[Bug c++/109551] New: Show location where implicit instantiation first required

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109551

            Bug ID: 109551
           Summary: Show location where implicit instantiation first
                    required
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Copied from https://github.com/llvm/llvm-project/issues/20129

template <class T> struct S1 {};
template <class T> struct S2: S1<T> {};
S2<int> s;
template <> struct S1<int> {};

We correctly reject this code:

src.cc:4:20: error: specialization of 'S1<int>' after instantiation
    4 | template <> struct S1<int> {};
      |                    ^~~~~~~
src.cc:4:20: error: redefinition of 'struct S1<int>'
src.cc:1:27: note: previous definition of 'struct S1<int>'
    1 | template <class T> struct S1 {};
      |                           ^~

But it would be nice if we also pointed to the location that required the
implicit instantiation, i.e. line 3:

src.cc:3:9: note: implicit instantiation first required here:
    3 | S2<int> s;
      |         ^


EDG does indicate where the implicit instantiation is, although it points to
the use of S1<T> as a base class of S2<int> on line 2 (maybe that's better?)

"src.cc", line 4: error: explicit specialization of class "S1<int>" must
          precede its first use (at line 2)
  template <> struct S1<int> {};
                     ^

1 error detected in the compilation of "src.cc".