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From: "ed at catmur dot uk" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/69623] Invalid deduction of non-trailing template parameter
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Date: Mon, 14 Mar 2022 14:26:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D69623

Ed Catmur <ed at catmur dot uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ed at catmur dot uk

--- Comment #6 from Ed Catmur <ed at catmur dot uk> ---
There is a fairly well-known idiom for extracting a specific element of an
argument pack:

auto f(auto..., auto a, auto...) { return a; }
template<auto> struct any { any(auto) {} };
template<std::size_t... I>
auto g(std::index_sequence<I...>, auto... a) { return f<any<I>...>(a...); }
template<unsigned I>
auto h(auto... a) { return g(std::make_index_sequence<I>(), a...); }

I believe the intent of http://eel.is/c++draft/temp#param-14.sentence-3 is =
that
this idiom should work; per http://eel.is/c++draft/temp#param-example-7 (as=
 in
comment #5) a function template secondary template parameter pack is only
disallowed if it is nondeducible.  In the original description of this tick=
et
(not comment #5, which I agree is disallowed) T is specifiable and U is
deducible, so both are OK.

> [...] A template parameter pack of a function template shall not be follo=
wed by another template parameter unless that template parameter can be ded=
uced from the parameter-type-list ([dcl.fct]) of the function template or h=
as a default argument ([temp.deduct]). [...]

Or is the intent that T must be explicitly specified, i.e. that f can be ca=
lled
as f<>() but not as f(), pace
http://eel.is/c++draft/temp#arg.explicit-4.sentence-3 ?

> If all of the template arguments can be deduced, they may all be omitted;=
 in this case, the empty template argument list <> itself may also be omitt=
ed.=