Re: [PATCH] c++: naming a dependently-scoped template for CTAD [PR104641]

[Jason Merrill <jason@redhat.com>]

On 3/9/22 10:39, Patrick Palka wrote:
> On Tue, 8 Mar 2022, Jason Merrill wrote:
> 
>> On 3/2/22 14:32, Patrick Palka wrote:
>>> In order to be able to perform CTAD for a dependently-scoped template
>>> such as A<T>::B in the testcase below, we need to permit a
>>> typename-specifier to resolve to a template as per [dcl.type.simple]/2,
>>> at least when it appears in a CTAD-enabled context.
>>>
>>> This patch implements this using a new tsubst flag tf_tst_ok to control
>>> when a TYPENAME_TYPE is allowed to name a template, and sets this flag
>>> when substituting into the type of a CAST_EXPR, CONSTRUCTOR or VAR_DECL
>>> (each of which is a CTAD-enabled context).
>>
>> What breaks if we always allow that, or at least in -std that support CTAD?
> 
> AFAICT no significant breakage, but some accepts-invalid and diagnostic
> regressions crop up, e.g. accepts-invalid for
> 
>    using type = typename A<T>::B; // no more diagnostic if typename resolves to a
>                                   // template at instantiation time
> 
> and diagnostic regression for
> 
>    template<class T, class = typename A<T>::B> void f();
>    // no more elaboration why deduction failed if typename resolves
>    // to a template

Ah, sure, the cost is that we would need to check for this case in 
various callers, rather than setting a flag in a different set of 
callers.  Fair enough.

> @@ -16229,6 +16237,12 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>  	      }
>  	  }
>  
> +	if (TREE_CODE (f) == TEMPLATE_DECL)
> +	  {
> +	    gcc_checking_assert (tst_ok);
> +	    f = make_template_placeholder (f);
> +	  }

How about calling make_template_placeholder in make_typename_type?

Jason