[Bug c++/89622] New: G++ prints notes, but no warning or error

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89622

            Bug ID: 89622
           Summary: G++ prints notes, but no warning or error
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

The following code compiles successfuly, but prints two mysterious notes:

# 1 "ambig.cc"
# 1 "ambig.h" 1

# 2 "ambig.h" 3


# 3 "ambig.h" 3
template<typename T>
struct X
{
  X() { }
  template<typename U> X(int, U&&) { }
  template<typename U> X(char, const X<U>&) { }
};

template<typename T>
X<T> wrap_X(X<T> x)
{
  return X<T>('a', x);
}
# 2 "ambig.cc" 2


# 3 "ambig.cc"
int main()
{
  X<void> x;
  wrap_X(x);
}


$ g++ ambig.ii
In file included from ambig.cc:1:
ambig.h: In instantiation of 'X<T> wrap_X(X<T>) [with T = void]':
ambig.cc:6:11:   required from here
ambig.h:8:24: note: candidate 1: 'X<T>::X(char, const X<U>&) [with U = void; T
= void]'
ambig.h:7:24: note: candidate 2: 'X<T>::X(int, U&&) [with U = X<void>&; T =
void]'


Adding -Wsystem-headers reveals that the notes are related to a warning that is
suppressed:

ambig.h: In instantiation of 'X<T> wrap_X(X<T>) [with T = void]':
ambig.cc:6:11:   required from here
ambig.h:14:10: warning: ISO C++ says that these are ambiguous, even though the
worst conversion for the first is better than the worst conversion for the
second:
ambig.h:8:24: note: candidate 1: 'X<T>::X(char, const X<U>&) [with U = void; T
= void]'
ambig.h:7:24: note: candidate 2: 'X<T>::X(int, U&&) [with U = X<void>&; T =
void]'


If the warning is suppressed then the notes should be too.

I get the same behaviour back to 4.3 so this doesn't seem to be a regression.