[Bug c++/80951] New: Deducing noexcept only works when also deducing something else

["rs2740 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80951

            Bug ID: 80951
           Summary: Deducing noexcept only works when also deducing
                    something else
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

Another issue (compare bug 80384 and bug 80908) with the extension making
noexcept(E) to be a deduced context
(https://gcc.gnu.org/ml/gcc-patches/2016-11/msg00665.html).

The following works:

void f() noexcept;

template<bool E, class R>
void g(R (*)() noexcept(E));

using t1 = decltype(g(f)); // OK

But not when the only thing being deduced is E:

template<bool E>
void h(void (*)() noexcept(E));

using t2 = decltype(h(f)); // error

<source>:12:24: error: no matching function for call to 'h(void (&)()
noexcept)'
 using t2 = decltype(h(f)); // error
                        ^
<source>:10:6: note: candidate: 'template<bool E> void h(void (*)() noexcept
(E))'
 void h(void (*)() noexcept(E));
      ^
<source>:10:6: note:   template argument deduction/substitution failed:
<source>:12:24: note:   couldn't deduce template parameter 'E'
 using t2 = decltype(h(f)); // error
                        ^
<source>:12:24: error: no matching function for call to 'h(void (&)()
noexcept)'
<source>:10:6: note: candidate: 'template<bool E> void h(void (*)() noexcept
(E))'
 void h(void (*)() noexcept(E));
      ^
<source>:10:6: note:   template argument deduction/substitution failed:
<source>:12:24: note:   couldn't deduce template parameter 'E'
 using t2 = decltype(h(f)); // error
                        ^