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From: "davveston at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/98820] New: Placeholder (auto) non-type template parameter
 wrongly deduced to 'const' for class type arguments
Date: Mon, 25 Jan 2021 11:02:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98820

            Bug ID: 98820
           Summary: Placeholder (auto) non-type template parameter wrongly
                    deduced to 'const' for class type arguments
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: davveston at gmail dot com
  Target Milestone: ---

As per [temp.param]/6 in N4861, for non-template parameters (including a ty=
pe
that contains a placeholder type):

"[...] The top-level cv-qualifiers on the template-parameter are ignored wh=
en
determining its type."

and as per [dcl.type.decltype]/1.2:

"For an expression E, the type denoted by decltype(E) is defined as follows=
:=20
- [...] naming a non-type template-parameter, decltype(E) is the type of the
template-parameter after performing any necessary type deduction [...]"

Thus, the following example should arguably be well-formed:

 #include <type_traits>

 struct A{};

 template <auto a>=20
 void f() {=20
     static_assert(std::is_same_v<decltype(a), A>);
 }

 template <auto const a>=20
 void g() {=20
     static_assert(std::is_same_v<decltype(a), A>);
 }

 int main() {
     constexpr A a{};
     f<a>();
     g<a>();
 }

However GCC rejects it as `decltype(a)` resolves to `A const` rather than `=
A`.

---

We may note that:

1) Clang accepts the program, and=20
2) Both Clang and GCC accepts the program when using a structural type dire=
ctly
instead of a placeholder type for the non-type template parameter; replacing
the definitions of `f` and `g` above with:

 template <A a>=20
 void f() {=20
     static_assert(std::is_same_v<decltype(a), A>);
 }

 template <A const a>=20
 void g() {=20
     static_assert(std::is_same_v<decltype(a), A>);
 }

3) Both clang and GCC accepts the program when using a placeholder type but
with a non-class type as template argument to it; replacing the definitions=
 of
`f` and `g` above with:

 template <auto a>=20
 void f() {=20
     static_assert(std::is_same_v<decltype(a), int>);
 }

 template <auto const a>=20
 void g() {=20
     static_assert(std::is_same_v<decltype(a), int>);
 }

and calling them as f<0>() and g<0>();

---

These standard rules are arguably somewhat confusing given that the template
parameter object itself (associated with the class-type non-type template
parameter) is `const` ([temp.param]/8).=