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From: "ensadc at mailnesia dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97700] New: Bogus error when a class containing a function
 pointer is used as a non-type template parameter
Date: Tue, 03 Nov 2020 14:14:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97700

            Bug ID: 97700
           Summary: Bogus error when a class containing a function pointer
                    is used as a non-type template parameter
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
                CC: jason at redhat dot com
  Target Milestone: ---

Originally discovered by cigien on StackOverflow. See
https://stackoverflow.com/questions/64655958/can-a-class-containing-a-funct=
ion-pointer-be-used-as-a-non-type-template-parameter

GCC does not like this program:

````

struct S=20
{=20
    void (*f)();=20
};=20

constexpr S s {[]{}};

template<S> struct X {};
using x =3D X<s>;

````

It complains with:

````
error: '<lambda()>::_FUN' is not a valid template argument of type 'void (*=
)()'
because '<lambda()>::_FUN' is not a variable
using x =3D X<s>;
             ^
````

As mentioned in the StackOverflow post, it appears that
`invalid_tparm_referent_p` (called by `get_template_parm_object`) fails to
account for the pointer-to-function case.=