[Bug c++/104326] New: Deduction failure for parameter pack in template template non-type parameter

[Bug c++/104326] New: Deduction failure for parameter pack in template template non-type parameter

[colavitam at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104326

            Bug ID: 104326
           Summary: Deduction failure for parameter pack in template
                    template non-type parameter
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: colavitam at gmail dot com
  Target Milestone: ---

Consider the following code:

```
template <typename... Args>
struct tuple {};

template <typename>
struct Test;

template <
    typename... Types,
    template <Types> typename... Outer,
    Types... Inner
>
struct Test<tuple<Outer<Inner>...>> {};

template <long T> struct O1 {};
template <long T> struct O2 {};
template <unsigned T> struct O3 {};

Test<tuple<O1<1>, O2<2>>> test1;
Test<tuple<O1<1>, O2<2>, O3<3>>> test2;
```

This code is rejected by gcc and accepted by clang.

gcc accepts the definition of test1 but rejects test2. It appears as though
Types is not being correctly expanded with the template template parameter.

[Bug c++/104326] Deduction failure for parameter pack in template template non-type parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104326

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think there is a c++ standard defect report about this case. The problem is
the conversion of unsigned and long.

[Bug c++/104326] Deduction failure for parameter pack in template template non-type parameter

[colavitam at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104326

--- Comment #2 from Michael Colavita <colavitam at gmail dot com> ---
It doesn't appear to be specific to the long and unsigned case. The problem
persists if you change unsigned to another type (or replace it with something
like a member pointer). I can't find an accepted case where the non-type
parameter type of O3 is different than the non-type parameter type of O2/O1.

e.g. the following still fails:

/* ... */
struct S {
    long field;
};

template <long T> struct O1 {};
template <long T> struct O2 {};
template <long S::* T> struct O3 {};

Test<tuple<O1<1>, O2<3>>> test1;
Test<tuple<O1<1>, O2<3>, O3<&S::field>>> test2;

[Bug c++/104326] Deduction failure for parameter pack in template template non-type parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104326

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Hmm, only clang is able to accept the code.
MSVC rejects it with:
<source>(13): error C3520: 'Types': parameter pack must be expanded in this
context


While ICC ICEs after an error message:
<source>(10): error: parameter pack "Types" was referenced but not expanded
      template <Types> typename... Outer,
                ^

<source>(11): error: parameter pack "Types" was referenced but not expanded
      Types... Inner
      ^

<source>(19): internal error: null pointer
  Test<tuple<O1<1>, O2<2>>> test1;
                            ^

[Bug c++/104326] Deduction failure for parameter pack in template template non-type parameter

[colavitam at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104326

--- Comment #4 from Michael Colavita <colavitam at gmail dot com> ---
This is a slightly different case, but if you change

-   template <Types> typename... Outer,
+   template <auto> typename... Outer,

it is accepted by both MSVC and clang (whereas gcc now rejects both test1 and
test2). ICC still ICEs.