I have been able to replicate this technique--using the example linked to godbolt from the blog post--working on clang 7 and failing on clang 8-11 (with the following error)... but it works again on clang 12 ;P. (I am bisecting overnight to see exactly what fixed it; if anyone sees this and is interested, come back to this thread tomorrow for a follow-up.)

    note: candidate template ignored: substitution failure [with N = 0]: function 'loophole' with deduced return type cannot be used before it is defined
    template <int N, typename T = decltype(loophole(A::tag<N>{}))> T get_type();
                                           ~~~~~~~~                  ^

    commit dd8297b0669f8e69b03ba40171b195b5acf0f963
    Author: Richard Smith <[email protected]>
    Date:   Fri Oct 30 18:30:56 2020 -0700

        PR42513: Fix handling of function definitions lazily instantiated from
        friends.

        When determining whether a function has a template instantiation
        pattern, look for other declarations of that function that were
        instantiated from a friend function definition, rather than assuming
        that checking for member specialization information on whichever
        declaration name lookup found will be sufficient.
 
     clang/include/clang/AST/Decl.h                 |  14 +++-
     clang/lib/AST/Decl.cpp                         |  61 ++++++++++++--
     clang/lib/Sema/SemaDecl.cpp                    |  84 +++++--------------
     clang/lib/Sema/SemaDeclCXX.cpp                 |  11 +++
     clang/lib/Sema/SemaTemplateInstantiateDecl.cpp | 110 ++++++++++++-------------
     clang/test/SemaTemplate/friend.cpp             |   7 ++
     6 files changed, 159 insertions(+), 128 deletions(-)

https://bugs.llvm.org/show_bug.cgi?id=42513