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From: "kretz at kde dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/100716] New: member function template parameter not printed
 in candidate list
Date: Fri, 21 May 2021 15:21:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100716

            Bug ID: 100716
           Summary: member function template parameter not printed in
                    candidate list
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kretz at kde dot org
                CC: paolo.carlini at oracle dot com
  Target Milestone: ---

template<typename T>
  struct A {
    template<typename U>
      void f() { }
  };

int main() {
  A<void>().f(0);
}

With -fpretty-templates this prints:

<source>: In function 'int main()':
<source>:8:6: error: no matching function for call to 'A<void>::f(int)'
<source>:4:12: note: candidate: 'template<class U> void A<T>::f() [with U =
=3D U;
T =3D void]'
[...]

>>From the diagnostics, it is not apparent what entity U applies to. Also 'wi=
th U
=3D U' is unnecessary noise. It should be:

<source>:4:12: note: candidate: 'template<class U> void A<T>::f<U>() [with =
T =3D
void]'

However,

template<typename T>
  struct A {
    template<typename U>
      void f(U) { }
  };

int main() {
  A<void>().f();
}

doesn't need to show the function template parameter and should be diagnosed
as:

<source>:4:12: note: candidate: 'template<class U> void A<T>::f(U) [with T =
=3D
void]'

However, to distinguish the two cases, cp/error.c would need to determine
whether the function template parameters are deducible. Alternatively, it w=
ould
have to look at the list of function parameters and check whether all templ=
ate
parameters are used somehow (even if not deducible). Not sure whether that's
worth the effort. I assume the current behavior was chosen because the majo=
rity
of function template parameters are deducible?

FWIW, I believe the fix for PR50828 was incorrect since it removes the abil=
ity
to control whether dump_template_parms called from dump_function_decl retur=
ns
early for primary templates or not. I think the flags need to be modified f=
or
printing the function's scope, but not for dump_function_decl.

I'll try to come up with a patch.=