3).  */
>   
>   static tree
> -check_noexcept_r (tree *tp, int * /*walk_subtrees*/, void * /*data*/)
> +check_noexcept_r (tree *tp, int *walk_subtrees, void *)
>   {
>     tree t = *tp;
>     enum tree_code code = TREE_CODE (t);
> -  if ((code == CALL_EXPR && CALL_EXPR_FN (t))
> -      || code == AGGR_INIT_EXPR)
> +
> +  if (unevaluated_p (code))
> +    *walk_subtrees = false;
> +  else if ((code == CALL_EXPR && CALL_EXPR_FN (t))
> +	   || code == AGGR_INIT_EXPR)
>       {
>         /* We can only use the exception specification of the called function
>   	 for determining the value of a noexcept expression; we can't use
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept70.C b/gcc/testsuite/g++.dg/cpp0x/noexcept70.C
> new file mode 100644
> index 00000000000..45a6137dd6f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept70.C
> @@ -0,0 +1,5 @@
> +// PR c++/101087
> +// { dg-do compile { target c++11 } }
> +
> +int f();
> +static_assert(noexcept(sizeof(f())), "");
> 
> base-commit: 763121ccd908f52bc666f277ea2cf42110b3aad9
>