Re: [PATCH] c++: address of NTTP object as targ [PR113242]

[Jason Merrill <jason@redhat.com>]

On 1/17/24 10:43, Patrick Palka wrote:
> On Mon, 15 Jan 2024, Jason Merrill wrote:
>> On 1/5/24 11:50, Patrick Palka wrote:
>>>
>>> invalid_tparm_referent_p was rejecting using the address of a class NTTP
>>> object as a template argument, but this should be fine.
>>
>> Hmm, I suppose so; https://eel.is/c++draft/temp#param-8 saying "No two
>> template parameter objects are template-argument-equivalent" suggests there
>> can be only one.  And clang/msvc allow it.
>>
>>> +	else if (VAR_P (decl) && !DECL_NTTP_OBJECT_P (decl)
>>> +		 && DECL_ARTIFICIAL (decl))
>>
>> If now some artificial variables are OK and others are not, perhaps we should
>> enumerate them either way and abort if it's one we haven't specifically
>> considered.
> 
> Sounds good, like so?  Shall we backport this patch or the original
> patch to the 13 branch?

Hmm, looks like this patch changes the non-checking default behavior 
from reject to accept; maybe just add a checking_assert (tinfo || fname) 
to your original patch?  OK with that change, for trunk and 13.

> -- >8 --
> 
> Subject: [PATCH] c++: address of class NTTP object as targ [PR113242]
> 
> invalid_tparm_referent_p was rejecting using the address of a class NTTP
> object as a template argument, but this should be fine.
> 
> This patch fixes this by refining the DECL_ARTIFICIAL rejection test to
> check specifically for the kinds of artificial variables we want to
> exclude.
> 
> 	PR c++/113242
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (invalid_tparm_referent_p) <case ADDR_EXPR>: Refine
> 	DECL_ARTIFICIAL rejection test.  Assert that C++20 template
> 	parameter objects are the only artificial variables we accept.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/nontype-class61.C: New test.
> ---
>   gcc/cp/pt.cc                                 | 13 +++++++---
>   gcc/testsuite/g++.dg/cpp2a/nontype-class61.C | 27 ++++++++++++++++++++
>   2 files changed, 37 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class61.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index b6117231de1..885c297450e 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -7212,12 +7212,14 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
>   	/* C++17: For a non-type template-parameter of reference or pointer
>   	   type, the value of the constant expression shall not refer to (or
>   	   for a pointer type, shall not be the address of):
> -	   * a subobject (4.5),
> +	   * a subobject (4.5), (relaxed in C++20)
>   	   * a temporary object (15.2),
> -	   * a string literal (5.13.5),
> +	   * a string literal (5.13.5), (we diagnose this early in
> +	     convert_nontype_argument)
>   	   * the result of a typeid expression (8.2.8), or
>   	   * a predefined __func__ variable (11.4.1).  */
> -	else if (VAR_P (decl) && DECL_ARTIFICIAL (decl))
> +	else if (VAR_P (decl)
> +		 && (DECL_TINFO_P (decl) || DECL_FNAME_P (decl)))
>   	  {
>   	    if (complain & tf_error)
>   	      error ("the address of %qD is not a valid template argument",
> @@ -7242,6 +7244,11 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
>   		     decl);
>   	    return true;
>   	  }
> +
> +	/* The only artificial variables we do accept are C++20
> +	   template parameter objects.   */
> +	if (VAR_P (decl) && DECL_ARTIFICIAL (decl))
> +	  gcc_checking_assert (DECL_NTTP_OBJECT_P (decl));
>         }
>         break;
>   
> diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class61.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class61.C
> new file mode 100644
> index 00000000000..90805a05ecf
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class61.C
> @@ -0,0 +1,27 @@
> +// PR c++/113242
> +// { dg-do compile { target c++20 } }
> +
> +struct wrapper {
> +  int n;
> +};
> +
> +template<const wrapper& X>
> +void f1() {
> +  static_assert(X.n == 42);
> +}
> +
> +template<const wrapper* X>
> +void f2() {
> +  static_assert(X->n == 42);
> +}
> +
> +template<wrapper X>
> +void g() {
> +  f1<X>();
> +  f2<&X>();
> +}
> +
> +int main() {
> +  constexpr wrapper X = {42};
> +  g<X>();
> +}