Re: [PATCH] c++: ahead of time variable template-id coercion [PR89442]

[Jason Merrill <jason@redhat.com>]

On 5/3/23 09:50, Patrick Palka wrote:
> This patch makes us coerce the arguments of a variable template-id ahead
> of time, as we do for other template-ids, which allows us to immediately
> diagnose template parameter/argument kind mismatches and arity mismatches.
> 
> Unfortunately this causes a regression in cpp1z/constexpr-if20.C: coercing
> the variable template-id m<ar, as> ahead of time means we strip it of
> typedefs, yielding m<typename C<i>::q, typename C<j>::q>, but in this
> stripped form we're directly using 'i' and so we expect to have captured
> it.  This is PR107437 but with a variable template instead of a class
> template.  I'm not sure how to fix this :(

Hmm, it does seem like strip_typedefs needs to be more conservative in a 
lambda.  We can probably stop stripping dependent function-scope 
typedefs in general without breaking things like cpp0x/decltype62.C.

> @@ -22097,7 +22099,7 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain)
>         /* We need to determine if we're using a partial or explicit
>   	 specialization now, because the type of the variable could be
>   	 different.  */
> -      tree tid = lookup_template_variable (tmpl, targ_ptr);
> +      tree tid = build2 (TEMPLATE_ID_EXPR, NULL_TREE, tmpl, targ_ptr);

Why?  I'd think we want to consider partial specializations based on the 
coerced arguments.

Jason