The paragraph that allowes place holder for deduced class type in template
parameter is [dcl.type.class.deduc]/§2

>A placeholder for a deduced class type can also be used in the
type-specifier-seq <http://eel.is/c++draft/dcl.type#nt:type-specifier-seq> in
the new-type-id <http://eel.is/c++draft/expr.new#nt:new-type-id> or type-id
<http://eel.is/c++draft/dcl.name#nt:type-id> of a new-expression
<http://eel.is/c++draft/expr.new#nt:new-expression>, as the
simple-type-specifier
<http://eel.is/c++draft/dcl.type.simple#nt:simple-type-specifier> in
an explicit
type conversion (functional notation)
<http://eel.is/c++draft/expr.type.conv>, or as the type-specifier
<http://eel.is/c++draft/dcl.type#nt:type-specifier> in the
parameter-declaration
<http://eel.is/c++draft/dcl.fct#nt:parameter-declaration> of a
template-parameter <http://eel.is/c++draft/temp.param#nt:template-parameter>
. <http://eel.is/c++draft/dcl.type.class.deduct#2.sentence-1>

I had not read non normative "Notes", and apparently the behavior seems to
be recognize by the standard [temp.func.order]/§3

>
[Note <http://eel.is/c++draft/temp.func.order#3.note-1>:
The type replacing the placeholder in the type of the value synthesized for
a non-type template parameter is also a unique synthesized type.
<http://eel.is/c++draft/temp.func.order#3.sentence-2>
 — *end note*]

This is a frustrating limitation because at least it makes impossible to
partially specialize any class template with a non-type template parameter
that uses a placeholder for deduced class type. Should not this be changed?

Le lun. 3 août 2020 à 00:44, Ville Voutilainen via Std-Discussion <
std-discussion_at_[hidden]> a écrit :

> On Mon, 3 Aug 2020 at 01:33, Jason McKesson via Std-Discussion
> <std-discussion_at_[hidden]> wrote:
> >
> > On Sun, Aug 2, 2020 at 2:30 PM Olivier Kannengieser via Std-Discussion
> > <std-discussion_at_[hidden]> wrote:
> > >
> > > No this is what I meant. 'A x' is a C++20 feature: place holder for
> deduced class type for the declaration of non type template parameter.
> >
> > No, "place holder for deduced class type for the declaration of non
> > type template parameter" is the C++17 feature that allows `auto x` to
> > work in template arguments. A template name by itself is not a
> > placeholder.
> >
> > C++20 allows `ConceptName auto x`, which does the above except with a
> > concept constraint as to `x`'s type.
> >
> > At no point does C++ permit `TemplateName x` to work in a template
> > parameter list. There were some Concept ideas that `TemplateName<auto>
> > x` might work, and maybe Concepts TS allowed you to drop the argument.
> > But C++20 does not.
>
> I'm not sure what you mean by "TemplateName x", but this certainly does
> work:
>
> template <class T> concept foo = /* whatever */;
>
> template <foo T> void f(); // or struct, class, etc.
>
> That foo is probably not what you mean by TemplateName, because it's a
> ConceptName.
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