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From: "renlin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97399] New: g++ 9.3 cannot compile SFINAE code with
 separated declaration and definition, g++ 7.3 compiles
Date: Tue, 13 Oct 2020 09:58:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97399

            Bug ID: 97399
           Summary: g++ 9.3 cannot compile SFINAE code with separated
                    declaration and definition, g++ 7.3 compiles
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: renlin at gcc dot gnu.org
  Target Milestone: ---

Created attachment 49362
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D49362&action=3Dedit
test case 1

For gcc_1.c++
gcc 7.3 compiles for this code
clang 7 compiles for this code
gcc 9.3 fails to compile with following message

Not sure if this is gcc's issue or clang.

```
<source>:29:16: error: no declaration matches 'constexpr
enable_if_t<((tmp*)this)->is_integral<E>(), bool> tmp::func(E, E) const'

   29 | constexpr auto tmp::func(E f_lhs, E f_rhs)

      |                ^~~

<source>:18:27: note: candidate is: 'template<class E> static constexpr
enable_if_t<((tmp*)this)->is_integral<E>(), bool> tmp::func(E, E)'

   18 |     static constexpr auto func(E f_lhs, E f_rhs)

      |                           ^~~~

<source>:12:8: note: 'struct tmp' defined here

   12 | struct tmp
```

Meanwhile for gcc_2.c++
gcc compiles without any issue.
clang gives the following error message
```
<source>:27:28: error: template parameter redefines default argument

template <class E, class =3D enable_if_t<tmp::is_integral<E>(), bool>>

                           ^

<source>:17:32: note: previous default template argument defined here

    template <class E, class =3D enable_if_t<tmp::is_integral<E>(), bool>>
```
It seems this is not an new issue, and might be duplicated.=