[Bug c++/97399] g++ 9.3 cannot compile SFINAE code with separated declaration and definition, g++ 7.3 compiles

[renlin at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97399

--- Comment #1 from Renlin Li  ---
Created attachment 49363
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49363&action=edit
test case 2

[Bug c++/97399] New: g++ 9.3 cannot compile SFINAE code with separated declaration and definition, g++ 7.3 compiles

[renlin at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97399

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=49362&action=edit
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.

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

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

  |^~~

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

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

  |   ^~~~

: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
```
:27:28: error: template parameter redefines default argument

template (), bool>>

   ^

:17:32: note: previous default template argument defined here

template (), bool>>
```
It seems this is not an new issue, and might be duplicated.