[PATCH] c++: unifying equal NONTYPE_ARGUMENT_PACKs [PR102547]

[Jason Merrill]

On 10/1/21 10:26, Patrick Palka wrote:
> On Fri, 1 Oct 2021, Jason Merrill wrote:
> 
>> On 10/1/21 09:46, Patrick Palka wrote:
>>> Here during partial ordering of the two partial specializations we end
>>> up in unify with parm=arg=NONTYPE_ARGUMENT_PACK<V0, V1>, and crash shortly
>>> thereafter because uses_template_parms calls potential_constant_expression
>>> which doesn't handle NONTYPE_ARGUMENT_PACK.
>>>
>>> This patch fixes this by checking dependent_template_arg_p instead of
>>> uses_template_parms when parm==arg, which does handle NONTYPE_ARGUMENT_PACK.
>>> We could also perhaps fix uses_template_parms / inst_dep_expr_p to better
>>> handle NONTYPE_ARGUMENT_PACK,
>>
>> Please.
> 
> Sounds good, like the following then?  Passes light testing, bootstrap
> and regtest on progress.
> 
> -- >8 --
> 
> 	PR c++/102547
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.c (instantiation_dependent_expression_p): Sidestep checking
> 	potential_constant_expression on NONTYPE_ARGUMENT_PACK.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/variadic-partial2.C: New test.
> 	* g++.dg/cpp0x/variadic-partial2a.C: New test.
> ---
>   gcc/cp/pt.c                                   |  4 +++-
>   .../g++.dg/cpp0x/variadic-partial2.C          | 16 ++++++++++++++
>   .../g++.dg/cpp0x/variadic-partial2a.C         | 22 +++++++++++++++++++
>   3 files changed, 41 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/variadic-partial2.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/variadic-partial2a.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 1dcdffe322a..643204103c5 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -27705,7 +27705,9 @@ instantiation_dependent_expression_p (tree expression)
>   {
>     return (instantiation_dependent_uneval_expression_p (expression)
>   	  || (processing_template_decl
> -	      && potential_constant_expression (expression)
> +	      && expression != NULL_TREE
> +	      && (TREE_CODE (expression) == NONTYPE_ARGUMENT_PACK
> +		  || potential_constant_expression (expression))

I'd prefer to loop over the elements of the pack, either here or 
(probably better) in potential_constant_expression.

>   	      && value_dependent_expression_p (expression)));
>   }
>   
> diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic-partial2.C b/gcc/testsuite/g++.dg/cpp0x/variadic-partial2.C
> new file mode 100644
> index 00000000000..df61f26a3c1
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/variadic-partial2.C
> @@ -0,0 +1,16 @@
> +// PR c++/102547
> +// { dg-do compile { target c++11 } }
> +
> +template<int... Vs>
> +struct vals { };
> +
> +template<class V, class T>
> +struct vals_client { };
> +
> +template<int V0, int V1, class T>
> +struct vals_client<vals<V0, V1>, T> { };
> +
> +template<int V0, int V1>
> +struct vals_client<vals<V0, V1>, void> { };
> +
> +template struct vals_client<vals<1, 2>, void>; //- "sorry, unimplemented..., ICE"
> diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic-partial2a.C b/gcc/testsuite/g++.dg/cpp0x/variadic-partial2a.C
> new file mode 100644
> index 00000000000..cc0ea488ad3
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/variadic-partial2a.C
> @@ -0,0 +1,22 @@
> +// PR c++/102547
> +// { dg-do compile { target c++11 } }
> +// A version of variadic-partial2.C where the partial ordering is performed
> +// on function templates instead of class templates.
> +
> +template<int... Vs>
> +struct vals { };
> +
> +template<class V, class T>
> +void f(V, T) { };
> +
> +template<int V0, int V1, class T>
> +void f(vals<V0, V1>, T) { };
> +
> +template<int V0, int V1>
> +void f(vals<V0, V1>, char) { };
> +
> +template void f(vals<1, 2>, char); //- "sorry, unimplemented..., ICE"
> +
> +int main() {
> +  f(vals<1, 3>{}, 'a'); //- "sorry, unimplemented..., ICE"
> +}
>