[Bug c++/59655] incorrect diagnostic on templatized function with lambda parameter

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59655

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another testcase, this time from Bug 85936:

template <class a> void f(int, a);
template <class a, class c> void g(int, a &&e, c) {
  f(0, [=] { e; });
}
void h() { g(0, h, 0); }


d.cc:1:25: error: ‘void f(int, a) [with a = g(int, a&&, c) [with a = void
(&)(); c = int]::<lambda()>]’, declared using local type ‘g(int, a&&, c) [with
a = void (&)(); c = int]::<lambda()>’, is used but never defined [-fpermissive]
 template <class a> void f(int, a);
                         ^

I'm not sure this one really is valid though.

Clang doesn't complain, and will link successfully if another TU instantiates
g<void(&)(), int> and has a definition for f<g::[lambda]> e.g. in another TU:

template <class a> void f(int, a) { }
template <class a, class c> void g(int, a &&e, c) {
  f(0, [=] { e; });
}

void i() { g(0, i, 0); }