Re: [PATCH] c++: deduction for dependent class type of NTTP [PR105110]

[Jason Merrill <jason@redhat.com>]

On 3/30/22 17:51, Patrick Palka wrote:
> Here deduction for the P/A pair V/a spuriously fails with
> 
>    types ‘A<T>’ and ‘const A<int>’ have incompatible cv-qualifiers
> 
> because the argument type is const, whereas the parameter type is
> non-const.
> 
> Since the type of an NTTP is always cv-unqualified, it seems natural to
> ignore cv-qualifiers on the argument type before attempting to unify the
> two types.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> 	PR c++/105110
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Ignore cv-quals on
> 	on the argument type of an NTTP before deducing from it.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/nontype-class52.C: New test.
> ---
>   gcc/cp/pt.cc                                 |  5 +++--
>   gcc/testsuite/g++.dg/cpp2a/nontype-class52.C | 13 +++++++++++++
>   2 files changed, 16 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class52.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 1acb5990c5c..cdd75d3b6ac 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -24271,8 +24271,9 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
>   	      && !(strict & UNIFY_ALLOW_INTEGER)
>   	      && TEMPLATE_PARM_LEVEL (parm) <= TMPL_ARGS_DEPTH (targs))
>   	    {
> -	      /* Deduce it from the non-type argument.  */
> -	      tree atype = TREE_TYPE (arg);
> +	      /* Deduce it from the non-type argument.  As above, ignore
> +		 top-level quals here too.  */
> +	      tree atype = cv_unqualified (TREE_TYPE (arg));
>   	      RECUR_AND_CHECK_FAILURE (tparms, targs,
>   				       tparm, atype,
>   				       UNIFY_ALLOW_NONE, explain_p);
> diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class52.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class52.C
> new file mode 100644
> index 00000000000..56163376afb
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class52.C
> @@ -0,0 +1,13 @@
> +// PR c++/105110
> +// { dg-do compile { target c++20 } }
> +
> +template<class> struct A { };
> +
> +template<auto> struct B { };
> +
> +template<class T, A<T> V> void f(B<V>);
> +
> +int main() {
> +  constexpr A<int> a;
> +  f(B<a>{});
> +}