TfRefPtr<TfRefBase> expected unqualified-id before ‘)’ token error when compiling with GCC 12.2.1 and -std=gnu++20

[expenses]

Description of Issue

usd/include/pxr/base/tf/refPtr.h:1304:25: error: expected unqualified-id before ‘)’ token
 1304 |     TfRefPtr<TfRefBase>() {
      |                         ^
usd/include/pxr/base/tf/refPtr.h:1311:31: error: expected unqualified-id before ‘)’ token
 1311 |     TfRefPtr<const TfRefBase>() {
      |                               ^

Steps to Reproduce

1. Compile USD on either the release (0c7b9a9) or dev (59eaff4) branch.
2. Try and include pxr/usd/sdf/layer.h using cxx_std_20 as a cmake compile feature

System Information (OS, Hardware)

• Arch Linux 6.1.7-arch1-1

If this is just a general case of 'USD doesn't compile with C++ 20' the that's fine, but I wonder how much effort it would take to fix that.

[expenses]

I tried just commenting out these lines:

/*
template <>
class TfRefPtr<TfRefBase> {
private:
    TfRefPtr<TfRefBase>() {
    }
};
template <>
class TfRefPtr<const TfRefBase> {
private:
    TfRefPtr<const TfRefBase>() {
    }
};
*/

and it now works fine on C++20 and C++23! Diff: release...expenses:USD:cpp23

I presume these lines are there for a reason (// Specialization: prevent construction of a TfRefPtr<TfRefBase>.). Maybe we can use the preprocessor to only compile them for C++17 and below?

[dubois]

It looks to me like invalid C++17 that accidentally happens to compile; I suspect the <TfRefBase> should be removed from the ctor definition:

template<> class TfRefPtr<TfRefBase> {
  TfRefPtr() {}
}

[sunyab]

Filed as internal issue #USD-7903